Impulse and Collisions
9.2 Impulse and Collisions
Learning Objectives
By the end of this section, you will be able to:
- Explain what an impulse is, physically
- Describe what an impulse does
- Relate impulses to collisions
- Apply the impulse-momentum theorem to solve problems
Impulse
For a force that is applied to an object over a time interval, the impulse is defined as:
Practice!
| Practice 9.2.1 |
|---|
 A ball bounces off the floor as shown. The direction of the impulse on the ball is … |
| (a) Straight up ⬆️ |
| (b) Straight down ⬇️ |
| (c) To the right ➡️ |
| (d) To the left ⬅️ |
| (e) Up and to the right ↗️ |
| Practice 9.2.2 |
|---|
| A 57.0-g tennis ball is traveling straight at a player at 18.0 m/s. The player volleys the ball straight back at 23.0 m/s. If the ball remains in contact with the racket for 0.0600 s, what average force acts on the ball? |
| (a) 4.75 N |
| (b) 17.1 N |
| (c) 56.05 N |
| (d) 21.85 N |
| (e) 38.95 N |
Discuss!
Consider how you would answer these questions. Then bring this to class for a group discussion.
A carnival game requires you to knock over a wooden post by throwing a ball at it. You are offered a very bouncy rubber ball and a very sticky clay ball of equal mass. Assume you can throw either one of them with equal speed and accuracy, but you are only allowed to throw one. Which ball (rubber or clay) do you throw and why?
| Pause & Predict 9.2.1 |
|---|
| A 2-kg block, initially moving in the positive x-direction at 3 m/s, collides with a wall and rebounds with a speed of 2 m/s. If the wall exerts a 400-N force on the block, how long is the block in contact with the wall? |
| (a) 0.005 s |
| (b) 0.010 s |
| (c) 0.015 s |
| (d) 0.020 s |
| (e) 0.025 s |
| Pause & Predict 9.2.2 |
|---|
| A 2-kg block, initially moving in the positive x-direction at 3 m/s, collides with a padded wall. The wall exerts a linearly changing force with a maximum of 640 N on the block. Then the block rebounds 0.035 s after the initial collision. What is the final velocity of the block? |
(a) 2.6 m/s  |
| (b) -2.6 m/s |
| (c) -8.6 m/s |
| (d) 8.6 m/s |
Practice!
| Practice 9.2.3 |
|---|
| A 700-kg car, driving at 29 m/s, hits a brick wall and rebounds with a speed of 4.5 m/s. What is the car’s change in momentum due to this collision? |
| (a) 2.3 × 104 kg•m/s towards the wall |
| (b) 1.7 × 104 kg•m/s towards the wall |
| (c) 2.3 × 104 kg•m/s away from the wall |
| (d) 1.7 × 104 kg•m/s away from the wall |
| Practice 9.2.4 |
|---|
| A 700-kg car, driving at 29 m/s, hits a brick wall and rebounds with a speed of 4.5 m/s. If the car was in contact with the wall for 0.30 s, what force did the wall exert on the car during the collision? |
| (a) 5.7 × 104 N |
| (b) 7.8 × 104 N |
| (c) 6.8 × 104 N |
| (d) 1.1 × 104 N |
| Practice 9.2.5 |
|---|
 A 65-kg bungee jumper, who is attached to one end of an 85-m long bungee cord that has its other end tied to a bridge, jumps off the bridge and toward the river below. When stretched, the bungee cord provides a force on the jumper that increases linearly as the cord is stretched and decreases linearly as the cord returns back to its equilibrium length. When the bungee cord is stretched to its maximum length, it exerts a 2.2 kN force on the jumper. If the bungee cord is stretched beyond its equilibrium length for a duration of 1.2 seconds, what is the impulse delivered to the bungee jumper? |
| (a) 2.6 kN•s |
| (b) 0.66 kN•s |
| (c) 1.8 kN•s |
| (d) 1.3 kN•s |
| (e) 0.55 kN•s |
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