Day 2 - Gravity:
Basketball on the Moon and Bouncing Balls

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Teacher's information:

Objects can store energy in many ways. A stretched or twisted rubber band stores elastic potential energy. The stored chemical energy in foods and fuels is a form of potential energy. In this activity, students explore a form of stored energy called gravitational potential energy.

A ball held at some distance above the ground possesses gravitational potential energy. The force needed to lift the object against gravity is the weight of the object, and work is done lifting it through a distance above the ground. The product of the object's weight and the distance lifted is known as the gravitational potential energy.

Thus, just before a ball is dropped it possesses gravitational potential energy. As it falls, it gains kinetic energy (energy of motion). When the ball collides with the floor, some of this kinetic energy is stored as elastic potential energy in the ball and the floor. The particles in the ball and the floor squeeze together like tiny springs. Because the material in the super ball springs back to its original shape after being deformed, more of the elastic potential energy is returned to the ball, causing it to rebound higher. The collision is said to be elastic. On the other hand, the type of material in a Nerf ball or clay ball causes it to absorb this potential energy and to return to its original shape slowly or not at all. Much of the energy is not returned to the motion of the ball, resulting in a low bounce. The collision is said to be inelastic.

Newton's equation for the force (in units of "Newtons") of gravity between two objects is:

F = G*m1*m2/d^2

m1 and m2 are masses (in kilograms) and d^2 is the distance (in meters) squared between the centers of the two objects (e.g. you and the center of the earth). G is a constant, the gravitational constant, which is 6.67*10^-11 when these units are used. Since this is a very small number, it means that gravity is a very weak force that you only really notice when there is a very large mass involved (like the earth or the moon).

So, we take our equation and plug and chug.

Mass of the Earth: 5.98 x 10^24 kg
Radius of the Earth: 6378 km
(from Welcome to the Planets) Mass of the moon: 7.34*10^22 kg