Work on stationary object?

[orangeblossom]

EK Physics #352.
EK says the following statement is not always true:
"No work is done on an object if that object remains stationary." (They also say that this answer choice tempts a lot of students.)

I distinctly remember my physics book mentioning the classic example of someone pushing against a wall for a hour, and no work is done b/c there is no displacement..... Is this wrong?

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[jHustle]

Well, if you are pushing on a block for an hour, isn't the block returning an equal force, which cancel each other out (no net force)? You're stationary, though work was done on the box, the box also did work on you. Or am I thinking about this the wrong way?

 

[ouroboros39]

So I had to look back to my physics textbooks for this, and while there was no specific statement, I think that, for example, creating an electric field on capacitor requires doing work on the electrons. So the object stays stationary, but the particles are "worked upon". Once again, I'm not sure, but it seems like this definition then includes heating an object as work, because the particles in the object are being worked upon.

(Someone should probably check if this makes sense.)

And Jhustle, the definition of work is Force/distance, so if neither object object moves then no work is done.

 

[BerkReviewTeach]

At the level of the MCAT, it would seem you could simplify this question to "does a stationary object experience a change in its energy?" If it does not move, then its kinetic energy remains at 0. If it does not move, then its potential energy does no change, unless the world moves around it. In the example above where an electric field is introduced around a stationary charged particle, its potential energy changed, so some form of work must have been done.

 

[orangeblossom]

At the level of the MCAT, it would seem you could simplify this question to "does a stationary object experience a change in its energy?" If it does not move, then its kinetic energy remains at 0. If it does not move, then its potential energy does no change, unless the world moves around it. In the example above where an electric field is introduced around a stationary charged particle, its potential energy changed, so some form of work must have been done.

Is that because W=qE? (a work formula that does not have "d" involved?)
If so, thank you. I understand now. Wow, I learn new things everyday!

 

[Testosterone]

EK Physics #352.
EK says the following statement is not always true:
"No work is done on an object if that object remains stationary." (They also say that this answer choice tempts a lot of students.)

I distinctly remember my physics book mentioning the classic example of someone pushing against a wall for a hour, and no work is done b/c there is no displacement..... Is this wrong?

Work is defined as a change in energy due to a force and can be calculated as W=F*d for all forces (mechanical, gravitational etc) except for frictional force. This is because frictional force increases the internal energy of systems to which it is applied to. If you rub your hand against an object, the object may be stationary, but work in the form of frictional force is still done on the object.

 

[daFendi]

Work is defined as a change in energy due to a force and can be calculated as W=F*d for all forces (mechanical, gravitational etc) except for frictional force. This is because frictional force increases the internal energy of systems to which it is applied to. If you rub your hand against an object, the object may be stationary, but work in the form of frictional force is still done on the object.

So friction does work in the direction to which it is opposing your movement; so W=Fd still stands, no? its just that net force happens to be not in direction in which movement is occuring.

I still dont understand how you can do work on something without moving it.

 

[Testosterone]

So friction does work in the direction to which it is opposing your movement; so W=Fd still stands, no? its just that net force happens to be not in direction in which movement is occuring.

I still dont understand how you can do work on something without moving it.

Yes, when an external force causes an object to move across a frictional surface, friction does work in the opposite direction and you can calculate that with W=Fd. That is why not all energy from the work done by the external force is transferred to kinetic energy because some of that is transferred into internal energy or heat. The example you gave is when an object is in motion.

What counts as internal energy and mechanical energy depends on what you define as the system and the surrounding. Take a piston for example. If we called our object, or system, a piston then all the work can be done inside the object without moving the object itself. In the case of a piston, this would entail increasing or decreasing the volume of the chamber. However if we define the weight of the piston as the system and the chamber as the surroundings then things would be different. You have to define what is your system first.

 

[Mornhavon]

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