# How can we jump when the ground does no work on us? [duplicate]

This may seem like a silly question, but how can we jump when the ground does no work on us?

When we jump off the ground, the ground does not move. This means the force from the ground on us does zero total work. Since the force does no work, we cannot gain any kinetic energy so we can't jump.

• The ground doesn't do any work on you, so your total energy stays the same. But you use your muscles to convert internal energy (from the food you ate) into kinetic energy. Dec 1, 2022 at 3:04
• Oh right, chemical potential energy into kinetic energy. Thank you! Dec 1, 2022 at 3:06
• @knzhou Actually I guess the ground not doing work on us is an approximation, because from newton's third law, if the ground push's us up then we push the ground down by a very infinitesimal amount due to the inertia of earth. This means that actually the ground dose do a very small amount of work on us increasing our total energy. Dec 1, 2022 at 3:09
• It is irrelevant if the ground does or not a little work. You don't need the work of an external force to increase the KE of a system. The work of the internal forces can change the KE. What the internal forces cannot do is to change the momentum of the center of mass. For this, you do need the external force exerted by the ground. But the change in momentum is given by the impulse of the force and this does not need a displacement. So the normal force provides the momentum of the COM. The internal forces take care of the KE (or most of it).
– nasu
Dec 1, 2022 at 4:13