Problem 12.

You are driving along the street at the speed limit (35mph) and 50 meters before reaching a traffic light you notice it becoming yellow. You accelerate to make the traffic light within the 3 seconds it takes for it to turn red. What is your speed as you cross the intersection? Assume that the acceleration is constant and that there is no air resistance.

Solution:

This is the motion with constant acceleration. If the acceleration of the car is a then we can write the expression for traveled distance:

We know the initial velocity , we also know that after 3 seconds the car travels distance 50 meters. Then we can find acceleration

Now we know acceleration, then we can find the final velocity: