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MCAT Work and Friction Problem

  • Thread starter Taminem
  • Start date
  • #1
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Homework Statement



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.

Homework Equations


W = Ffr*d


The Attempt at a Solution


The answer given by the book is D but I do not understand why A is the incorrect answer. I thought if friction is doing work against the direction of motion of the block it would just equal exactly as what is stated - force of friction direction parallel to motion times the distance? As for D, wouldn't it be equivalent to A since whatever changes in potential and kinetic energy would entail and equivalent change in friction?

I'm really new to physics, so I'm missing very key concepts that I am not able to piece together. So thank you for your help and patience.
-Taminem
 

Answers and Replies

  • #2
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do you have any image of how the block was sliding ?

work done by friction depend on distance no displacement --- maybe thats why A is wrong coz problem may be referring to displacement
 
  • #3
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Hi cupid.callin,
Thanks for the reply. Turns out there is no image of how the block was sliding. However, your comment made me reread the equation for work and realize why D is the answer and not A. Since, d in the work equation is displacement and not distance, like the phenomenon of rubbing two hands together, no displacement is done since the positions of the hands in the initial state and final state are the same, but work is still done in the form of increasing the internal energy/temperature of the hands at the expense of kinetic mechanical energy. Thus, work done by frictional forces is not just Ffr * d but the change in Eint and mechanical energy.

Thanks for your reply! It was really useful.
 

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