Did I Do Any Work Now?

Classical Mechanics Level 2

A heavy box was kept on a rough floor, initially at rest. I started moving it in a circular path, and then brought it to rest when it completed one revolution.

Fill in the blank with the most appropriate choice:

The work performed ________.

Must be positive May be positive Must be zero May be zero

1 solution

Aditya Kumar
Apr 28, 2016

It is given that the floor is rough. Hence, work is done against friction. The work done is given out as heat energy. Hence, work done is $\boxed{\text{positive}}$ .

Moderator note:

How can we show from first principles that the work done is positive?

@Calvin Lin here the system is the man and work is done by friction on the man. Also heat is released by the man hence work done is positive.

Aditya Kumar - 5 years, 1 month ago

Simple explanation ! Thought the same !+1

Rishabh Tiwari - 5 years ago

Can someone explain this mathematically?

Kazem Sepehrinia - 3 years, 12 months ago

This and previous problem should be thrown out. The only difference is smooth vs rough. Apparently someone believes "smooth" floor means zero friction. Is smooth now a technical term ? I don't think so. We call plenty of things smooth which of course still have some friction when pushed across.

Tony Miller - 2 years, 5 months ago

The question was unclear : the total work performed on the box - including friction - must be zero. The work performed BY THE MAN was positive, but the man was not in the question.

Arthur Jouniaux - 1 year, 9 months ago

I think the answer should be "May be positive", because you didn't define exactly what you meant by "rough floor", unless you explicitly state that there is friction.

Milly Choochoo - 5 years, 1 month ago

The word rough indicates the presence of friction.

Aditya Kumar - 5 years, 1 month ago

Did I Do Any Work Now?

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