# I need help with a physics problem?

A 0.300-kg toy car moving with a speed of 0.860 m/s collides with a wall. The figure below shows the force exerted on the car by the wall over the course of the collision. What is the magnitude of the velocity, or final speed, of the car after the collision?

Here is the figure:

http://imgur.com/RNUsoVG

Thank you!

Here is the figure:

http://imgur.com/RNUsoVG

Thank you!

## 2 Answers

- Integrate force over time to find the impulse conveyed to the car by the wall. Then divide by mass to find the change in velocity.00
- The "impulse" (or "change of momentum") is equal to the area under the F/t graph.

Use your knowledge of the areas of triangles and rectangles to find the area.

See what I mean here:

https://www.flickr.com/photos/61682497@N...

Click for better resolution.

I get impulse

Δp = 0.312 N·s = 0.312 kg·m/s

since 1 N·s = 1 kg·m/s.

Since the impulse is applied in a direction opposite the initial momentum, the final momentum is

final p = initial p - Δp

final m*v = initial m*v - Δp

0.300kg * v = 0.300kg * 0.860m/s - 0.312kg·m/s = -0.054 kg·m/s

v = -0.180 m/s ← as it rebounds away from the wall

so speed = 0.180 m/s ◄ speed is always positive.

Hope this helps!30

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