SECTION 3: Newton’s 3rd Law and Momentum

LEARNING GOALS State Newton’s 3rd Law of Motion. Identify action and reaction forces. Calculate momentum. Recognize when momentum is

conserved.

NEWTON’S 3RD LAW Newton’s 3rd Law of

Motion: 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”.

ACTION AND REACTION FORCES DON’T CANCEL! If the action and reaction forces

occurred on the same object, they would result in zero motion. The action and reaction forces

must occur on different objects.

ACTION AND REACTION FORCES DON’T CANCEL!Example: A swimmer pushes the water. She “acts” on the water. The water “reacts” by moving and pushing her forward.

MOMENTUM Momentum: property of a

moving object that equals its mass times its velocity. Momentum is related to how

much force is needed to change an object’s motion.

MOMENTUM EQUATION

p = momentum (kg x m/s)

m = mass (kg)

v = velocity (m/s) Just like velocity, momentum

has size and direction!

p = mv

FORMULA SHEET

p = mvm = p/vv = p/m

EXAMPLE 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 = force in N

mvf = final momentum in kg x m/s

mvi = intital momentum in kg x m/s

t = time

F = (mvf – mvi) / t

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 its mass, velocity, or both change.

Momentum can be transferred from one object to another.

LAW OF CONSERVATION OF MOMENTUM Law of Conservation of Momentum: the total momentum of objects before a collision is equal to the total momentum of objects after a collision. Total momentum is not lost or

gained during a collision

LAW OF CONSERVATION OF MOMENTUM

LAW OF CONSERVATION OF MOMENTUM