Newton’s Third Law of Motion Take notes on page 51 of your notebook

First and Second Laws The first law states that an object in motion will stay in motion unless acted upon by a force. Aristotle thought that constant force must be applied, but that did not explain an arrow flying through the air! The second law states that the force needed to change the motion of an object depends on the object’s mass and its acceleration. It takes a lot more force to stop a truck than to stop a bicycle!

Third Law of Motion “For every action force, there is a reaction force equal in strength and opposite in direction.” All forces come in pairs The forces act on separate objects The motion of both objects is affected

Example: throwing a ball

Momentum Momentum is mass times speed. Your mass times your speed is equal and opposite to the ball’s mass times its speed. This is the Law of Conservation of Momentum

The speed of throwing the ball and the mass of the ball affect your backward acceleration (recoil)

Let’s do a problem! Example: An astronaut in space throws a 2-kilogram wrench away from him at a speed of -10 m/sec. If the astronaut’s mass is 100 kilograms, at what speed does the astronaut move backward after throwing the wrench? Solution: (1) You are asked for the speed. Since the astronaut is in space, we can ignore friction. (2) You are given the mass and speed of the wrench and the mass of the astronaut. (3) This is enough to apply the law of conservation of momentum. m 1 x v 1 + m 2 x v 2 = 0 (4) Plug in numbers. [2 kg × (-10 m/sec)] + [(100 kg) × v 2 ] = 0 v 2 = +20 / 100 = +0.2 m/sec The astronaut moves backward at a speed of +0.2 m/sec.

Newton’s Laws Formulas Force F = m x a Force equals mass times acceleration Applies to the second law Applies to one object Momentum p = m x v Momentum equals mass times speed Applies to the third law Applies to two objects

Assignment Complete the Newton’s Laws skill sheets and attach to page 49.