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Newton’s Third Law of Motion & Momentum

12-3 pg 372-377

Newton’s 3rd 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 object

 For every action there is an equal and opposite reaction

Action and Reaction forces

 Action force = The force exerted by the first object

 Reaction force = The equal and opposite force the second object exerts back on the first object

Bumper Cars   Action force - your car

hitting the other car

  Reaction force - the force the other car exerts on your car

  These are equal in size and opposite in direction

Hammer hitting a nail  Hammer exerts a

force on the nail driving it into the wood (action)

 The nail exerts a force on the hammer causing it to stop moving (reaction)

Action-Reaction Forces and Motion  Swimmers use their arms to push against

the water to create an action force

 This action force pushes the water in the same direction of the action force

 The reaction force of the water acts on the swimmer and pushes them forward

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Action-Reaction Force   The example of the

swimmer shows that not all action-reaction forces cancel each other out

  The reason these forces do not cancel is because they act on different objects

  The action force acts on the water. The reaction force acts on the swimmer.

Momentum  The product of an object’s mass and its velocity  p = m x v  Use p to represent momentum

because m = mass

 An object has a large momentum if the product of the mass and velocity is large.

Momentum (p)

 Momentum = mass x velocity

 Units for momentum = kgm/s

 Units are not the same as units for force which is kgm/s2

Momentum (p) = m x v Momentum = Mass x velocity The truck has much more mass than the mini cooper. Therefore, its momentum will be much larger and the mini cooper would be destroyed if they were to collide.

p = 13,600 kg x 2 m/s = 27,000 kgm/s

p = 1,150kg x 2m/s = 2,300 kgm/s

Find the momentum of an object if its mass is 7 kg and its

velocity is 6 m/s

 Given information  m = 7 kg  v = 6 m/s

 p = m x v

 p = 7 kg x 6 m/s

 p = 42 kgm/s

Law of Conservation of Momentum

 If no net force acts on a system, then the total momentum of the system does not change.

 In a closed system, the loss of momentum of one object equals the gain in momentum of another object, momentum is conserved

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Pool

 When you play pool the billiard balls have momentum

 They transfer their momentum when they collide