My question is regarding work by a force and it is one which is giving me many issues in understanding when work done by force is 0.
My question can be described as a multiple choice question.
Q) No work is done by a force on an object if -
a) the force is always perpendicular to velocity
b) the force is always perpendicular to acceleration
c) the object is stationary but point of application of force moves on the object
d) the object moves in such a way that the point of application of force remains fixed
The answers are (a), (c) and (d)
Now obviously I know (a) is true. Can somebody please explain :
What the OPTIONS (c) and (d) mean. I'm unable to understand the options clearly. What does THIS POINT OF APPLICATION OF FORCE REFER TO? Is it the force or the point on which force is acting?
Relate (c) and (d) to a sphere pure rolling. I know in pure rolling velocity of point of contact is 0 and hence point of contact doesn't move and hence work done by friction is 0. Now how to analyse using (c) or (d)? Which option does sphere rolling satisfy? Is it (c) or (d)?
Relate (c) and (d) with a pulley of mass M and moment of Inertia I about axis of rotation assuming string doesn't slip on pulley. I'm unable to analyse this situation in any way. How is work done by both tensions on a ROTATING PULLEY with mass 0? (The string doesn't slip on pulley and pulley has mass M and moment of inertia I about axis).Which option does pulley rotating satisfy? Is it (c) or (d)?
Examples for (c) and (d). I want to first understand (c) and (d) and then relate it with many other examples
I'd really like a SIMPLE explanation highlighting the above issues in order to analyse WHEN WORK DONE BY A FORCE IS 0 without any problems.