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3 Evidence A popular question of interest in the 1870s was whether horses ever had all four hooves off the ground at the same time when running. Artists often painted horse...

3 Evidence A popular question of interest in the 1870s was whether horses ever had all four hooves off the ground at the same time when running. Artists often painted horses with their front legs pointing forwards and their rear legs backwards, but no-one knew if this actually happened. Photographer Eadweard Muybridge decided to gather evidence to answer this question by setting up a series of cameras that were triggered by a thread as a galloping horse passed. The resulting images show that all four of the horse’s hooves do leave the ground, but only when the hooves are underneath its body, not outstretched as the artists had been depicting. The evidence caused artists to change the way in which they drew horses. Why is photographic evidence compelling? 4 4  DNA traces left at the scene of a crime can provide evidence in a trial. What does the DNA evidence actually prove? The giant squid has been the subject of myth for thousands of years, yet almost nothing was known about it as the only evidence of its existence was from dead specimens washed up on the shore or fragments found in the stomachs of sperm whales. The rst observation of live animals did not occur until the beginning of the 21st century. Was it necessary to see a live animal in order to prove its existence? The possibility of climate change is a major threat to the human race. Many people believe that climate change is caused by humans; however, providing conclusive evidence that can persuade all scientists and politicians alike has proven difcult, and so the issue remains controversial. Why might scientists and politicians be persuaded by different forms of evidence? 4 5  E V I D E N C E Introduction Scientists try to explain how and why things happen. In physics, we are concerned with the way the universe works, and physicists Key concept: Relationships develop theories to explain the underlying mechanisms of nature. Some theories and hypotheses may seem to be common sense Related concept: Evidence whereas other theories may make claims that seem bizarre. The test of the truth of these theories is whether there is sufcient evidence to Global context: Identities and support them. relationships Theories make predictions about the outcome of experiments and suggest how one factor may change another. It is important to measure the extent and the nature of these effects. In this chapter we will see some of the different ways in which variables can be related. In the 1960s scientists For this reason, the key concept of this chapter is relationships. theorized the existence of the Higgs boson; however, In this chapter, we will also see how scientic evidence has changed the theory could not be the way we think about the universe. Rather than a never-changing conrmed until the particle’s emptiness, we now believe that the universe exploded into existence discovery in 2012. Nobel in the Big Bang and has been expanding ever since. Because scientic Prizes cannot be awarded until there is sucient evidence caused us to rethink the identity of the universe, the global evidence, so the prize was context is identities and relationships. not given until 2013 Statement of inquiry: Experiments and measurements provide evidence to support or disprove scientic claims. 4 6  M E A S U R E M E N T Why do we do experiments? One of the most important aspects of science is that of developing ideas or theories and then testing them with experiments. In Chapter 9, Development, we see how to design an experiment with a view to testing a hypothesis, but how do we draw conclusions from the results of an experiment? Most experiments involve measurements. Rather than looking at a table of measurements, it is often helpful to plot a graph of them as this makes it easier to spot a trend in the data. Usually we plot the independent variable (the quantity which you actively change) on the x-axis and the dependent variable (the one which you are investigating how it changes) is plotted on the y-axis. Imagine an experiment in which you investigate how the mass of a ball bearing affects the time it takes for it to roll down a slope. You might make a hypothesis that a heavier ball bearing will roll down the slope in less time than a lighter one because the force of gravity is greater on the heavier ball bearing. The results of your experiment might look like this: Mass of ball bearing (g) Time taken (s) 1 1.07 2 0.96 5 1.04 10 0.99 20 1.01 1.08 1.06 1.04 )s( nekat 1.02 emit 1 0.98 0.96 0.94 0 5 10 15 20 25 mass of ball bearing (g) The results of the experiment are the evidence which either supports or contradicts the hypothesis. If you look at the values on the y-axis you can see that all of the balls rolled down the ramp in about one second. So does this mean that the mass of the ball bearing has no effect on the time taken for it to roll down the slope? The experiment suggests that this might be the case, but the evidence is not very strong. 