Newton’s 3rd Law and Momentum

Newton’s 3rd Law

When one object exerts a force on a second object, the second object exerts a force on the first that is equal in strength and opposite in direction.

Action and Reaction

• Another way of stating Newton’s 3rd Law is “to every action force there is an equal and opposite reaction force”.

• Example – You exert a downward force on a trampoline. The trampoline exerts an equal force upward on you.

Action and Reaction Forces Don’t Cancel!

• Action and reaction forces work on different objects, so they are not balanced.

Example – A swimmer pushes the water. She “acts” on the water. The water “reacts” by moving and pushing her forward.

Momentum

• Momentum – how much force is needed to change an object’s motion.

• Depends on an object’s mass and velocity• Momentum is given the symbol p.

Momentum Equation

p = mv

p = momentum in kg * m/sm = mass in kg

v = velocity in m/s

Just like velocity, momentum has size and direction!

Example Momentum Problem

At the end of a race, a sprinter with a mass of 80 kg has a speed of 10 m/s. What is the sprinter’s momentum?

Force and Changing Momentum

• If you catch a baseball, your hand might sting (even with a glove). This is because the baseball exerted a force on your hand when it came to a stop and its momentum changed.

• This can also be calculated!

Calculating the Force Using Momentum

F = (mvf – mvi) / t

F = force in Nmvf = final momentum in kg * m/s

mvi = intital momentum in kg* m/s

t = time

Example of Calculating Force Using Momentum

What is the force exerted by a catcher’s glove on a 0.15 kg baseball moving at 35 m/s that is stopped in 0.02 s?

Law of Conservation of Momentum

• The momentum of an object does not change unless it’s mass, velocity, or both change.

• Momentum can be transferred from one object to another.

• Example – A bowling ball traveling down the lane has momentum. That momentum is transferred to the pins as the ball strikes the pins.