Physics Circular Motion Question?

S

Shnitzel

New member
I can give you 2 very simple explanations :)

First of all, the formula of work is actually: Work = Force * Distance * cos(angle)
when the angle, is the angle between the force vector and the displacement vector.
When an object is in a circular motion, the force is adjacent to its speed at any given moment.
cos(90) = 0.

Another thing about work, is that the work of a force is equal to the change in energy of an object:
Work = delta Energy
The energy of the rock is kinetic energy => mv^2/2
since we know the mass of the rock doesn't change, and the speed of the rock also doesn't change (otherwise the rock would quit this circular motion) - it's obvious that the change in it's kinetic energy is 0
(Change in Energy = Energy at the end - Energy at the beginning //<- Both are the same value).
Therefore, Work = 0.

I apologize for any English mistakes :)
 
Hi,

I was doing a question and I got the wrong answer. Here is the question:

A stone of mass m is attached to a string and moves round in a horizontal circle of radius R at constant speed V. The work done by the pull of the string on the stone in one complete revolution is..

4 answers were given by 2 were completely illogical. The two that were left were "zero" and "2(pi)m(v^2)". Using the formula for work done (force x distance) I logically came at the latter, however I found out that the correct answer is indeed zero. I am completely lost as to how zero is the answer. I was wondering if someone could clarify this.

Thanks a lot guys!
 
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