## Gravitational Forces on the Earth

• We are used to calculating the Earth's gravitational force on objects in everyday life:
```          F  =  m * g        downwards
```

• But this is just a simplified version of the actual Law of Gravitation, which states that any two objects attract each other with a force
```                  m1 * m2
F  =   G * ---------
R^2
```
where
```         G   =  6.67 x 10^(-11)  N * (kg^2) / (m^2)
m1  =  mass of first object, in kg
m2  =  mass of second object, in kg
R   =  distance between objects, in m
```

• It turns out that gravity is very weak -- as indicated by the small value of the gravitational constant G. In fact, the gravitational force is so small that it's very difficult to measure the force between any two ordinary, everyday objects (such as people or cars). Scientists have had to resort to ingenious and delicate equipment to measure such tiny forces.

• On the other hand, gravity is the dominant force between objects on the very largest scales: planets, stars and galaxies. So, even though it's weak, it still plays a big part in the universe.

Viewgraphs

Viewgraph 1

Viewgraph 2

Viewgraph 3

Viewgraph 4

Viewgraph 5

Viewgraph 6

Despite the tiny size of the gravitational force between two ordinary objects, it is possible to measure this force, even with low-tech equipment. Take a look at this experiment, which shows motions due to the gravitational force between chunks of metal and rocks.

This material comes from experiments by John Walker. You can find his descriptions of gravitational experiments at http://www.fourmilab.ch/gravitation/foobar/