When I jump in the air I have a sudden increase in kinetic energy, this energy must have come from the work done due normal force on the system (me).

My question is since the work done is (force)$\times$(displacement of point of contact) Why does the normal force do work since the point of contact (At my foot) has no displacement during the jump.

This question is similar to my question, but I do not agree with the answer. The answer there uses an internal forces argument. I mentioned the system to be me, so internal forces (muscles) should not have any role in my increase in kinetic energy.


1 Answer 1


The normal force does not do any work because, as you said, it is not being applied over some distance. The energy/work comes from your muscles (from the chemical energy stored in you body), where the forces are applied over some distance. The normal force just gives something to push off from.

As a simpler model system, you can think of it like a mass attached to a vertical compressed spring that is then suddenly released off of the ground. Even though the spring is pushing on the ground, the spring force is what does work on the mass. The potential energy stored in the spring is partially converted into the kinetic energy of the mass.

I mentioned the system to be me, so internal forces (muscles) should not have any role in my increase in kinetic energy.

"Internal" and "external" are just labels we apply to forces depending on how we define the system. They do not impact any of the physics. A force can do work and change the kinetic energy of something regardless of the label we put on it. So, even if you say your muscles are part of the system, the muscles will still do work, and they are still responsible for giving you kinetic energy.

  • $\begingroup$ If you take the spring to also be in the system .Only the normal force and gravity act on this system .What causes the increase in KE this case? Also "spring force is what does work on the mass " This effect should not be considered since it is in internal force in the spring-mass system . $\endgroup$ May 21, 2021 at 6:15
  • $\begingroup$ @Glowingbluejuicebox Why do you think something internal to the system can't increase the kinetic energy of parts of the system? That's just a difference in labels; the physics doesn't change just because you labeled something as "internal" or "external". $\endgroup$ May 21, 2021 at 11:50

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