4 7 E V I D E N C E L TA Communication skills Presenting data in a graph The scale of a graph does not necessarily have to start from the origin; however, the graph will appear very different if this is the case. The graph in the example on the previous page could be plotted with the y-axis starting from zero and it would look like this: 1.2 1 )s( 0.8 nekat 0.6 emit 0.4 0.2 0 0 5 10 15 20 25 mass of ball bearing (g) It is clearer that all the balls rolled down in about one second, but it is harder to see any trends within that range as most of the graph is empty. Scientists often choose the axes of graphs to make the points spread over most of the graph but they are not the only ones to communicate data using graphs. Many communicators choose the axes of graphs to emphasize a point. For example, if a magazine sold 91,000 copies in a month and its nearest rival sold 83,000 copies, different axes can make the sales look very different at rst glance. 100,000 90,000 80,000 70,000 htnom 92,000 60,000 rep 90,000 dlos 50,000 htnom 88,000 seipoc 40,000 rep 86,000 dlos 30,000 84,000 seipoc 20,000 82,000 10,000 80,000 0 Magazine Rival 78,000 Magazine Rival If the origin is included, then it becomes In this char t it appears that the magazine clear that both magazines sold ver y has vastly outsold its rival similar numbers of copies 4 8  Presenting data Two rival companies publish their yearly sales revenue (in millions of US dollars). The gures for the previous years are shown in the table below. Year Company A Company B 2012 439 507 2013 472 486 2014 508 459 2015 524 452 2016 556 493 2017 587 574 1. Imagine that you work for Company A. Try to present the data in such a way that emphasizes that your company is the best. 2. Now imagine that you work for Company B. How might you change the presentation to show your company to be more successful? 3 Why is it important for scientists to try to present their data in as unbiased a way as possible? M E A S U R E M E N T What constitutes strong evidence? Testing a die With a perfect die you should have an equal chance of rolling any of the numbers on its faces. A weighted die has an increased chance of rolling one of the numbers (often a six). If you take a die and roll it once, does this tell you anything about whether it is weighted or not? If you now roll it six times, the chances that each roll will give you a different number are about 1.5%. Does this mean that the die is weighted? If you then roll the die more times and record the results in a table, how many times would you need to roll the die before you had enough evidence to say whether or not the die is weighted? When evaluating the strength of evidence scientists consider its reliability and validity. Validity is whether the experiment properly investigates the variables it set out to in a fair way. In order for an experiment to be valid, the independent variable should be investigated over a suitable range and all the relevant control variables should be accounted for. If we had only investigated ball bearings of masses 10g, 11g and 12g, then the investigation would not have given valid results because the range of masses would have been too limited and would 4 9 E V I D E N C E not have enabled a sound conclusion to be drawn. If we had not kept the length of the ramp the same, then the measured times would have been longer for longer ramps and the results would have been invalid as the ramp length would have affected the measured times. Reliability is a term used to describe whether subsequent experiments are likely to agree with the original experiment. A reliable experiment would always give similar results. We can consider reliability in two ways: Reliability of the trend: If all your data follow a good trend with no data points far off your line of best t, then it would be reasonable to assume that if you took another data point it would also lie close to the trend line. This means that the trend is reliable. Reliability of the data: It is important to repeat the experiment. If you took a certain data point three times and got similar results each time, then we could assume that if we repeated the experiment a fourth time, the results would probably also be similar. The data can therefore be described as reliable. On the other hand, if your results vary signicantly each time, then they are not reliable. Our earlier experiment on rolling different balls down a ramp seems to be valid, but we cannot say if the results are reliable or not unless we repeat our measurements. If we do this, we might get data like this: Mass of ball Time taken (s) Average bearing (g) 1st reading 2nd reading 3rd reading 1 1.09 1.02 1.05 1.053 2 0.94 1.07 1.02 1.01 5 1.09 1.02 0.95 1.02 10 1.02 0.93 0.98 0.977 20 1.04 0.95 0.95 0.98 We are now able to see that the data are in fact