Work Done by Friction

[ShySpliceosome]

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I have a question from the EK 1001 Physics book:

Question #354: A block slides to a stop along a table top. Which of the following statements is true?

A. The work done by friction is equal to the force of friction times the distance that the block slid.

B. The work done on the block is equal to the force of friction times the distance that the block slid.

C. The work done on the table is equal to the force of friction times the distance that the block slid.

D. The work done on the block is equal to the sum of its changes in kinetic and internal energies.

To me, they all seem correct! D is the correct answer. EK's explanation for why D is correct confused me even more:

"When considering the energy transfer due to friction, we must take into account internal energy change. The frictional force times the distance is equal to the change in mechanical energy: fd = delta(KE) + delta(PE), but this does not take into account the internal energy change of the block. Work by friction does not equal fd."

That last sentence particularly confuses me. 😕 I don't understand how W is not equal to fd in this case. I would appreciate it if someone could explain to me why A, B, and C are wrong. Thanks in advance!

 

[DocMC]

I have a question from the EK 1001 Physics book:

Question #354: A block slides to a stop along a table top. Which of the following statements is true?

A. The work done by friction is equal to the force of friction times the distance that the block slid.

B. The work done on the block is equal to the force of friction times the distance that the block slid.

C. The work done on the table is equal to the force of friction times the distance that the block slid.

D. The work done on the block is equal to the sum of its changes in kinetic and internal energies.

To me, they all seem correct! D is the correct answer. EK's explanation for why D is correct confused me even more:

"When considering the energy transfer due to friction, we must take into account internal energy change. The frictional force times the distance is equal to the change in mechanical energy: fd = delta(KE) + delta(PE), but this does not take into account the internal energy change of the block. Work by friction does not equal fd."

That last sentence particularly confuses me. 😕 I don't understand how W is not equal to fd in this case. I would appreciate it if someone could explain to me why A, B, and C are wrong. Thanks in advance!

Hmm. B sounds true, because wouldn't there be negative work? But A definitely sounds correct, since Wf = Fn x d . D) Is also correct. C doesn't sound right either; I don't think the block is doing any work on the table, because in that case the table would have a displacement. I stay away from EK.

 

[eradicator]

By definition total W is defined as a change in total Energy