Question

a car slams on its brakes, coming to a complete stop in 4.0 s. the car was traveling north at 60.0 mph. calculate the acceleration.
mi/h/s
-20
-22
-15

Explanation:

Step1: Recall the acceleration formula

Acceleration \( a \) is defined as the change in velocity \( \Delta v \) divided by the time interval \( \Delta t \), so \( a=\frac{\Delta v}{\Delta t} \).

Step2: Determine initial and final velocities

The initial velocity \( v_i = 60.0\space \text{mph} \) (north), and the final velocity \( v_f = 0\space \text{mph} \) (since it comes to a stop). So the change in velocity \( \Delta v=v_f - v_i=0 - 60.0=- 60.0\space \text{mph} \).

Step3: Calculate acceleration

The time interval \( \Delta t = 4.0\space \text{s} \). Substitute into the acceleration formula: \( a=\frac{- 60.0}{4.0}=- 15\space \text{mi/h/s} \)? Wait, no, wait: Wait, \( 60\div4 = 15 \), but with a negative sign because it's deceleration. Wait, but let's check again. Wait, \( \Delta v=0 - 60=-60 \), \( \Delta t = 4 \), so \( a=\frac{-60}{4}=- 15 \)? Wait, but maybe I made a mistake. Wait, no, wait, 60 divided by 4 is 15, but the options have -15, -20, -22. Wait, wait, maybe the units? Wait, no, the formula is correct. Wait, initial velocity is 60 mph, final is 0, time is 4 s. So acceleration is \( (0 - 60)/4=-15\space \text{mi/h/s} \). Wait, but let's check the calculation again. \( 60\div4 = 15 \), so with the negative sign, it's -15. So the acceleration is -15 mi/h/s.

Answer:

-15