Impulse and Momentum How can you karatechop through

Impulse and Momentum How can you karate-chop through a stack of wood? Why is follow-through important when swinging a club, racket, or bat? When a sports announcer says a team has momentum- what do they mean?

What is Momentum? • Harder to stop a truck than a skateboard- truck has more momentum • Momentum=inertia in motion momentum=mass x velocity p=mv

What makes momentum big? • You can have large momentum if you have…? • Large mass • Large velocity • Can you think of a way for the truck and the skateboard to have the same momentum?

Momentum in a collision • Think of hitting a baseball- what is the momentum of the bat and ball before the collision? • What is the momentum of the bat and ball after? • Draw vector diagram of each momentyes…momentum is a vector

Problem solving- momentum • Determine the momentum of a 60 kg player moving eastward at 9 m/s p = m*v = 60 kg*9 m/s p = 540 kg • m/s, east

Problem solving- momentum • Determine the momentum of a 1000 kg car moving southward at 20 m/s p = m*v = 1000 kg*20 m/s p = 20 000 kg • m/s south

Problem solving- momentum • A car possesses 20 000 units of momentum. What would be the car's new momentum if. . . a. its velocity were doubled. b. its velocity were tripled. c. its mass were doubled (by adding more passengers and a greater load) d. both its velocity were doubled and its mass were doubled.

Solutions- Momentum A. p = 40 000 units (doubling the velocity will double the momentum) B. p = 60 000 units (tripling the velocity will triple the momentum) C. p = 40 000 units (doubling the mass will double the momentum) D. p = 80 000 units (doubling the velocity will double the momentum and doubling the mass will also double the momentum; the combined result is that the momentum is doubled twice quadrupled)

Impulse • If the momentum of an object changes, either its mass or its velocity must change • Since we don’t usually change the mass of objects, what must have happened?

Impulse • Remember Newton’s 2 nd Law? • If the velocity changed, there was acceleration, then there must have been a force • How long that force acted is also important Impulse=Ft= force x time Impulse is the change in momentum so: Ft= (mv) This is called the impulsemomentum theorem

Increasing momentum • To increase momentum the most, what do we need to do? • Apply the largest force for the longest period of time • Thus the concept of “follow through”

Decreasing momentum • A volunteer? ? ? • Come up to the front of the class and catch this falling mass with your hand. • You will apply a force over a certain time (impulse) to change the momentum

Decreasing Momentum • Now- place the back of your catching hand on the lab table and I will drop the mass again • What will happen? • Why? • Write the impulsemomentum theorem and use it to explain

Questions? ? ? • When you jump, why do you bend your knees upon landing? • In boxing, why do you “ride” with the punch? • When a dish falls, would the impulse be more if it falls on concrete than if it falls on carpet? No- the change in momentum is the same so the impulse is the same!

Conservation of Momentum • In the absence of any external force, the momentum of a system remains unchanged • Forces internal to the system do not matter

What does it mean to be a system? • In an isolated system, no net external forces are acting • Think of financial systems…if Jack has $100 and gives $50 to Jane, both would have $50 and the total amount of $100 in the system is conserved.

Examples • What is the momentum before? After? • Which moves faster, the cannon or cannonball- why?

Examples

Elastic collisions • When objects collide without being permanently deformed or generating heat • Perfect “bounce”

Inelastic Collisions • Colliding objects stick together after the collision • Could have permanent deformation or heat generation • MOMENTUM IS STILL CONSERVED!

Outside forces • Most collisions will involve some outside forcefriction, air resistance, etc • BUT- these forces generally small enough to have conservation of momentum

Problem-Solving • A 6 kg fish swims 1 m/s towards and swallows a 2 kg fish that is rest. Momentum is conserved. What is the velocity of the fish after lunch?

Solution • • Momentum before=momentum after m 1 v 1=m 2 v 2 (6 kg)(1 m/s)+(2 kg)(0 m/s)=(6 kg+2 kg)v 2 v 2=6/8=0. 75 m/s

Try on your own…beware of signs!!!!! • What if the little fish wasn’t still but was actually swimming away from the big fish at 0. 5 m/s when he got swallowed. What was the velocity after lunch? • What if the little fish was really stupid and was swimming towards the big fish at 2 m/s when he was swallowed- what was the velocity after lunch?