Motion and Stability: Forces and Interactions - Gravity
Gravity is a force with direction
Earth pulls every object toward its center. “Down” simply means toward Earth’s center, which is why a dropped ball always falls toward the ground and why people on the opposite side of the planet also fall “down.” Gravity acts at a distance - nothing has to touch. Earth’s pull keeps the Moon in orbit, and the Sun’s much larger pull keeps the planets in orbit around it.
Mass versus weight
Mass is the amount of matter in an object and stays the same everywhere. Weight is the pull of gravity on that mass, so an astronaut weighs less on the Moon, where gravity is weaker, while the mass is unchanged.
Traps to watch for
- “Gravity only pulls heavy things.” False - gravity pulls every object with mass, including specks of dust.
- “There is no gravity in space, so astronauts float.” False - astronauts in orbit are falling around Earth, and the Moon orbits because of gravity.
- “The bigger object falls faster.” In a fair comparison without air resistance, objects land together.
- Confusing the upward push of a table or floor with gravity; that push is a support force, not a gravitational pull.
Worked example
A ball is thrown straight up. Describe gravity at every moment: it pulls down on the way up (slowing the ball), at the very top (it does not switch off), and on the way down (speeding the ball up). To compare locations, ask “which body pulls harder?” A ball dropped on the Moon falls more slowly and with less force than on Earth, but its mass is identical.