# An electric toy car with a mass of #4 kg# is powered by a motor with a voltage of #2 V# and a current supply of #7 A#. How long will it take for the toy car to accelerate from rest to #5 m/s#?

##### 1 Answer

#### Explanation:

Here is a reference for Power and Energy

When the car reaches the speed of

where V is the speed of the car (in

The power (in Watts) being supplied to the motor is:

where I is the current (in Amperes) and E is the voltage (in Volts)

One of the ways to breakdown the Watt into other units is:

We are not given an efficiency for the motor, therefore, we must assume that it is 100% efficient. This is not a bad assumption, because it is quite easy to obtain 98% efficiency from an electric motor.

The following equation will give a consistent set of units where 100% of the power supplied to the electric motor went into the kinetic energy of the car:

where t is the time of the acceleration in seconds.

Solving for t: