Exploring Forces
Gravitational force
Investigate why everything you throw up comes back down.
The simplest test (Activity 5.8). Throw a ball straight up. It comes down. Throw it harder. It still comes down. Whatever you throw, in any direction, ends up back on the ground.
Everything falls towards the Earth, so the Earth must be pulling it. The force with which the Earth pulls objects towards itself is the gravitational force, also called the force of gravity, or just gravity.
Gravity works without touching the object, so it is a non-contact force.
But it is different from the other two in one big way:
| Force | Can it pull? | Can it push away? |
|---|---|---|
| magnetic | yes | yes |
| electrostatic | yes | yes |
| gravitational | yes | never |
Gravity is always attractive: it only ever pulls.
Watch the path. Drop something and it falls straight down. Throw it straight up and it rises, slows down, stops for an instant at the top, then falls straight back, getting faster as it comes down. Moving up or down under gravity like this is called vertical motion.
And that answers Sonali's question. Coming down the hill, something really was pulling her bicycle: the gravitational force of the Earth.
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