Newton’s Third Law of Motion and Momentum Chapter 12.3

A force cannot exist alone. Forces always exist in pairs.

Newton’s Third Law According to Newton’s third law of motion, whenever on object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. These two forces are called action and reaction forces.

Action and Reaction Forces The force your bumper car exerts on the other car is the action force. The force the other car exerts on your car is the reaction force. These two forces are equal in size and opposite in direction.

Action and Reaction Forces Pressing your hand against the wall also produces a pair of forces. Your hand exerts a force against the wall (action force). The wall exerts an equal and opposite force against you hand (reaction force).

Action and Reaction Forces The action force of a hammer hitting a nail drives the nail into a piece of wood. The reaction force of the nail on the hammer stops the motion of the hammer.

Action and Reaction Forces and Motion What are the action and reaction forces involved in swimming? The swimmer exerts an action force on the water. The water moves in the direction of the action force. The water also exerts an equal and opposite reaction force on the swimmer and pushes the swimmer through the water.

Action-Reaction Forces Do Not Cancel Why don’t the action-reaction forces acting on the swimmer cancel each other and produce a net force of zero? They do not act on the same object. The action force acts on the water, the reaction force acts on the swimmer. Only when equal and opposite forces act on the same object do they result in a net force of zero.

Momentum If a marble and a bowling ball were rolling slowly toward you, which would be harder to stop? The bowling ball is harder to stop because it has more mass. If the marble was moving 100 times faster, which would be harder to stop?

Momentum Momentum = the product of an object’s mass and its velocity. An object has a large momentum if the product of its mass and velocity is large. 𝒑=𝒎×𝒗 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚=𝑚𝑎𝑠𝑠 ×𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦

Which has more momentum, a 0. 046 kg golf ball with a speed of 60 Which has more momentum, a 0.046 kg golf ball with a speed of 60.0 m/s or a 7.0 kg bowling ball with a speed of 6.0 m/s? Momentum of golf ball Momentum of bowling ball 𝑚=0.046 𝑘𝑔 𝑣=60 𝑚/𝑠 𝑝=? 𝑝=𝑚𝑣 0.046 𝑘𝑔 ×60 𝑚/𝑠 2.8 𝑘𝑔∙𝑚/𝑠 𝑚=7.0 𝑘𝑔 𝑣=6.0 𝑚/𝑠 𝑝=? 𝑝=𝑚𝑣 7.0 𝑘𝑔 ×6.0 𝑚/𝑠 42 𝑘𝑔∙𝑚/𝑠

The momentum for any object that is not moving is zero The momentum for any object that is not moving is zero. Momentum of space shuttle before takeoff? 𝑝=𝑚𝑣 𝑝=2,051,113 kg x 0 m/s 𝑝 = 0 kg∙m/s

Conservation of Momentum According to the 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.

Newton's Third Law NFL Football Science of Football

REVIEW For every action force there is a a. reaction force b. net force c. friction force d. unbalanced force

REVIEW 2. Momentum depends on a. force only b. velocity and friction c. weight and mass d. mass and velocity