Free fall motion

When an object accelerates because of the force due to gravity, with no other forces acting on it, we say that the object is in free fall.

When we make the assumption that an object is in free fall, we are neglecting any other forces, like the force of air resistance.

Courtesy: NASA - Galileo and Apollo 15

This video vas taken on the surface of the moon by Commander David Scott during the Apollo 15 mission. In the absence of air resistance, a hammer and a feather dropped from the same height at the same instant were shown to hit the ground at the same time. The objects have different mass and different weight, but have the same acceleration due to the force of gravity.

The acceleration due to gravity is relatively very constant on the surface of the Earth. This map shows gravitational anomalies in units of mGal, where 1 mGal is about a millionth of the average strength of the Earth's surface gravity.

g = 9.80665 m/s2

Note that the value of g is positive, it is the magnitude of the acceleration vector. The direction of the acceleration vector is toward the center of Earth.

Practice problem

Two rocks are simultaneously thrown from from a cliff. Rock A is dropped (starts from rest) while Rock B has an initial upward velocity of 2.0 m/s. If the cliff is 80 m high, how long after Rock A hits the ground does Rock B hit the ground?

What are the final velocities of the rocks? Note: when we consider the final velocity of a falling object, we want the velocity just before it lands.