Chapter 2-5 Physics Concepts PDF

1 Chapter 2 Galileo's discovery: Objects of different weight fall to the ground at the same time in the absence of air resistance. A moving object needs no force to keep it moving in the absence of friction....

1 Chapter 2 Galileo's discovery: Objects of different weight fall to the ground at the same time in the absence of air resistance. A moving object needs no force to keep it moving in the absence of friction. The use of inclined planes for Galileo's experiments helped him to understand the concept of inertia Aristotle motion – Natural motion :Straight up or straight down for all things on Earth, Beyond Earth, motion is circular – Example: The Sun and Moon continually circle Earth. Violent motion – Produced by external pushes or pulls on objects – Example: Wind imposes motion on ships. Force is a push or a pull. Formula is Force = mass * acceleration. Unit = kg m/s2 ( or Newton) Inertia is a property of matter to resist changes in motion. depends on the amount of matter in an object (its mass). Newton’s first law of motion Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force. Scalar quantity a quantity whose description requires both magnitude ( just the number with units) Example: Mass, temperature, pressure, area, volume,speed,distance Vector quantity – example: Force, velocity,acceleration,displacement a quantity whose description requires both magnitude (how much) and direction (which way) can be represented by arrows drawn to scale, called vectors – length of arrow represents magnitude and arrowhead shows direction  2 Net Force Nellie Newton hangs from a rope as shown. Which side has the greater tension? Because of the different angles, different rope tensions will occur in each side. Nellie hangs in equilibrium, so her weight is supported by two rope tensions, adding vectorially to be equal and opposite to her weight There are three forces acting on Nellie: – her weight, mg, a tension in the left-hand side of the rope, and a tension in the right- hand side of the rope. Equilibrium rule The vector sum of forces acting on a nonaccelerating object equals zero. Equilibrium rule applies to vector quantities and not to Scalar quantities. In equation form: F = 0. Equilibrium: a state of no change with no net force acting Static equilibrium:Example: hockey puck at rest on slippery ice Dynamic equilibrium:Example: hockey puck sliding at constant speed on slippery ice You are riding in a vehicle at a steady speed and toss a coin straight upward. Where will the coin land? Due to the coin's inertia, it continues sideways with the same speed as the vehicle in its up-and-down motion, which is why it lands in your hand Support force (normal force) is an upward force on an object that is opposite to the force of gravity. Example: A book on a table compresses atoms in the table, and the compressed atoms produce the support force.  3 Chapter 2 worksheet 4  5 total distance covered Average speed = time interval Example: Drive a distance of 200 km in 2 h and your average speed is 100 km/h. Instantaneous speed is the speed at any instant. When you ride in your car, you may speed up and slow down with speed at any instant that is normally quite different than your average speed. Your instantaneous speed is given by your speedometer Average velocity = displacement / time Example: 50 mi /h along east direction Constant velocity means the direction as well the speed is not changing. It also means that acceleration is zero when there is a constant velocity. change in velocity Change in speed (increase or decrease) per time is Acceleration = acceleration, so slowing is acceleration. time interval Change in direction is acceleration (even if speed 2 Unit of acceleration is m/s stays the same), so rounding a curve is acceleration. EXAMPLE: 50 m/ s2 speeding Free fall Falling under the influence of gravity only-with no air resistance Freely falling objects on Earth accelerate at the rate of 10 m/s2 (more precisely, 9.8 m/s2). The distance covered by an accelerating object starting from rest is Distance = (1/2) ´ acceleration ´ time ´ time Under free fall, when acceleration is 10 m/s2, the distance fallen is – 5 m/s after 1 s. – 20 m/s after 2 s., 45 m/s after 3 s. And so on.  6 End of chapter 58. What is the acceleration of a car that moves at a steady velocity of 100km/h for 100s. explain your answer 64. If air resistance can be ignored, how does the acceleration of a ball that has been tossed straight upward compare with its acceleration if simply dropped 71. If there were no air resistance, why would it be dangerous to go outdoors on rainy days 72. As speed increases for an object in a free fall, does acceleration