Asked by: Mike Miller, Malta
As Einstein showed, it's all relative and depends on your own speed.
If you jump backwards from the skateboard more slowly than the skateboard's forward speed, then relative to a friend standing by watching, you'll still appear to be moving forward, as your jump hasn't cancelled out all your forward speed.
If, however, you jump at a higher speed than your forward speed, your friend will see you moving backwards.
Robert is a science writer and visiting professor of science at Aston University.