# An electric toy car with a mass of #6 kg# is powered by a motor with a voltage of #7 V# and a current supply of #6 A#. How long will it take for the toy car to accelerate from rest to #4 m/s#?

##### 1 Answer

#### Explanation:

We can suggest two solutions.

Energy conservation approach leads to following solution.

The beginning kinetic energy of a toy car is zero. Ending energy is

This is exactly the amount of work (

Power of the electric motor, that is its *voltage* times *current* (

Therefore,

and

Different approach is related to analyzing the force and distance.

Assume, the time to accelerate from rest (

That means, acceleration equals to

The distance covered by a toy car equals to

The force of the electric motor that accelerates a toy car equals to its mass times acceleration (Second Newton's Law)

The work performed by a motor equals to force times distance:

The power of the electric motor *voltage* by *current* (*amperage*):

The work electric motor performs during the time

Therefore,