Describe how the forces change on a parachutist while they fall.

It is important to remember that there are four main concepts to keep in mind while thinking about the parachutist. To remember them, it might be helpful to use the acronym GADE: Gravity, Acceleration, Drag, Equilibrium. It is important to explain each stage in detail, and show you understand how forces change the parachutist's speed. In a session (or an exam answer), force diagrams can help to make these concepts more real, and make sure you understand them.

First, when the parachutist jumps, they experience the force of Gravity pulling them downwards. Newton's Law says that the total (net) force on an object is proportional to it's acceleration, so this force causes the parachutist to accelerate down. However, as they fall, the air starts to push back, so they experience a Drag force pushing them upwards trying to slow them down, which gets larger the faster they go. So the faster they go, the smaller the total force pulling them down is, and the smaller their acceleration, until they reach a speed where they are in Equilibrium - the Drag force equals the force of Gravity, and they stop Accelerating. This means they fall at a constant speed, called the 'terminal velocity'.

Now that we have looked at how the forces on the parachutist work without a parachute, we can see what will happen when the parachute opens. The parachute has a very large area, so it will increase the drag force (imagine trying to pull a parachute through water vs pulling the parachutist by herself). Because the force of Gravity is always the same, the total force is now upwards, so she will slow down. As she slows, the drag force gets smaller until it reaches a speed where it is equal to gravity and there is no total force - so a new equilibrium, and a new terminal velocity, has been reached.