newton’s 3 rd law and momentum. newton’s 3 rd law when one object exerts a force on a second...
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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.
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