Gravity: a pull towards the Earth
Gravity is a pull between two masses. The Earth is so big that its pull on things near it is strong. Every object on or near the Earth is pulled towards the centre of the Earth. For us, that means "down".
The pull is a force. It acts on an object whether it touches the ground or not. Drop a pen: gravity pulls it down. Hold it still: gravity still pulls, and your hand pulls up with an equal force.
Direction of weight
The pull of gravity on an object is its weight. Its direction is always vertically downwards, towards the centre of the Earth. This holds even on a slope.
A plumb line is a string with a small heavy bob. It hangs along the direction of gravity. Builders use it to check that a wall is vertical. Water in a bucket also keeps a flat, horizontal surface, which is at right angles to the plumb line.
Weight and mass: W = m × g
W = m × g. Mass m is in kilograms. The number g is the gravity strength. On Earth it is about 9.8 N per kg (some books use 10). So a 1 kg object weighs about 9.8 N.
The Moon pulls about 1.6 N per kg, so on the Moon the same object weighs only about 1.6 N. Mass stays the same, weight changes with g. A spring scale reads weight.
Centre of gravity
An object has weight in every little part of it. But we can act as if all of the weight pulls at one point. That point is the centre of gravity (CG).
- For a uniform ruler or a ball, it is the middle.
- For an odd shape like a letter L, find it by hanging the shape from a pin, drawing the plumb line, then hanging it from another pin and drawing a second line. The lines cross at the CG.
- A hanging object rests with its CG right below the pin.
You can balance an object on one finger if the finger is below its CG. A low CG makes a thing hard to tip over, like a tractor or a toy that rocks and stands up again.
Try it
Cut an odd shape from cardboard. Push a pin through one corner into a wall or a cork and let the shape swing. Tie a thread with a small nut to the pin and mark the line on the cardboard. Repeat with another corner. The crossing of the two lines is the centre of gravity. Check by balancing the shape on your pencil tip at that point.
Key formulas and definitions
- Weight W = m × g
- g ≈ 9.8 N/kg on Earth (≈ 10 for quick sums), about 1.6 N/kg on the Moon
- Weight acts vertically downwards, towards the centre of the Earth
- Mass m = W / g
- Centre of gravity: the point where the whole weight seems to act
Worked examples
1. Find the weight of a 5 kg bag on Earth. Use g = 9.8 N/kg.
W = m × g = 5 × 9.8 = 49 N, straight down.
2. A box weighs 196 N on Earth (g = 9.8). What is its mass?
m = W / g = 196 / 9.8 = 20 kg.
3. A girl has a mass of 40 kg. Find her weight on Earth (g = 10) and on the Moon (g = 1.6).
Earth: 40 × 10 = 400 N. Moon: 40 × 1.6 = 64 N.
Common mistakes
- Thinking gravity points "along the slope" on a ramp. It always points straight down.
- Writing weight in kg. Weight is a force in newtons.
- Using W = m × g with mass in grams. Change grams to kg first.
- Thinking the centre of gravity must be inside the object. For a ring, it is in the empty middle.