Keiko is riding a skateboard 2 m/s, west. How long will it take her to reach a velocity of 8 m/s, west if her acceleration is 2 m/s2?

Hello!

By definition, acceleration is the measure of a velocity change in time. In a particular case when the acceleration is constant, the velocity of an object is changed uniformly (linearly) with time.

If we denote the initial velocity as the constant acceleration as a and the time since the initial moment as t, then the new speed V at time t will be

In our problem the velocities and the acceleration are given, and we need to solve the above equation for t. It is simple:

This is the answer. Note that it is important that the initial velocity and the final velocity have the same direction (west). If they would have opposite direction, one of them should be taken with a minus sign, and the acceleration might be negative.