Question: If gravity is a constant why do people speed up when they are falling? The jump from space (Felix Baugartner) was my inspration for this question. Thanks

Stephen Davitt answered on 7 Nov 2016:

First great question and I love that video!

The short answer to your question is that yes gravity is constant (or at least we pretend it is, but thats a very long conversation and it barely changes anyway), but it is not a constant speed but a constant acceleration. Now depending on how much physics you’ve done this could require further explaining if not stop here. Otherwise I’ll try explain it in without getting too complex.

So here goes…

Speed or ‘velocity’ is the name we give to how fast you are moving, it is a measurement of how far you can travel in a certain time. When walking during 1 second we might travel a distance of 2 meters, so we call this travelling 2 meters per second (2m/s). When you move at the same speed it is said your speed is constant.

Acceleration is the name we give to when you change your speed, slowing down or speeding up. If you move from walking to running you can either slowly speed up or suddenly begin to sprint, in order to know the difference acceleration also has to indicate how fast the change in speed happened. We measure acceleration in meters per second per second (m/s/s). if you have an acceleration of 2m/s/s, every second your speed will increase by 2m/s until you stop accelerating.

Gravity is an acceleration and is 9.81m/s/s so if we take Felix for example he jumps and starts with zero speed 0m/s he begins to be pulled downwards and accelerated by gravity 9.81m/s/s. One second later his speed is now 9.81m/s, one second more his speed would be 19.62m/s and so on. So when we say that gravity is constant its not the Felix would travel at a constant speed, but that his speed would continue to increase by a constant amount every second.

I hope this makes some sense and answers your question.