# Energy behind door knocking?

How do we estimate the energy released from knocking on a wooden table or a door?

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It depends on the knocking technique. Do you knock with a finger, or do you knock with a full hand? I would estimate the released energy with the kinetic energy of the finger/hand. If you approximate a hand with a sphere of 8 cm diameter and density of water, and the velocity with 0.1 m/s. You get $$E \approx m v^2= 0.25 kg * (0.1 m/s)^2 = 3 mJ$$ which is an absolute upper limit

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