A student enters an elevator holding a heavy box, stands there and takes the elevator up...
A student enters an elevator holding a heavy box, stands there and takes the elevator up for 3 minutes. After he gets out, he claims “I did a lot of work while I stood and held the box”. Is this true?
A box slides along surface with a coefficient of kinetic friction of 0.3. Is it true that friction is doing positive work?
Homework Answers
Yes , the student did a positive work on the box .The student applied a holding force on the box and this is the force that caused the box to go upwards with the elevator.And,since, Work done = (Force) . (Displacement ),So the student is true.
No, Friction is doing negative work . As friction tends to oppose direction of motion ,so it is acting in opposite direction of the box sliding. So,in this case force and displacement are in opposite direction to each other. As work done W =Fscos(theta) .and here, theta= 180degree ,So cos (theta) = -1.So work done by friction is negative.
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