Free Fall

In class we covered freely falling objects.

Ignoring frictional forces, such as air resistance, objects near the surface of the Earth will acclerate towards the Earth at a rate of -9.8 m/s^2.

Example 1:

A rock dropped from a 100 meter cliff will acclerate downwards towards the ground below.

We can calcualte the time it takes to fall in the following way.

Note in the calculation above that I used -100 meters for the displacement. Remember displacement is a vector and in free fall problem I will assign a negative sign to any downward pointing vector such as acceleration (-9.8 m/s^2) and the velocity of any downward moving object.

Example 2:

If we assume a ball thrown up from the surface of the Earth has an initial velocity of 50 m/s,  it too will also experience the same acceleration as the rock in the previous problem except, in this case, the ball's positive inital velocity will continually reduced each second it rises. The ball will loose 9.8 m/s of its upward velocity each second until its velocity reaches 0 m/s, 5.1 seconds after it was initially thrown. At that instant (v=0m/s), the ball will start to fall back to the Earth gradually gaining speed as it falls.

Assuming the ball lands back where it started, the ball will impact the ground with a velocity of -50m/s at t=10.2 seconds. The time it took to travel from its apex to the ground is the same as the time it took to travel from the ground to its apex.

Homework: Read the Free Fall section:
Problems page 40: 33,35,37,39,42