reasonably reliable. The variation in each set of readings is between 0.07 and 0.14s which is much smaller than the measured times which are all about 1s. This variation is about the same as the total variation in the times between all the different ball bearings. The evidence does not show a signicant variation in the time taken for the different ball bearings to roll down the slope, and so the evidence contradicts the hypothesis. This experiment is similar to one conducted by Galileo in which he dropped balls from the Leaning Tower of Pisa. Galileo’s experiment showed that balls of different masses fell at the same rate. Similarly, the different ball bearings roll down the slope at the same speed. Even though a ball bearing with twice the mass of another has twice the weight pulling it downwards, using Newton’s equation F = ma, we can see that if the force is doubled and the mass is also doubled, then the 5 0 acceleration will remain the same. As a result, the ball bearings will all roll down the slope with the same acceleration and will reach the bottom in the same time. elbairav Data-based question: Car testing A car manufacturer is testing a new design of car. They want to tnedneped know how much CO is emitted for every kilometer it drives. 2 –1 They test it three times and get measurements of 147gkm , –1 –1 157gkm and 143gkm 1. What is the average amount of CO emitted per kilometer independent variable 2 driven? Linear: The graph is a –1 straight line but does not 2. The manufacturer states that the car emits less than 150gkm. pass through the origin Is this a reliable statement? Measuring height In your class, ask three people to independently measure the same person’s height using a meter rule. Do all three measurements agree? How reliable are your measurements? You may have noticed that in the ball bearing experiment there appears elbairav to be a slight downwards trend in the data. Even though the times do b not vary by very much, the lighter ball bearings seem to take longer to gradient = a tnedneped roll down the slope. To investigate this further, you would need to be able to show a difference in the time taken by the lightest ball bearings and that taken by the heaviest ones. Since the difference in times is only about 0.07s, you would need a timer that is capable of timing to the nearest millisecond. Light gates connected to a data-logger can do this. independent variable An electromagnet which releases the ball bearing at the exact time the Directly propor tional: The timer starts would also help to make the timing more accurate. If you graph is a straight line were to do this then you might be able to verify that the lighter ball through the origin bearings do indeed roll down the slope a little bit more slowly. This is because the air resistance acts on them and slows lighter ball bearings more than the heavier ball bearings. Of course, different experiments would give different graphs showing elbairav different trends. Sometimes a graph of your data will show a straight line trend. Such a trend is described as linear. If your graph has a linear tnedneped trend, then the gradient of the graph is the same at all places. This makes it easy to nd the gradient and also the intercept with the y-axis. Sometimes, the straight line trend passes through the origin (or at least very close to it). Such a trend is described as directly proportional. independent variable Other experiments might give a trend which is not a straight line. Such Non-linear: The graph is trends can be described as non-linear. In these cases you could further cur ved describe whether the gradient of the graph is increasing or decreasing. 51 E V I D E N C E A B C D A student makes a hypothesis that the time between the rst and second bounce of a ball is proportional to the height from which it is dropped. Design and carry out an experiment which gathers evidence to test this hypothesis. Using the evidence, establish whether or not the hypothesis is correct. L TA Communications skills Using and interpreting a range of discipline -specic terms and symbols When quoting experimental measurements or any other numerical result, two important considerations are precision and accuracy. Accuracy refers to whether the measurement is right or not. An accurate result will reect the true value. Sometimes in experiments it is hard to assess whether a measurement is accurate if you do not know what the result is meant to be. However, the equipment you use can be tested for accuracy. For example, you could measure a known mass on a balance to test if the balance is accurate. Precision refers to the number of signicant gures given in your measurement. If you were asked the time and said that it was about ten to eleven, this is a relatively imprecise answer. On the other hand, 10:51 and 14 seconds is a very precise answer. Numerical answers can be both precise and accurate or inaccurate and imprecise. They can be precise but inaccurate, or indeed imprecise but accurate. 