Physics : work and friction

[coyfish]

What work is being done on a car moving with a constant speed alone a straight, level road?

I none
II work done on the car
III work done by the car

The answer is II and III.

The reasoning in the book makes the statement: "some kind of work is being done by the car--this is why it moves." It makes the distinction that the net force is zero but there are forces acting. I don't follow. According to entropy a moving object can have no forces acting on it thus it continues to move. W=FD. Thus F = 0 and even though D is increasing W would = 0. . . Or W=deltaKE which would also be 0.

The next question is regarding friction. I decided to assume no friction here and missed it. I missed another theoretical type question where I did assume friction. Neither that or this passage said anything regarding friction. The other question was about a slide and this one is about a car. So when do we assume friction and when don't we . . .

I was doing verbal most of the day so I guess im stuck in "make absolutely no assumption mode."

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[PiBond]

What work is being done on a car moving with a constant speed alone a straight, level road?

I none
II work done on the car
III work done by the car

The answer is II and III.

The reasoning in the book makes the statement: "some kind of work is being done by the car--this is why it moves." It makes the distinction that the net force is zero but there are forces acting. I don't follow. According to entropy a moving object can have no forces acting on it thus it continues to move. W=FD. Thus F = 0 and even though D is increasing W would = 0. . . Or W=deltaKE which would also be 0.

The next question is regarding friction. I decided to assume no friction here and missed it. I missed another theoretical type question where I did assume friction. Neither that or this passage said anything regarding friction. The other question was about a slide and this one is about a car. So when do we assume friction and when don't we . . .

I was doing verbal most of the day so I guess im stuck in "make absolutely no assumption mode."

if the car is driving along an outside road, then there will be friction from the road (and wind resistance) opposing the motion of the car as it moves and this will slow the car down. therefore, the car's engine must supply a force in order to keep the car moving at a constant velocity. thus, the road is doing work on the car and the car is also doing work to keep it moving at the same speed.

in real world examples like this (ie the level road being traveled on by the car), we have to consider the realities experienced by an object. however, if there's an explicit experiment being done in a controlled environment, then we generally just use what we're given and ignore the realities unless we're told otherwise.

 

[coyfish]

I still don't understand the logic in the book stating: some kind of work is being done by the car--this is why it moves.

Not everything moving has work being done . . .

 

[TieuBachHo]

You said that the next question regarded to friction, so I presumed that there was a friction force acting on the car, right? It must either mention somewhere in the passage or in the schematic representation. Now if there was a friction force then it's a non-conservative force which decelerated the car; therefore, the work here was obviously not zero.

So it makes sense when they say that there must be some kind of force being done on the car which makes it move along its path. If not, then the car wouldn't be moving at all due to friction. Eventhough, the net force is zero (no acceleration -- constant speed) but work must be done on the car to offset its dissipated heat due to friction.

PS: Look up both conservative and non-conservative forces.

 

[coyfish]

You said that the next question regarded to friction, so I presumed that there was a friction force acting on the car, right? It must either mention somewhere in the passage or in the schematic representation. Now if there was a friction force then it's a non-conservative force which decelerated the car; therefore, the work here was obviously not zero.

So it makes sense when they say that there must be some kind of force being done on the car which makes it move along its path. If not, then the car wouldn't be moving at all due to friction. Eventhough, the net force is zero (no acceleration -- constant speed) but work must be done on the car to offset its dissipated heat due to friction.

PS: Look up both conservative and non-conservative forces.

Nope there was no friction mentioned anywhere in this passage or related questions. Pibond is right though. I looked at more friction assumption problems and in real life conditions such as this you assume friction unless told otherwise.

But that doesn't really change my question in the 2nd post. The explanation in the back of the BR book made that statement where its moving = thus must have work. I don't understand that.

In terms of friction / net force it makes sense but they didn't state it that way.

 

[eldoctor]

Nope there was no friction mentioned anywhere in this passage or related questions. Pibond is right though. I looked at more friction assumption problems and in real life conditions such as this you assume friction unless told otherwise.

But that doesn't really change my question in the 2nd post. The explanation in the back of the BR book made that statement where its moving = thus must have work. I don't understand that.

In terms of friction / net force it makes sense but they didn't state it that way.

i think you're getting hung up on their word choice, which is admittedly poor. I think by "moving" they may have meant "displacement", so that W = Fd cos() is not actually zero, since the displacement is some positive value and the Force is also positive to counteract the force of friction and air resistance. We can ignore the angle since this is a level road.

 

[Isoprop]

The NET or TOTAL work is zero. That doesn't mean zero work is being done by different forces.

The question is asking what work is being done. Not what is the net amount of work done. The car is doing work. Friction is doing work. And air resistance is doing work. The scalar sum of all the work equals zero.

You can almost always safely assume no friction when it comes to calculations (unless instructed otherwise by the question/passage). But when they ask you to apply physics concepts in the real world with simple or no computations, you should consider everything.

 

[coyfish]

The NET or TOTAL work is zero. That doesn't mean zero work is being done by different forces.

The question is asking what work is being done. Not what is the net amount of work done. The car is doing work. Friction is doing work. And air resistance is doing work. The scalar sum of all the work equals zero.

You can almost always safely assume no friction when it comes to calculations (unless instructed otherwise by the question/passage). But when they ask you to apply physics concepts in the real world with simple or no computations, you should consider everything.

Thanks for your help guys. I understand how this relates to the problem but what I was confused about is the wording that BR uses. It seems like it was just poorly worded and it seems that is the case.


"some kind of work is being done by the car--this is why it moves."

What I assume they mean is that + work must be done to overcome frictions - work . . . thus maintaining the net force = 0.

I previously thought that the statement indicated that movement = work which is not true in all cases.