Newton s Laws of Motion

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1 Newton s Laws of Motion Newton s Laws Forces Mass and Weight Serway and Jewett 5.1 to 5.6 Practice: Chapter 5, Objective Questions 2, 11 Conceptual Questions 7, 9, 19, 21 Problems 2, 3, 7, 13

2 Newton s First Law (Law of Inertia) An isolated object, free from external forces, will continue moving at constant velocity, or remain at rest. (actually due to Galileo.) Earlier, Aristotle believed objects were naturally at rest, and needed a continuing push to keep moving. Galileo realised that motion at constant velocity is natural, and only changes in velocity require external causes. Objects in equilibrium (no net external force) also move at constant velocity.

3 Newton s Second Law r F net = r ma F net (or F total ) is the vector sum of all forces acting on the particle of mass m. The acceleration a is parallel to the total force, and proportional to it. The proportionality constant is the particle s mass. Newton defines mass as a measure of an object s inertia.

4 Forces A force is a push or pull that tends to cause motion (more exactly, changes in motion) From the Second Law, force should have units of 1 kg m/s Force is a vector. 2 = 1 newton (N) In Newton s dynamics, all influences on the motion of a particle are expressed as forces exerted by something on the particle.

5 Contact Forces : direct contact is required examples - normal forces, friction, air resistance, buoyancy,... Non-Contact Forces : gravity, electromagnetic forces. The gravitational force is also called weight and is measured in newtons. Weight is proportional to mass : F w = mg, where g is the gravitational field (and is also the acceleration of an object in free fall).

6 Weight and Mass Weight is a force; it can be measured using a spring scale On Earth, a baseball weighs 2.40 N On the moon, a baseball weighs 0.40 N

7 Mass is a measure of inertia : on the earth or on the moon, a 24.5 N force applied to the baseball will give it an acceleration of 100 m/s 2 (its mass is m = F/a = kg) We can compare masses with a balance, because of the remarkable property : weight mass Weights are equal when masses are equal F r g,1 F r g, 2

8 Newton s Law of Gravitation Gravity is a universal attractive force between masses. M The force is proportional to each mass, and inversely proportional to the the square of the distance between them: F = G Mm 2 r r m

9 Quiz Astronauts in orbit 600 km above the earth are seen floating motionless inside the International Space Station. They can do this because: a) They have no mass in space. b) There is no gravity in space. c) The station is not an inertial reference frame.

10 Newton s Third Law (action and reaction) Whenever object A exerts a force on object B, object B simultaneously exerts an equal, opposite force back on A.

11 Newton s Third Law : examples What is the reaction to the following forces? Action Reaction 1.) F r g Gravity pulls block down 2.) Table pushes up on block

12 Quiz Action 1.) F r g Gravity pulls block down This action force above is: A) A force exerted on the block by the table B) A force exerted on the table by the block C) Neither of these

13 Quiz A 140-kg wrestler and a 90-kg wrestler try to push each other backwards out of the ring. At first they are motionless as they push; then the large wrestler moves the other one backwards. Compare the forces they exert on each other. Which statement is correct? a) The forces are always equal. b) The larger wrestler always exerts a larger force. c) When they are motionless, the forces are equal; they start to move when the large wrestler exerts a larger force on his opponent than his opponent exerts back on him.

14 Newton s Three Laws Summary 1. Objects naturally move at constant velocity in a straight line 2. Accelerations are caused by unbalanced forces 3. Forces come in action-reaction pairs, with the action and reaction always on different objects. Gravity : r r = mg F g Weight is a force, mass is a measure of inertia. All familiar forces require direct contact, except for gravity and electromagnetism.

The Concept of Force. field forces d) The gravitational force of attraction between two objects. f) Force a bar magnet exerts on a piece of iron.

The Concept of Force. field forces d) The gravitational force of attraction between two objects. f) Force a bar magnet exerts on a piece of iron. Lecture 3 The Laws of Motion OUTLINE 5.1 The Concept of Force 5.2 Newton s First Law and Inertial Frames 5.3 Mass 5.4 Newton s Second Law 5.5 The Gravitational Force and Weight 5.6 Newton s Third Law 5.8