increase also? 23. Consider these measurements : 10m , 10m/s, 10m/s2. What do these measure ? ( speed/ distance/ acceleration) 84. Give an example in which your speed could be zero while your acceleration is non zero 91. If you drop an object, its acceleration toward the ground is 10m/s2. If you throw it down instead, would its acceleration after throwing be greater than 10m/s2. Why or why not?  7 Chapter 3 worksheet  8 Chapter 4 9  10 CHAPTER 4 Acceleration is directly proportional to net force. To increase the acceleration of an object, increase the net force acting on it. Acceleration ~ net force Force of friction Depends on the kinds of material and how much they are pressed together. Is due to tiny surface bumps and to "stickiness" of the atoms on a material's surface. Friction is an opposing force Two types of friction: Static friction : friction at rest ; Kinetic friction: friction in motion What factors does friction depend upon? Difference between mass and weight? Mass: The quantity of matter in an object. It is also the measure of the inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way. A measure of the inertia of a material object Independent of gravity Greater inertia  greater mass Unit of measurement is the kilogram (kg) Weight: Usually the force upon an object due to gravity. (W = mass * acceleration due to gravity) – Usually the force on an object due to gravity – Scientific unit of force is the newton (N) – Unit is also the pound (lb) 1 kilogram weighs 10 newtons (9.8 newtons, to Mass versus weight be precise). On the Moon and Earth: Relationship between Weight of an object on the Moon kilograms and pounds: is less than on Earth. – 1 kg = 2.2 lb = 10 Mass of an object is the same N at Earth's in both locations. surface – 1 lb = 4.45 N  11 Newton's second law (the law of acceleration) relates acceleration and force. The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object. In equation form: acceleration = Net force / mass Example: – If net force acting on object is doubled  object's acceleration will be doubled. – If mass of object is doubled  object's acceleration will be halved. Free fall and Newton’s 2nd law The greater the mass of the object… – the greater its force of attraction toward the Earth. – the smaller its tendency to move, that is, the greater its inertia. – So, acceleration of both sets of bricks is the same. (Twice the force on twice the mass gives the same acceleration g!) – The acceleration of both sets of bricks is the same, 10 m/s2 (more precisely, 9.8 m/s2). Non free fall When an object falls downward through the air it experiences – force of gravity pulling it downward. – air drag force acting upward. The condition of nonfree fall – occurs when air resistance is nonnegligible. depends on two things: speed and frontal surface area. When the object is moving fast enough so that air resistance builds up to equal the force of gravity. Then no net force; No acceleration ;Velocity does not change Terminal speed – occurs when acceleration terminates (when air resistance equals weight and net force is zero). Terminal velocity – same as terminal speed, with direction implied or specified. 12 13  14 Chapter 5 Interaction – is between one thing and another. – requires a pair of forces acting on two objects. Example: interaction of hand and wall pushing on each other Force pair—you push on wall; wall pushes on you. Newton’s Third law of motion Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. A soccer player kicks a ball with 1500 N of force. The ball exerts a reaction force against the player's foot of ……………………. Action vs Reaction Force Action and reaction forces – one force is called the action force; the other force is called the reaction force. – are co-pairs of a single interaction. – neither force exists without the other. – are equal in strength and opposite in direction. – always act on different objects. Action: Object A exerts a force on object B. Reaction: Object B exerts a force on object A. Example: Action—rocket (object A) exerts force on gas (object B). Reaction—gas (object B) exerts force on rocket (object A). Example: When you step off a curb, Earth pulls you downward. The reaction to this force is you pulling on the earth 15 48. You push a heavy car by hand. The car in turn pushes back with an opposite but equal force on you. Doesn’t this mean that the forces cancel each other making acceleration impossible? Why or Why not 62. A stone is suspended at rest by a string. Draw force vectors for all the forces that act on the stone. Should the vectors have a zero resultant? Why or Why not? 66. A stone is sliding down a friction free incline. Identify the forces and draw the force vectors.