Gravity, resistance and mechanisms
Physics · Key Stage 2 · The School
Why things fall
GRAVITY is a force of attraction between any two masses. The Earth is enormous, so the pull between it and everything on it is what we notice — an unsupported object falls towards the Earth’s centre. Weight is the name for that pull, which is why the same object weighs less on the Moon while containing exactly as much matter.
Forces that push back
Moving things meet resistance. AIR RESISTANCE pushes back on anything moving through air, and grows with speed and with the area facing forwards — which is how a parachute works. WATER RESISTANCE does the same in water, so boats and fish are shaped to cut through it. FRICTION acts between surfaces sliding on each other; it slows things down, and it is also what lets you walk and stops a bike.
Getting more from a smaller force
A MECHANISM lets a small force do a big job. A LEVER — a spanner, scissors, a wheelbarrow — trades distance for force: push a longer way with less effort. A PULLEY changes the direction of your pull and, with more than one wheel, shares the load. GEARS pass turning from one wheel to another; a small gear driving a large one turns it more slowly but with more turning force.
Take two sheets of the same paper. Drop one flat and screw the other into a tight ball, then drop both from the same height. The ball lands well first. Their MASS is identical, so gravity pulls equally on both — what differs is air resistance, which grows with the area facing forwards. Now do it again with a book and the flat sheet resting on top of it, and they fall together, because the book clears the air out of the way.