Science 8 - Newton's First Law of Motion - PDF

SCIENCE 8  NEWTON’S FIRST LAW OF MOTION  Lesson Objectives State Demonstrate Newton’s first Explain how Differentiate mass and how a body law of motion mass is related...

SCIENCE 8  NEWTON’S FIRST LAW OF MOTION  Lesson Objectives State Demonstrate Newton’s first Explain how Differentiate mass and how a body law of motion mass is related responds to and describe to inertia weight changes in inertia motion SYSTEMS THINKING HABITS  Newton's First Law of Motion Newton's First Law of Motion states that an object in motion tends to stay in motion unless an external force acts upon it. Similarly, if the object is at rest, it will remain unless an unbalanced force acts upon it. Newton's First Law of Motion is also known as the Law of Inertia.  Newton's First Law of Motion What Newton's First Law is saying is that objects behave predictably. If a ball is sitting on your table, it isn't going to start rolling or fall off the table unless a force acts upon it to cause it to do so. Moving objects don't change their direction unless a force causes them to move from their path.  Newton's First Law of Motion As you know, if you slide a block across a table, it eventually stops rather than continuing forever. This is because the frictional force opposes the continued movement. If you throw a ball out in space, there is much less resistance. The ball will continue onward for a much greater distance. LAW OF INERTIA: RESISTANCE TO CHANGE The object at rest remain at rest An object at in constant motion remains to be in that state of motion unless acted upon by an external force. An object continues to be at rest if it is at rest or moving if it is moving unless an unbalanced force acts on it to change its state. The poperty of objects when it opposes changes in motion is called inertia In the law of inertia, objects do not accelerate on their own; instead, a net external force acts on them to oppose the tendency of resistance so that the objects will accelerate. Acceleration is the rate of change of velocity. In other words, it is the change in velocity over a given time interval. Velocity is the rate at which an object changes position with time.  MASS AS A MEASURE OF INERTIA Mass-the amount of matter of an object contains. International System (SI) for mass is kilogram (kg) Weight- the amount of gravitational force that an object experiences. The SI unit for weight is newton (N)  MASS AS A MEASURE OF INERTIA On Earth, mass and weight is directly proportional to each other. If the mass of an object is doubled, it is also double the same if the mass is halved, the weight will also be halved. Mass- fundamental quantity that measures the amount of matter Weight- measure of the gravitational pull of Earth of an object mass.  MASS AS A MEASURE OF INERTIA weight (w) = mass (m) x acceleration due to gravity (g) w= mg If the pull of gravity changes, the weight of an object changes but its mass does not. Consider the mass and weight of a person on Earth and Moon where acceleration due to gravity is only 1/6 as strong on Earth. If the person has a mass of 50 kg, how much he/she weigh on the Moon?  MASS AS A MEASURE OF INERTIA weight (w) = mass (m) x acceleration due to gravity (g) w= mg SOLUTION: On Earth: Mass= 50 kg gravity on Earth= 9.8 m/s2 weight on Earth= mass x gravity weight on Earth= 50 kg x 9.8 m/s2 = 490 kg m/s2 or 490 N  MASS AS A MEASURE OF INERTIA weight (w) = mass (m) x acceleration due to gravity (g) w= mg SOLUTION: On Moon: Mass= 50 kg gravity on Moon= 1/6 (9.8 m/s2) = 1.6333 m/s2 weight on Moon= mass x gravity weight on Moon= 50 kg x 1.6333 m/s2 = 81.67 kg m/s2 or 81.67 N  MASS AS A MEASURE OF INERTIA Therefore, the person weigh more if he/she is on Earth than if he/she is on the Moon. Mass remains to be constant for any object regardless of its location, but its weight may change depending on the pull of gravity that acts on it.  TRY TO SOLVE.. 1. The weight of a 1 kg mass at the surface of the Earth is… g = 9.8 m/s2 Known : Mass (m) = 1 kg The acceleration due to gravity at the surface of the Earth (g) = 9.8 m/s2 Wanted: weight (w)  TRY TO SOLVE.. Solution : w=mg m = mass (The SI unit of mass is the kilogram, kg) g = acceleration due to gravity (The SI unit of g is m/s2) w = weight (The SI unit of w is kg m/s2 or Newton) w = (1 kg)(9.8 m/s2) = 9.8 kg m/s2 = 9.8 Newton  TRY TO SOLVE.. A physical science test book has a mass of 2.2 kg a. What is the weight on the Earth? b. What is the weight on Mars (g = 3.7 m/s2 ) SUMMARY  SUMMARY Newton’s first law, the law of inertia, states that a stationary object remains to be at rest or an object moving at constant velocity continues to be moving at constant speed and direction unless imposed by an external force that gives a net unbalanced force. Mass is a measure of inertia. The greater the mass of an object, the greater it resists changes in its states of motion. Weight is the product of mass and acceleration due to gravity  Newton's Second Law of Motion Newton's Second Law of Motion states that when a force acts on an object, it will cause the object to accelerate. The larger the object's mass, the greater the force will need to be to cause it to accelerate. This Law may be written as force = mass x acceleration or: F=m*a  Newton's Second Law of Motion Another way to state the Second Law is to say it takes more force to move a heavy object than it does to move a light object. Simple, right? The law also explains deceleration or slowing down. You can think of deceleration as acceleration with a negative sign on it. For example, a ball rolling down a hill moves faster or accelerates as gravity acts on it in the same direction as the motion (acceleration is positive). If a ball is rolled up a hill, the force of gravity acts on it in the opposite direction of the motion (acceleration is negative or the ball decelerates).  Newton's Second Law of Motion 1. A 1 kg object accelerated at a constant 5 m/s2. Estimate the net force needed to accelerate the object. Known : Mass (m) = 1 kg Acceleration (a) = 5 m/s2 Wanted : net force (∑F)  Newton's Second Law of Motion 2. Mass of an object = 1 kg, net force ∑F = 2 Newton. Determine the magnitude and direction of the object’s acceleration Known : Mass (m) = 1 kg Net force (∑F) = 2 Newton Wanted : The magnitude and direction of the acceleration (a)  Newton's Second Law of Motion a = ∑F / m a=2/1 a = 2 m/s2 The direction of the acceleration = the direction of the net force (∑F) LAW OF ACCELERATION: THE INFLUENCE OF FORCE AND MASS In all the instances, motion keeps on changing because of the action of forces applied. NET FORCE- sum of all forces acts on an object. acceleration is directly proportional to the net force, and both should have the same direction; and acceleration is inversely proportional to inertia or the mass a= netforce/ mass  SAMPLE PROBLEM: An object with a mass of 5.50 kilograms accelerate at a rate of 4.00 m/s2 when an unknown net force is applied to it. Calculate the net force acting on this object. Given: mass (m)= 5.50 kg acceleration(a)= 4.00 m/s2  SAMPLE PROBLEM: SOLUTION: To calculate the net force: Fnet= ma Fnet= 5.50 kg x 4.00 m/s2 Fnet= 22 kg m/s2 or 22 N  SAMPLE PROBLEM: SOLUTION: To calculate the net force: Fnet= ma Fnet= 5.50 kg x 4.00 m/s2 Fnet= 22 kg m/s2 or 22 N  ANSWER THIS! 1. An object accelerate at 2.25 m/s2 when a force of 7.80 N is applied to it. What is the mass of the object? 2.