# The brakes are applied on a car traveling at 30. m/s [fwd]. The car stops in 3.0s. What is its displacement during this time?

## The answer is 45 m [fwd], but I do not understand how to get to this answer.

Dec 26, 2016

You can use the equations of motion to find the displacement, as shown below.

#### Explanation:

If we are assuming that the acceleration is uniform (which I believe must be the case), you can use the following equation of motion, as it does not require that you know, or first calculate the acceleration:

$\Delta d = \frac{1}{2} \left({v}_{i} + {v}_{f}\right) \Delta t$

This basically says that the displacement $\Delta d$ is equal to the average speed $\frac{1}{2} \left({v}_{i} + {v}_{f}\right)$ multiplied by the time interval $\Delta t$.

Insert the numbers

$\Delta d = \frac{1}{2} \left(30 + 0\right) \left(3\right) = 15 \left(3\right) = 45 m$