The pull that speeds things up
From experience we know that an object falling from a height gets faster as it goes. That is, the object accelerates.
Why is there an acceleration?
For an acceleration to arise, a force must act on the object.
When an object falls from a height, the force acting on it is the earth's gravitational force.
The acceleration produced by that force is called the acceleration due to gravity. Its symbol is g.
The value of g
g ≈ 9.8 m s−2
at the earth's surface, at sea level
This means that as an object falls from a height, its velocity increases by 9.8 m s−1 every second.
Going upwards
When an object travels vertically upwards, what happens instead is that its velocity decreases by 9.8 m s−1 every second. So for an object moving upwards the value taken for the acceleration due to gravity is −9.8 m s−2.
How the sign is decided
| Motion | What velocity does | g |
|---|---|---|
| Falling down | Increases | +9.8 m s−2 |
| Rising up | Decreases | −9.8 m s−2 |
The sign does not change because g changes. Gravity always pulls downwards — what changes is which direction we chose to call positive.
To keep the arithmetic simple
In problems, g = 10 m s−2 is often used instead. The numbers come out cleanly and the physics is identical. If a question says to use it, use it.