1. Assess the following statements to determine their accuracy and precision. The world’s population is about ten billion people. The Moon orbits the Earth every 27.322 days. –1 The speed of light is 289,792,458ms There are over a million different languages spoken on Earth. 5 2  W A V E S What is the Doppler eect? Scientists interpret the evidence from experiments to compare the experimental results to hypotheses made from scientic theories. However, gathering evidence and data can be a challenge. In 1842, a physicist named Christian Doppler made a hypothesis that waves which were emitted from a source would have a different wavelength if the source were moving. He thought that this might explain why stars in the sky were different colors. (It didn’t!) He predicted that the effect of moving the source would change the observed wavelength and frequency by a fraction that was proportional to the relative velocity of the source and the observer. This is now called the Doppler effect. In 1845, a young physicist named Christoph Buys Ballot attempted to demonstrate this effect. He lived near a railway and was familiar with the idea that the whistle of a steam train changed pitch as it went past. However, gathering convincing evidence was hard. The train’s whistle varied naturally in pitch so he could not reliably rule this cause out. Nor did he have the measuring equipment that we have today to measure the frequency of sound waves. Instead, he used musicians. Since a change in the frequency of a wave would cause the pitch to change, musicians who were well trained in recognizing the pitch of notes were good detectors of the change of frequency of sound. He obtained the use of a steam train for a day and hired six trumpeters. He stood three trumpeters on the platform and put the three others on the train. He got the trumpeters on the train to take it in turns to play a note as the train went past the platform: when one played a note, others were able to verify that the note was at a constant pitch. The trumpeters on the platform had to listen to the note played, although it was quite difcult to hear the trumpet over the sound of the train. Timing the trumpeter so that he played one note as he went past the station was also difcult. Regardless, the trumpeters on the platform agreed that when the train was moving towards them, the trumpet sounded at a higher pitch, and when it was moving away from them, it sounded lower. Buys Ballot ’s evidence of the Doppler eect was convincing because it was obser ved by musicians who were independent of the scientic process 53 E V I D E N C E Buys Ballot gathered sufcient evidence to show that the Doppler effect did indeed occur, although he was not able to show that the change in frequency was proportional to speed. Nowadays it is easy to observe the Doppler effect, for example by listening how the sound of the siren on a passing ambulance or police car will change in pitch as it goes by. This is because the Doppler effect shifts the sound upwards in pitch (higher frequency) when the vehicle is coming towards you and when it is moving away from you, the pitch is lower (lower frequency). As an ambulance passes at high speed, the pitch of the siren may appear to change. This is due to the Doppler eect 1. The trumpeters played a note with a frequency of 698Hz. If the –1 speed of sound is 340m s , using the physics you learned in Chapter 1, Models, calculate the wavelength of the sound waves coming from the trumpet. 2. Calculate the time period between successive waves. –1 3. The train traveled at 16 ms. How far would the train travel in the time of one time period? 4. For a person standing on the station, the wavelength of the waves (calculated in question 1) would be shorter by an amount calculated in question 3 because each successive wave is emitted at a closer distance by that much. Calculate the wavelength of the waves as heard by a person on the station. 5 4  A S T R O P H YS I C S Hubble’s law In 1919, an astronomer named Edwin Hubble started working at the Mount Wilson Observatory in California. The telescope there had just been completed and, at the time, was the biggest telescope in the world. One of his rst discoveries was that there were other galaxies. At the time the universe was thought only to extend to the edge of our own galaxy, the Milky Way. Ten years later, astronomers knew of almost 50 galaxies. Hubble made measurements of their distances and, using the Doppler effect, the speed at which they were traveling away from us. Stars consist mainly of hydrogen. Because they are hot, the hydrogen emits light of a certain color. This is very similar to the way a ame test can be used to identify elements in chemistry. A certain color of light corresponds to a particular wavelength of light, and Hubble could measure the specic wavelengths of light emitted from these distant galaxies. If the galaxy were moving towards us, the frequency of the waves would be higher and the light