# What forces are experienced when someone running near the speed of light jumps?

Suppose I am running near the speed of light, say 0.87c so that my gamma factor is exactly 2. What forces will I and the ground experience if I jump? Imagine it's a normal two-footed jump where I take off vertically but maintain all forward velocity. And how will gravity act on me?

My guess: I will exert some amount of force on the ground, say F Newtons. But from the ground's reference frame my acceleration is doubled so it experiences 2F Newtons of force. The reaction force is 2F Newtons also, which is 'translated' to F Newtons in my frame of reference, launching me upwards. I would experience half the acceleration from gravity, but I would also experience length contraction so I think I've jumped normal height but actually I've jumped double height. The impact with the ground would be F Newtons from my reference frame but 2F from the ground's.

Suppose now I'm vibrating on the spot at 0.87c, and I jump. Does anything change in this scenario?

Edit: Assume I'm on an infinite plane and acted upon by gravity, g=9.8m/s^2.

• You must have pretty good running shoes. ;) You'd circumnavigate the planet pretty quickly at that speed. And circular motion at that speed at the radius of the Earth requires about 4.3 billion g centripetal acceleration. BTW, relativistic length contraction only happens in the direction of motion. – PM 2Ring Jun 11 at 20:01
• @PM2Ring Edited for clarity on the Earth size issue. – otah007 Jun 11 at 20:33