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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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I guess you can mesuare it in db scale, and assuming that most of the energy goes to phonons and hence it will be sound waves that can be measured.

The sound intensity can be easily translated into terms of the energy you invested to make the knock.

check this link out

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