would be shifted towards the blue end of the spectrum. On the other hand, if the galaxy were moving away from us, the light’s frequency would be lower and the light would appear to be red-shifted. galaxy moving away from Earth As distant galaxies move away from the Ear th, their light is red-shifted. Measuring this red-shift enables astronomers to determine the galaxy ’s speed In 2004, the Hubble telescope took this picture of the most distant galaxies Hubble discovered that the light from most galaxies was red-shifted. in the universe. These He was able to measure the amount by which the light was red- galaxies are moving away shifted and could therefore determine the velocity at which the from us at ver y fast speeds galaxies were moving away. He discovered that the velocities of the as the universe expands. As galaxies are directly proportional to the distance that they are from a result, the light from these galaxies is signicantly red- us. This is now known as Hubble’s law. shifted 5 5 E V I D E N C E 1 megaparsec or Mpc is Data-based question: Edwin Hubble’s data 16 3.09 × 10 m or 3.26 million light years. 20,000 ) 1– 15,000 s mk( ν 10,000 ,yticolev 5000 0 0 10 20 30 distance, d (Mpc) This is a graph of Edwin Hubble’s original data. The gradient of this –1 –1 graph is called Hubble’s constant. It has units of km s Mpc 1. Find the gradient of this graph. 2. Comment on the reliability of the trend. –1 –1 3. The accepted value of Hubble’s constant is 72 km s Mpc. What does this suggest about the validity of Hubble’s original experiment? L TA Transfer skills As an example, a person ve standard What constitutes evidence? deviations above the average height would In physics the strength of evidence can be be about 210 cm tall. assessed through statistics. In order to consider Many different subject disciplines deal with an experimental result to have proved evidence and have different ways of assessing something, the chances of getting that result what constitutes strong or weak evidence. through random chance has to be shown Think about and research what might to be less than 1 in 3.5 million. This is often constitute strong or weak evidence in the called the 5–σ test (sigma σ is the Greek letter following subjects: s so this test is also referred to as the 5-sigma test) where σ is the standard deviation. The mathematics probability of nding something ve standard history deviations from the average is so rare that this is set as the denition of scientic proof. philosophy. 5 6 Data-based question: Using supernovae to test Hubble’s law A supernova (lower left) appears as bright as the rest of its galaxy for just a few weeks A supernova is the explosive end to a 1. Plot a graph of the data with distance in –1 star’s life. For a few weeks, the dying star Mpc on the x-axis and speed in km s on the outshines its galaxy. Supernovae are useful y-axis. tools for astronomers because they can be 2. Describe the trend of the data. used to calculate the distance to that galaxy. 3. Add a line of best t to your graph. The Measurements of the red-shift of the light gradient of the graph is the Hubble constant. coming from the galaxy can then be used to Find the value of the gradient. test Hubble’s law. 4. Comment on the reliability of the trend. The table below shows the distance in megaparsecs to some supernovae as well as the speed at which the galaxy is moving away. –1 Supernova Distance (Mpc) Speed (km s ) SN2007s 66.4 4,500 SN2008l 75.0 5,670 SN2007au 87.2 6,270 SN2007bc 93.3 6,570 SN2008bf 97.6 7,530 SN2007f 109.1 7,260 SN2007co 116.1 7,980 SN2007bd 131.1 9,600 SN2008af 142.6 10,230 SN2007o 156.4 10,980 57 E V I D E N C E A S T R O P H YS I C S What does Hubble’s law say about the origin of the universe? At the time of Hubble’s investigations, most astronomers believed in a static universe. In that model, the universe was unchanging and had existed forever. Hubble’s discovery, on the other hand, showed that the universe was expanding. This implied that at an earlier point in the universe’s history, it would have been smaller and denser, and, as a result, hotter. Because the velocity of galaxies was found to be directly proportional to their distance from us, this was consistent with the idea that the universe started from a single event. Galaxies that were twice as far away were found to be traveling at twice the speed which meant that they had been traveling for the same time. Hubble’s discovery led to the development of the Big Bang model of the universe. In this model, all of space and time started from an innitesimally small point and exploded outwards into the universe that we see today. What other evidence is there for the A S T R O P H YS I C S Big Bang? Although Hubble’s law provided good evidence for the Big Bang, it was only one piece of evidence and some astronomers were not convinced that the universe had to have started in this way. Some believed that matter was created in some parts of the universe and used up in other