Why a tucked gymnast spins faster
Anything that is spinning has a quantity physicists call angular momentum, and unless something outside pushes or drags on it, the amount stays the same. It depends on how fast the object spins and how far its weight is spread out from the centre. So if a spinning gymnast, diver or figure skater pulls their arms and legs in close, the spread gets smaller, and the spin has to get faster to make up for it. Stretching out slows the spin down again. Nobody pushes them; they change speed simply by changing shape.
Our gymnastics page covers the tuck, pike and straight positions that use this. A falling cat uses the same physics to twist round and land on its feet, bending its body in the middle and turning one half and then the other.
Every push pushes back
A FORCE is a PUSH or a PULL. When the forces on something are BALANCED, as in a tug of war with two equal teams, nothing changes: a still object stays still and a moving one keeps moving. When they are UNBALANCED, the object will ACCELERATE, speeding up, slowing down or changing DIRECTION. Isaac Newton also realised that every force comes with an equal force pushing back the other way. A rocket works by pushing hot gas out of its engines at tremendous SPEED; the gas pushes the rocket forward with the same force, called THRUST, even in space where there is nothing to push against. Our spacecraft page covers where that takes them.
Falling at full speed
GRAVITY pulls everything towards the ground, and that pull is your WEIGHT. Air pushes back on anything moving through it, a force called DRAG, which grows the faster you go. A skydiver speeds up until the drag matches their weight, then stops getting faster, at a top speed of around 200 kilometres an hour lying flat. In 2012 Felix Baumgartner jumped from a balloon about 39 kilometres up, where the air is so thin there is almost no drag, and fell faster than the speed of sound before opening his parachute. A raindrop falls much more gently: drag stops it speeding up once it reaches around thirty kilometres an hour, so rain does not hurt when it lands.
Whether something will FLOAT or SINK depends on whether the water it pushes aside weighs more or less than it does. A ball rolls further on a smooth floor than on a carpet because of FRICTION, and it rolls faster down a steeper SLOPE.
Stretch, squash and twist
Forces can change an object's shape as well as its speed. You can STRETCH an elastic band, SQUASH a sponge and TWIST a towel. In 1676 Robert Hooke, the same scientist who named the cell, worked out that a SPRING stretches in proportion to the force pulling it, double the force for double the stretch, a rule still used to design weighing scales and car suspension. MAGNETISM is a force that can act without touching at all, pulling iron and steel from a distance, and our physics page covers the rest of the laws of MOTION.