Question

An electric toy car with a mass of `3 kg` is powered by a motor with a voltage of `4 V` and a current supply of `7 A`. How long will it take for the toy car to accelerate from rest to `4 m/s`?

Answer 1

I got `0.86s`

Explanation:

In reaching `4m/s` the car will have a Kinetic Energy of:
`K=1/2mv^2=1/2*3*4^2=24J`

The Power deliverd by the motor is:

`P=VI=4*7=28W` that means that it delivers `28J` each second.

The toy car will need `24J` so using a proportion we get:

`1"sec":28J=x"sec":24J`

or

`x=24/28=0.857~~0.86s`