#### Question

The driver of a car traveling at a speed of 18 m/s slams on the brakes and comes to a stop in 4 s. If we assume that the car's speed changed at a constant rate (constant net force): What was the car's average speed during this 4 s interval?

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#### Step 1

1 of 2

Since the car starts at speed of 18 m/s, then it comes to rest, and because the net force on the car is constant, then the average velocity of the car is

$v_{\text{avg}} = \frac{v_f+v_i}{2} = \frac{0 + 18}{2} = 9 \; \mathrm{m/s}$

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