Physics Work and Friction

[FutureDoc2]

---

Members don't see this ad.

Hey guys, I'm trying to differentiate between positive work and negative work when friction occurs. Can anyone help with this? Just slightly confuse.

 

[SaintJude]

Is this related to a specific MCAT scenario?

Anyway:

Frictional force is always opposing the relative motion of the body. When a body is dragged along a rough surface, the frictional force will be acting in the direction opposite to the displacement. The angle between the frictional force and the displacement of the body will be 180o. Thus, the work done by the frictional force will be negative.

Work done by a force is negative if the applied force has a component in a direction opposite to the displacement.

 

[FutureDoc2]

Is this related to a specific MCAT scenario?

Anyway:

Frictional force is always opposing the relative motion of the body. When a body is dragged along a rough surface, the frictional force will be acting in the direction opposite to the displacement. The angle between the frictional force and the displacement of the body will be 180o. Thus, the work done by the frictional force will be negative.

Work done by a force is negative if the applied force has a component in a direction opposite to the displacement.

Thanks for your reply! I understand exactly what you are saying, but sometimes I end up confused. For instance, example 3.2a TBR phys question is "The work done on a block by friction as it slides on a rough surface is: A-negative and directly proportional to uk." I understand this (consistent with everything you mentioned above), however I do not understand how 3.2b "The work done on a road surface, as a car skids across it, is: C. Positive and directly proportional to the skidding distance" In which direction is friction? Would friction be negative or positive? Just need some clarification. Thanks for all of your help!

 

[Erythropoietin]

in the 1st case, friction is doing the work on the block so it's negative.
in the 2nd case, the car's tires are doing work on the road surface, which is positive

 

[SaintJude]

in the 1st case, friction is doing the work on the block so it's negative.
in the 2nd case, the car's tires are doing work on the road surface, which is positive

Yup, I mean I agree. And in direct response to your question, friction will still be negative, because it is still (as always) in the opposite direction of displacement. In this case, the friction is opposing the work done by the car's tires.

 

[FutureDoc2]

Ooohh that makes perfect sense! Thanks guys!