Momentum

It was first introduced by Isaac Newton. Momentum is symbolized by the letter ‘p’ It means inertia in motion.

Momentum = mass * velocity (Velocity has direction, therefore momentum is a vector) p = m * v Unit of p = kg * m/s

Therefore, the amount of momentum is based on the amount of mass of the object and its speed.

A moving object has larger momentum if 1. mass is large or 2. speed is large At rest, an object has No (zero) momentum. Why?

Question— Which has more momentum: a truck moving downhill or a roller skate moving down hill at the same speed? Will the trucker always have more momentum than the roller skate? Can the truck and the roller skate ever have the same momentum?

Therefore, if velocity changes, so does the momentum and the body will be accelerated, therefore a net force is present.

Impulse Impulse = change in momentum I =  mv I = F * t Unit = (kg*m/s 2 ) * s N * s Therefore Ft=mv

--longer time a force is exerted on something…the more change in momentum (greater the impulse) --If you wish to change the momentum of an object, exert an impulse on it. To do this, force and time are important. There are several ways to change the momentum of an object.

Case 1 – Increase Momentum by… ~Increasing the force on an object ~(or) extend the time of contact Ex: hitting a baseball – swing hard & follow through

Case 2 -- Decrease Momentum over a long time by… ~if longer time, yields a smaller force Ex: in your car – would you rather hit a concrete wall or a haystack? (haystack because it extends the time of contact )

Ex: padded dashboards, airbags, catch ball by moving arm backward with the ball, jumping to the ground by bending knees, falling on wooden floor vs. concrete

Question—If the boxer is able to make the duration of the impact five times as long by riding with the punch, by how much will the impact be reduced? --The force of impact will be 5 times less than if you didn’t pull back.

Case 3 – Decreasing Momentum Over a short time by… ~If the time is short…Force of impact is large Ex. Car hits wall

Ex. Karate chop breaks bricks (Bounce makes force even greater!)

Bouncing Impulses are greater when bouncing takes place because…you need an impulse to bring the object to a stop as well as to send it back in the opposite direction. Question? Which would hurt more… if a flower pot fell on your head, or if it hit and bounced back? Ans: Hit and bounced back

Examples: If the mass of an object is 8kg and its velocity is 1.4 m/s, what is its momentum? What is the velocity of an object whose mass is 3.2 kg and momentum is 2.41 N*s? What is the mass of an object whose momentum is 8.21 N*s and whose velocity is 4.4 m/s?

What is the impulse on an object when the force exerted is -22 N and is exerted over a time of 31.1 seconds? If the impulse on an object is 85 N*s, and the force exerted on it is 14 N, how long was the force exerted on the object? What is the force exerted on an object is the impulse on it is 42 N*s and the force is exerted over a time of 53 seconds?