parts so that although galaxies were moving away from us, the universe was not expanding overall. To settle this dispute further evidence was required. The Big Bang model of the universe predicts that at earlier times in the universe’s history, it was more compact and therefore hotter. Evidence of hotter, earlier stages in the universe’s history would support the Big Bang theory. In 1964, Arno Penzias and Robert Woodrow Wilson were testing sensitive microwave receivers when they found an unexplained signal. Since this signal was detected all the time, regardless of the direction in which they pointed the receiver, they assumed that this was background noise and was due to some faulty wiring in the detector. They checked the wiring and everything else that could account for this signal but found no cause. Having ruled out all possible sources of the noise from Earth, they concluded that the microwave signal was coming from outer space. 5 8  Penzias and Wilson with their microwave receiver Penzias and Wilson had detected the radiation given off by the hot universe at a much earlier stage in its history. About 400,000 years after the Big Bang, the universe had cooled to about 3000 °C. At this stage the universe became transparent and the light emitted from the hot universe was able to travel through space. Since then, the universe has expanded signicantly and the wavelengths of the photons have been stretched along with it. What would have been visible or infrared light when it was emitted has now been “stretched” into microwaves. A S T R O P H YS I C S What will happen in the universe’s future? If the universe had a distinct beginning in the Big Bang, then it is reasonable to ask what the future of the universe will be. This is harder for scientists to answer denitively since the future is yet to happen. This does not stop scientists from measuring and making predictions based on their measurements. If there is enough matter in the universe, then the gravitational pull on this matter could cause the universe eventually to collapse back in on itself in a Big Crunch. On the other hand, if there is not enough matter, perhaps the universe would expand outwards forever. In 1998, astronomers measuring distant supernovae came to a different conclusion. Their measurements suggested that the universe was accelerating. The mysterious force which causes this acceleration is referred to as dark energy but its nature is not known. The nature of dark energy and indeed whether it even exists at all is one of the most important questions in modern physics. 5 9 E V I D E N C E Summative assessment Statement of inquiry: Experiments and measurements provide evidence to support or disprove scientic claims. Introduction Some speed cameras make use of the Doppler effect in radar guns to provide evidence of cars breaking the speed limit. This assessment will examine the physics of radar guns and the strength of the evidence that they provide. A B Using the radar gun C D The radar gun emits radio waves of a known frequency. These bounce off the moving car and back to the radar gun which detects them. The frequency of these waves is measured. If the car is moving towards the radar gun, the detected frequency is higher than the original frequency. 1. State the word used to describe what happens when waves bounce off a surface. 10 2. The radar gun uses radio waves with a frequency of 1.8 × 10 Hz. 8 –1 The radio waves travel at the speed of light (3 × 10 m s ). Calculate the wavelength of these radio waves. 3. The Doppler shift of the radio waves depends on the speed of the car compared to the speed of the radio waves. A 100% change in frequency occurs if the car is traveling at the same speed as the waves; a 50% change occurs if the speed of the car is half that of the radio waves. Explain why only a very small change in frequency would be expected to be detected from this radar gun. –1 a) A car is traveling towards the radar gun at 50 km h. Express this as a fraction of the speed of light. b) The change in the detected frequency will be this fraction of This speed camera uses the original frequency. Calculate the change in frequency. radar to detect speeding cars c) Describe how the wavelength of the received radio waves has changed from the emitted wavelength. d) Describe how the change in frequency would be different if the car was traveling away from the radar gun. 6 0 A B Calibrating the speed camera C D The radar gun is tested on cars traveling at known speeds. The graph below shows the frequency of the detected radio waves against the speed of the car. 4,000 4. a) Describe an experiment that might be used to produce 3,500 )zH( this graph. 3,000 ycneuqerf b) Identify one suitable control variable in this 2,500 experiment and explain how it might be 2,000 controlled. ni 1,500 e g na h c 5. Explain why the radar gun could not be used to measure 1,000 the speed of the car. Describe a different method that 500 could be used to establish the car’s speed. 