1. 7. Momentum and impulse Momentum: 𝑝=𝑚𝑣 (unit: kg m s-1)
Change in momentum (∆𝑝) is called impulse (𝐼):
∆𝑝=𝐼=𝐹𝑡
Momentum is ALWAYS conserved!

2. Ex 1: What is the momentum of a hockey puck with mass 170 g moving at 10 m s-1?

3. Ex 2: Which action requires more force, to block the puck (m = 0,17 kg, u = 10 m s-1) or to catch it? Both actions take 0.10 s.
(041217)

4. Ex. 3: A force of 100 N from a racket acts on a tennis ball (m = 0
Ex. 3: A force of 100 N from a racket acts on a tennis ball (m = kg, u =20 ms-1) for s. Calculate:
The impulse
The change in velocity
The final velocity

5. 8. Collisions
Total momentum before and after the collision are the same!
Inelastic collisions:
Momentum is conserved
Formula: 𝑚 1 𝑢 1 + 𝑚 2 𝑢 2 = 𝑚 1 𝑣 1 + 𝑚 2 𝑣 2
Elastic collisions:
”Perfect bounce”
Momentum and kinetic energy conserved
Totally inelastic collisions:
”Stick together”
Formula becomes: 𝑚 1 𝑢 1 + 𝑚 2 𝑢 2 = (𝑚 1 + 𝑚 2 )𝑣
The direction is important!

6. Example 1: An ice skater (m = 49 kg) at rest catches a ball (m = 1 kg) with velocity 10 m s-1. What happens?
Totally inelastic: 𝑚 1 𝑢 1 + 𝑚 2 𝑢 2 = (𝑚 1 + 𝑚 2 )𝑣
𝑚 1 =49 kg, 𝑢 1 =0 m s-1
𝑚 2 =1 kg, 𝑢 2 =10 m s-1
49 kg∙0 m s −1 +1 kg∙10 m s −1 =(49 kg+1 kg)∙𝑣
𝑣= 1 kg∙10 m s −1 49 kg+1 kg =0.2 m s −1
Answer: the skater (holding the ball) will start moving with 0.2 m s-1 in the same direction as the ball that hit her

7. Example 2: A car (m = 1000 kg, v = 100 km h-1) and a truck (m = kg, v = 80 km h-1) collide head on and stick together. What happens?
Totally inelastic: 𝑚 1 𝑢 1 + 𝑚 2 𝑢 2 = (𝑚 1 + 𝑚 2 )𝑣
𝑚 1 =1000 kg, 𝑢 1 =100 km h-1
𝑚 2 = kg, 𝑢 2 = - 80 km h-1
1000 kg∙100 km h −1 −15000 kg∙80 km h −1 =(1000 kg kg)∙𝑣
𝑣= 1000 kg∙100 km h −1 −15000 kg∙80 km h − kg kg =−68.75 km h −1
Answer: The combined wreck will move at 69 km h-1 in the direction of the truck

8. P. 23: 7.1, 7.2, 7.3, 7.4, 7.5, (extra 7.7)
P. 27-: 8.1 (b) means the first puck stops the energy from the push is converted to kinetic energy shared between them (extra 8.9)