Chapter 6

1. Chapter 6 Momentum and Collisions

2. Let’s Play… …What Would You Rather Be Hit With!!!! Today’s Choices Are… …Mr. Friel’sDry Erase Marker!!! …Mr. Friel’sWhiffle Ball !!! …Mr. Friel’s 10.00 kg shot put!!! Now Choose!!!!!

3. Why did you make your decision? Now…the Bonus Round!!! Who would you rather have throw the object? …Mr. Friel or …Aroldis Chapman, 105 mph flamethrower for the Cincinnati Reds

4. What factors were involved in your decision? - mass - velocity Remember, Fnet= ma = m (Δv/Δt) - it takes force to alter the motion of an object Ex. Randy Johnson Pitch

5. SI Momentum (p) – possessed by any object in motion (must have mass and velocity) p = mv - SI Units are kg m / s - Vector quantity, the direction of the momentum is the same as the velocity

6. Conservation of Momentum and Impulse As long as no external force (friction) acts on an object in motion, momentum is conserved (Δp = 0) Any change in momentum due to an outside force is known as impulse.

7. Impulse F = ma F = m (Δv/Δt), multiply both sides by Δt Impulse = FΔt = mΔv = Δp Vector quantity, the direction is the same as the direction of the force.

8. Impulse-Momentum Theorem • The theorem states that the impulse acting on the object is equal to the change in momentum of the object. • If the force is not constant, use the average force applied

9. Why can we belly flop onto our beds, but not onto the kitchen floor, and other pertinent questions of our time… Think about how an airbag works in a car - increases Δt - decreases ΔF What if you hit the steering wheel? - ouch… What are some other objects that take advantage of impulse?

10. Let’s do another quick example, everyone climb up onto your chair… …now jump off… …how did you land? …what happed to your knees?

11. Conservation of Momentum, Example • The momentum of each object will change • The total momentum of the system remains constant

12. When objects collide, total momentum change (impulse) = 0 Initial momentum (pB + pA) is equal to final momentum (pB’ + pA’) Notice how Δp for object A and Δp for object B are exact opposites. ΔpA = - ΔpB

13. Law of conservation of momentum – the momentum of any closed, isolated system does not change - no objects in or out - no net external forces

14. Conservation of Momentum • Mathematically: • Momentum is always conserved for the system of objects • p1init + p2init = p1final + p2final

15. Types of Collisions • Momentum is conserved in any type of collision • Collisions are one of the following: • Perfectly elastic • Perfectly inelastic • Somewhat elastic

16. Perfectly Elastic Collisions • Only happens in a closed system (no friction in collision, no thermal energy loss). • Both momentum andkinetic energy conserved in a perfectly elastic collision.

17. Perfectly Inelastic Collisions • Inelastic collisions • Kinetic energy is not conserved • Perfectly inelastic collisions occur when the objects stick together • Not all of the KE is necessarily lost • In a perfectly inelastic collision, the final velocity is the same for both objects • Example of Perfectly Inelastic Collision - Office Linebacker

18. More About Perfectly Inelastic Collisions • When two objects stick together after the collision, they have undergone a perfectly inelastic collision • Conservation of momentum becomes • Because the masses have stuck together after colliding and are moving at the same velocity

19. More Types of Collisions • Perfectly elastic collision • Both momentum and kinetic energy are conserved • Actual collisions • Most collisions fall between perfectly elastic and perfectly inelastic collisions • In this case, kinetic energy is not conserved either.

20. More About Elastic Collisions • Both momentum and kinetic energy are conserved • Typically solved using systems of equations – two equations, two unknowns

21. Glancing Collisions • For a collision of two objects in three-dimensional space, the conservation of momentum principle implies that the total momentum of the system in each direction is conserved

22. Glancing Collisions • The “after” velocities have x and y components • Momentum is conserved in the x direction and in the y direction • Apply conservation of momentum separately to each direction