Question

A car moves with a speed of 80 m/s. If the driver used the brakes to decrease the speed, so it decreases by 2 m/sec^2. What is its speed after 12 seconds from using the brakes?

Answer 1

I found `56m/s`

Explanation:

Here you can use the cinematic relationship:

`color(red)(v_f=v_i+at)`

Where:
`t` is time,
`v_f` is the final velocity,
`v_i` the initial velocity and
`a` acceleration;
in your case:
`v_f=80-2*12=56m/s`

Answer 2

The car's speed after twelve seconds of using brakes is `56`m/s

Explanation:

If the car's acceleration due to braking is `2`m/s², that means that every second, the car's speed decreases by `2`m/s:

`2`m/s²`=2`m/s/s

Therefore, if the car's speed is `80`m/s when the brakes are used, its speed is `78`m/s one second later and, after `12` seconds, its speed will be `12*2=24`m/s lower.

`80-24=56`