Gravity

What is gravity?

Gravity is a force of attraction between any two objects that have mass. It is a non-contact force.

• Every object pulls on every other object.
• The pull is only noticeable when at least one object is very massive, like a planet or a star.
• The pull gets weaker as objects get further apart.

Gravity holds you on the ground, makes rain fall, keeps the Moon orbiting Earth and keeps Earth orbiting the Sun.

Mass and weight

$$W=m\times g$$

• $W$ = weight (N)
• $m$ = mass (kg)
• $g$ = gravitational field strength (N/kg)

On Earth, $g\approx 9.8$ N/kg. This is the same number as the acceleration due to gravity, 9.8 m/s². The units N/kg and m/s² are actually equal.

Falling objects

Without air, everything falls with the same acceleration, $g$. With air, there is also air resistance.

1. Just after jumping, a skydiver's only big force is weight. She accelerates at about 9.8 m/s².
2. As she speeds up, air resistance grows. Net force gets smaller, so acceleration gets smaller.
3. Eventually air resistance equals weight. Net force is zero. She falls at a steady top speed: terminal velocity.
4. She opens the parachute. Air resistance jumps up, now bigger than weight. She slows down.
5. Slower speed means less air resistance, until it balances weight again at a new, much lower terminal velocity. Safe landing.

Gravity in space

The Moon is constantly falling towards Earth. But it is also moving sideways so fast that it keeps missing. That curved path is an orbit. Satellites and the International Space Station orbit Earth the same way.

Astronauts in the space station feel "weightless". Gravity there is still about 90% as strong as on the ground. They float because they and their station are falling together around Earth.

Quick summary

• Gravity is a non-contact force of attraction between masses.
• Mass (kg) is the amount of matter and never changes. Weight (N) is the force of gravity.
• $W=mg$. On Earth $g\approx 9.8$ N/kg.
• Your weight changes on other planets. Your mass does not.
• Falling objects reach terminal velocity when air resistance equals weight.
• Orbits happen when an object falls towards a planet while moving sideways fast enough to keep missing it.

Quiz