1. Impulse and Momentum
AP Physics 1

2. Momentum

3. Momentum
The product of a particle’s mass and velocity is called the momentum of the particle:
Momentum is a vector, with units of kg m/s.
A particle’s momentum vector can be decomposed into x- and y-components.
Slide 9-21

4. The cart’s change of momentum px is
QuickCheck 9.1
The cart’s change of momentum px is
–20 kg m/s.
–10 kg m/s.
0 kg m/s.
10 kg m/s.
30 kg m/s.
Slide 9-22 4

5. The cart’s change of momentum px is
QuickCheck 9.1
The cart’s change of momentum px is
–20 kg m/s.
–10 kg m/s.
0 kg m/s.
10 kg m/s.
30 kg m/s.
px = 10 kg m/s  (20 kg m/s) = 30 kg m/s
Negative initial momentum because motion is to the left and vx < 0.
Slide 9-23 5

6. Impulse

7. Impulse = Momentum Momentum is defined as “Inertia in Motion” Ns
Consider Newton’s 2nd Law and the definition of acceleration Units of Impulse: Units of Momentum: Ns
Kg x m/s
Momentum is defined as “Inertia in Motion”

8. Impulse is the Area
Since J=Ft, Impulse is the AREA of a Force vs. Time graph.

9. Impulse During a Collision
A large force exerted for a small interval of time is called an impulsive force.
The figure shows a particle with initial velocity .
The particle experiences an impulsive force of short duration t.
The particle leaves with final velocity .
Slide 9-26

10. Conservation of Momentum

11. Collisions
A collision is a short-duration interaction between two objects.
The collision between a tennis ball and a racket is quick, but it is not instantaneous.
Notice that the right side of the ball is flattened.
It takes time to compress the ball, and more time for the ball to re-expand as it leaves the racket.
Slide 9-24

12. Atomic Model of a Collision
Slide 9-25

13. How about a collision?
Consider 2 objects speeding toward each other. When they collide Due to Newton’s 3rd Law the FORCE they exert on each other are EQUAL and OPPOSITE. The TIMES of impact are also equal. Therefore, the IMPULSES of the 2 objects colliding are also EQUAL

14. How about a collision?
If the Impulses are equal then the MOMENTUMS are also equal!

15. QuickCheck 9.8
A mosquito and a truck have a head-on collision. Splat! Which has a larger change of momentum?
The mosquito.
The truck.
They have the same change of momentum.
Can’t say without knowing their initial velocities.
STT1.2
Slide 9-60 15

16. QuickCheck 9.8
A mosquito and a truck have a head-on collision. Splat! Which has a larger change of momentum?
The mosquito.
The truck.
They have the same change of momentum.
Can’t say without knowing their initial velocities.
STT1.2
Momentum is conserved, so pmosquito + ptruck = 0.
Equal magnitude (but opposite sign) changes in momentum.
Slide 9-61 16

17. Momentum is conserved!
The Law of Conservation of Momentum: “In the absence of an external force (gravity, friction), the total momentum before the collision is equal to the total momentum after the collision.”

18. Several Types of collisions
Sometimes objects stick together or blow apart. In this case, momentum is ALWAYS conserved.
When 2 objects collide and DON’T stick
When 2 objects collide and stick together
When 1 object breaks into 2 objects
Elastic Collision = Kinetic Energy is Conserved
Inelastic Collision = Kinetic Energy is NOT Conserved

19. Collisions and Explosion

20. Example
A bird perched on an 8.00 cm tall swing has a mass of 52.0 g, and the base of the swing has a mass of 153 g. Assume that the swing and bird are originally at rest and that the bird takes off horizontally at 2.00 m/s. If the base can swing freely (without friction) around the pivot, how high will the base of the swing rise above its original level? How many objects due to have BEFORE the action? How many objects do you have AFTER the action? 1 2
m/s
0.024 m

21. Example
Granny (m=80 kg) whizzes around the rink with a velocity of 6 m/s. She suddenly collides with Ambrose (m=40 kg) who is at rest directly in her path. Rather than knock him over, she picks him up and continues in motion without "braking." Determine the velocity of Granny and Ambrose.
How many objects do I have before the collision?
How many objects do I have after the collision? 2 1
4 m/s

22. Momentum in Two Dimensions
The total momentum is a vector sum of the momenta of the individual particles.
Momentum is conserved only if each component of is conserved:
Slide 9-86

23. Collisions in 2 Dimensions
The figure to the left shows a collision between two pucks on an air hockey table. Puck A has a mass of kg and is moving along the x-axis with a velocity of +5.5 m/s. It makes a collision with puck B, which has a mass of kg and is initially at rest. The collision is NOT head on. After the collision, the two pucks fly apart with angles shown in the drawing. Calculate the speeds of the pucks after the collision. vA
vAsinq
vAcosq
vBcosq
vBsinq vB

24. Collisions in 2 dimensionsvA
vAsinq
vAcosq
vBcosq
vBsinq vB

25. Collisions in 2 dimensions