Adapted from University of California, Bourns College of Engineering Momentum and Impulse Dorsey Physics Adapted from University of California, Bourns College of Engineering

Let’s start with everyday language What do you say when a sports team is on a roll? They may not have the lead but they have ___________

Let’s start with everyday language MOMENTUM! A team that has momentum is hard to stop.

What is Momentum? An object with a lot of momentum is also hard to stop Momentum = p = mv Units: kg∙m/s m=mass v=velocity Momentum is also a vector (it has direction)

Let’s practice A 1200 kg car drives west at 25 m/s for 3 hours. What is the car’s momentum? Identify the variables: Mass= Velocity= Time=

Let’s practice A 1200 kg car drives west at 25 m/s for 3 hours. What is the car’s momentum? Identify the variables: Mass = 1200 kg Velocity = 25m/s, west Time = 3 hours Actually 3.0e4kgm/ss

Let’s practice A 1200 kg car drives west at 25 m/s for 3 hours. What is the car’s momentum? Identify the variables: Mass = 1200 kg Velocity = 25m/s, west Time = 3 hours P = mv = 1200 x 25 = 30000 kg m/s, west Actually 3.0e4kgm/ss

How hard is it to stop a moving object?

How hard is it to stop a moving object? To stop an object, we have to apply a force over a period of time. This is called Impulse Impulse = FΔt Units: N∙s F = force (N) Δt = time elapsed (s)

How hard is it to stop a moving object? Impulse = FΔt Units: N∙s

How hard is it to stop a moving object? Using Newton’s 2nd Law we get FΔt= mΔv Which means Impulse = change in momentum

Why does an egg break or not break? An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why? FΔt= mΔv Ask students why

Why does an egg break or not break? FΔt= mΔv In both cases, m and Δv are the same. If Δt goes up, what happens to F, the force? Right! Force goes down. A pillow increases the time the egg takes to stop! Since m change in v is the same in both then F*delta t When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops.

Why does an egg break or not break? FΔt= mΔv Since m change in v is the same in both then F*delta t When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops.

Practice Problem A 57 gram tennis ball falls on a hard floor. Its velocity changes from -1.2 m/s to +1.2 m/s in 0.02 s. What is the average force on the ball? Identify the variables: Mass = 57 g = 0.057 kg Δvelocity = +1.2 – (-1.2) = 2.4 m/s Time = 0.02 s using FΔt= mΔv Fx (0.02 s) = (0.057 kg)(2.4 m/s) F= 6.8 N What is always the first step. Draw the picture Draw the arrows, it starts as going ot Change in V is important here What is the second

Car Crash Would you rather be in a head on collision with an identical car, traveling at the same speed as you, or a brick wall? Assume in both situations you come to a complete stop. Take a guess http://techdigestuk.typepad.com/photos/uncategorized/car_crash.JPG

Car Crash (cont.) Everyone should vote now And the answer is….. Raise one finger if you think it is better to hit another car, two if it’s better to hit a wall and three if it doesn’t matter. (or some variation of this)

Car Crash Let the MythBusters handle this!

Car Crash (cont.) The answer is… Both are the same! Look at FΔt= mΔv In both situations, Δt, m, and Δv are the same! The time it takes you to stop depends on your car, m is the mass of your car, and Δv depends on how fast you were initially traveling. This is from knowing that it is the force that hurts us

Egg Drop connection FΔt= mΔv How does this apply to the egg drop? Which of these variables can you control? FΔt= mΔv Which of them do you want to maximize, which do you want to minimize

From the California Standards Test 20000 Copyright © 2004 California Department of Education.

From the California Standards Test Momentum is conserved if the total energy in the system is conserved. Copyright © 2004 California Department of Education.

From the California Standards Test Answer 1m/s Copyright © 2004 California Department of Education.