Newton' First and Third Law

1. Newton' First and Third Law By: Asha Saujani Krupa Patel Erin Parcell

2. Newton's First Law • An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. The Law of Inertia resisting state of motion • Simple Terms: • things keep doing what they were doing until they were interrupted

3. one more time..... • Once moving at a steady speed In a straight line… It will continue moving…. At a steady speed…. In a straight line….. Once standing still it will stay still…..

4. Rules • Objects resist accelerations. • Left to themselves, objects don't speed up, don't slow down, and don't change direction. • It requires an unbalanced force to change the velocity of an object. • If an object has a constant velocity, then no unbalanced forces are pushing on it. • A force on an object tends to change it velocity. If no force pushes or pulls on an object (or if the forces on the object cancel each other out) then the object's velocity stays the same. • No unbalanced force means no acceleration. • No force means no acceleration.

5. If no forces act on an object (or if all of the forces that do act cancel each other out), then the object will not: • speed u,slow down, change directions • If an object is not accelerating, then either: • no forces are pushing on it, or • all of the forces that are pushing on it balance each other exactly. • Objects made of matter have inertia. • The velocity of a free body is constant in time. (David Layzer, Constructing the Universe, Scientific American Library) • If Fnet = 0, then a = 0. • If there is no net force on an object it is in equilibrium, and vice versa.

6. Inertia • Newton's First Law says that any object made of matter has the property that it resists accelerations. The name of this property is inertia - inertia is the property of matter that causes objects to not "want to" accelerate. It is important to understand that inertia is not a force - inertia does not push an object along. Inertia is a property of matter, like color, density, etc. • all objects made of matter obey Newton's First Law - regardless of how much inertia they have. • Newton's First Law does not depend on how much inertia an object has - all objects made of matter have inertia, and they all obey Newton's First Law. In a little while, when we get into Newton's Second Law, the amount of inertia an object has will become important - but not now.

7. Net force The net force on an object is the vector sum of all of the forces that push or pull on the object. • can change velocity of an object.. • Examples: two forces act on a object at one time… the force to the left is strong so the net force on the left will cause the object to change velocity it may appear as is there only one force at acting on the object … they are two separate forces but one is stronger What are the two forces?

8. What is a Force? • A force is an interaction between 2 objects involving a push or a pull.Forces are vectors - they have a direction in space.Common units of force: pounds, Newtons What is an "object" Anything made of matter is an “object”. Equilibrium An object that is not accelerating is said to be “in equilibrium”.If an object is in equilibrium, the net force on it is zero.

9. Riding in a Car • Problems with the Common Explanation: • First of all, there is nothing pushing you back in your seat! (Acceleration is a concept, not an object, and only objects can push you. See Thoughts on Force... for more discussion.) • Correct Explanation: • You were at rest, you remain at rest. The car accelerates forward, and you stay where you are. • Accelerating from Rest - "G Forces" • Situation: • Your car is at rest. Suddenly you push down on the "accelerator" and the accelerates forward. You feel that you are pushed back into your seat. • Common Explanation: • "G Forces", caused by the acceleration, push you back into your seat.

10. Decelerating • Common Explanation: • You are thrown forward. • Problems with the Common Explanation: • What throws you? • Correct Explanation: • You are moving with a constant velocity. You keep that velocity (while the car slows down) until something stops you.

11. POP QUIZ!!! http://www.batesville.k12.in.us/physics/PhyNet/Mechanics/Newton1/First_Law_Quiz.html#First%20Law%20Quiz

12. Newton’s Third Law • For every action, there is an equal and opposite reaction.

13. Applying This Law to Life • While you are sitting in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

14. Forces • Forces always come in pairs. • The direction is opposite. • They are equal in size. A baseball forces the bat to the left and the bat forces the ball to the right, but they exert the same amount of force.

15. Action and Reaction Forces • “Action” and “Reaction” are names of forces. • “For every action force, there is … a reaction force” means:Forces ALWAYS occur in pairs.Single forces NEVER happen. • Since a force is an interaction between objects, two objects are involved in every force. Call the objects A and B:Action force: “A pushes B”Reaction force: “B pushes A”

16. Equal • In Newton’s Third Law, “equal” means:Equal in size.The action and reaction forces are EXACTLY the same size.Equal in time.The action and reaction forces occur at EXACTLY the same time.

17. Mass • The same force acting on objects of different mass will produce different accelerations! • FnetFnetma=m= aSameForce

18. Internal Forces • Internal ForcesIt is possible for Newton’s Third Law action/reaction forces to cancel - if they act on different parts of the same object.These forces are called “internal forces”.

19. Pop Quiz http://www.batesville.k12.in.us/physics/PhyNet/Mechanics/Newton3/Third_Law_Quiz.html

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