0 6. Describe the trend of this graph. 0 5 10 15 20 25 30 35 –1 speed (m s ) 7. Comment on the reliability of the data. Explain how might the reliability be improved. 8. Find the gradient of the graph. 9. The detector is only capable of measuring the frequency of the –1 waves to the nearest 100Hz. If the speed limit is 20ms , what speed would the radar gun have to detect in order to be condent that the car was going faster than this speed limit? 10. A car causes the radar gun to detect a shift of 1800Hz (measured –1 to the nearest 100 Hz). The speed limit is 40kmh but it is normal not to prosecute a speeding driver unless their speed is 10% greater than this. Evaluate the evidence and decide whether there is enough evidence to suggest that the car was speeding. A B Avoiding being caught by speed cameras C D Some motorists install radar detectors to detect the radio waves coming from the radar gun. This warns them if there is a speed trap ahead and gives them time to slow down to avoid being caught. 11. Discuss how you might design a radar gun to avoid this problem. 12. Comment on the ethics of avoiding speed cameras. 13. Many speed cameras have a back-up measurement of the speed. This works by taking two photos separated by a known interval of time. Lines on the road, a set distance apart, help to determine the position of the car in each picture. a) Explain why it is important to have a back-up measurement when gathering evidence of a car exceeding the speed limit. b) Describe, using scientic language appropriately, how the two photos may be used to determine the speed of the car. 61  4 Movement Movement is the act of changing from one place or situation to another. The Helios probes were launched in the 1970s to orbit and study the Sun at close range. Their orbits pass closer to the Sun than Mercury, so they get very hot. As they fell into their orbit, the Sun’s gravity accelerated them to high speeds. The −1 probes set the speed record for the fastest man-made object at 252,792 km h. −1 By contrast, the fastest speed attained on the surface of Mars is only 0.18 km h. What are the challenges of traveling at high speeds on other planets? 6 2 The growth rate of plants Some motion is imperceptible. The movement of the atoms can be very slow. This in this molten gold gives it its heat but the distance that the saguaro cactus may grow to atoms travel is so small that we cannot see it. The speed at over 18m, but it may take which the light travels to our eyes is the fastest possible speed centuries to reach its full in the universe, so fast that the time taken for the light to height growing at a rate of reach us is imperceptibly small. As this gold cools down, how only a couple of centimeters will the atoms’ motion change? per year. Which plants grow the fastest? Plate tectonics causes continental drift which occurs at just a couple of centimeters per year. Sometimes all the continents move together and form one big supercontinent such as the continent Rodinia which is thought to have broken apart about 700 million years ago. The last time that all the land was linked in one land mass was about 200 million years ago when the supercontinent Pangaea was surrounded by an ocean called Panthalassa. In about 200 million years’ time, the continents may once again join together to form this conguration called Pangaea Ultima. What evidence do we have that the continents were once all joined together? 63  M O V E M E N T Introduction Movement has been central to human progress over the centuries. We have crossed oceans to reach new continents, and navigated Key concept: Change across land and sea to nd food and resources, or just to explore the unknown. Related concept: Movement Human migration and invading armies have caused the movement Global context: Orientation in not just of people, but also of language, culture and technology. space and time As a result they have shaped the world around us. Movement also requires navigation so that we do not get lost. In this chapter we will look at how we measure and describe motion, and how humans and other animals use magnetic elds to keep track of where we are. Because movement and navigation are linked, the global context of this chapter is orientation in space and time. Movement is the change from one state of being to another. For a moving object, it is the location that might change or its orientation if the object is rotating. Such a change in position will also occur over a period of time. Therefore, the key concept for this chapter is change. This magnetic liquid moves in response to a magnetic eld. The spikes form along the eld lines Statement of inquiry: Movement enables humans and animals to change their surroundings for the better. 6 4 Scientists have shown that honey bees can sense magnetic elds. Other insects, birds, mammals, sh and even bacteria appear to be able to sense magnetic elds. Some scientists even believe that humans have the capability of detecting magnetic elds