The velocity of a car changes from 20 ms east to 5 ms east in 5 seconds. What is the acceleration of the car?

To find the acceleration, we use the formula ( a = \frac{\Delta v}{\Delta t} ), where ( \Delta v ) is the change in velocity and ( \Delta t ) is the change in time. The change in velocity ( \Delta v ) is ( 5 , \text{ms}^{-1} - 20 , \text{ms}^{-1} = -15 , \text{ms}^{-1} ). Over the time interval of 5 seconds, the acceleration is ( a = \frac{-15 , \text{ms}^{-1}}{5 , \text{s}} = -3 , \text{ms}^{-2} ). Thus, the car's acceleration is ( -3 , \text{ms}^{-2} ), indicating it is decelerating.