# Homework Help: Work Energy Motion

1. Nov 29, 2005

### teken894

Here's a simple question that I get confused in explaining..

Can work be done on something if there is no motion?

This was a test question...and of course I wasn't thinking clearly..and I said that work can be done since motion is relative to something, work can be done... I thought, for example, that a box sitting on earth has no motion, but the sun is "doing" work on the earth, and so does work on the box too...
However, I forgot that--the box--relative to the sun has motion...So my thought was wrong...

So..my real question now is...is work relative, since it depends on the relative motion(for displacement)? If work is relative, and work is the transfer of energy from one object to another...then is energy a relative variable too?

My other question is: can an object have no motion...because I thought motion was similar to matter..in that it can't be destroyed...only transferred..

I need help in connecting work, energy, and motion in a effective and clear way...because right now I'm confusing the concepts..any help is appreciated...

2. Nov 29, 2005

### Staff: Mentor

Yes, work and energy are frame-dependent quantities. The work done by a force and the kinetic energy of an object both depend on the relative velocity of the frame in which the measurements are made.

An object can certainly have zero speed with respect to some frame, if that's what you are asking. (I don't know what you mean by "motion" being similar to matter.) The speed of an object is not an intrinsic property of the object.

3. Nov 29, 2005

### stuplato

The equation for work would help explain...

4. Nov 29, 2005

### mezarashi

Strangely enough, work and energy are relative concepts. Take for example a car speeding by you. To you standing on the road, the car has kinetic energy 1/2mv^2. But to the guy sitting on the car, the situation is different. Work is similar. Try the case of a you and a box sitting on an elevator.

Also a note on the sun. The sun is not actually doing work on the Earth in fact (not due to gravity atleast). As work is defined as the dot product of the force vector and displacement vector. In circular motion, the instantaneous displacement or velocity vector is always perpendicular to the centripetal force.

Motion or velocity is not conserved. Classically, mass, energy, momentum, and charge are conserved. Motion can be created and 'destroyed'.

5. Nov 29, 2005

### Staff: Mentor

teken894: I merged the two threads that you created.

6. Nov 29, 2005

### teken894

Yes...centripetal force does not do work...but the sun is doing some work since the earth orbit is not perfectly uniform...

Here's what I understand so far...energy, work, and motion are relative to some frame of reference...

..I would like to know if motion is a form of energy (I would guess its kinetic energy), and if it is, then it too is conserved? This why I said motion is conserved..in some form of energy.

Sorry about the double post...i posted this in the wrong forum first...

Last edited: Nov 29, 2005
7. Nov 29, 2005

### Staff: Mentor

Right. Centripetal force does no work only if there is no centripetal displacement. Of course, since the earth's orbit is not quite circular, but elliptical, gravity does positive work on the earth when the earth approaches and negative work when the earth recedes.

8. Nov 29, 2005

### teken894

Can there be no motion?

Is there any way that there can be "no motion"?
and is there such a thing as absolute motion? Then I would think that all of physics is actually relative to something..and there is not "absolute" measurement of anything.

9. Nov 30, 2005

### Staff: Mentor

There can be "no motion" (meaning: speed = zero) with respect to some frame of reference. In modern physics there's no such thing as absolute motion.
Some quantities (such as kinetic energy and work) depend on the frame in which they are measured; some do not.