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# Can Friction do Positive WorK?

Registered User Posts: 868 Member
edited November 2006
I saw a question that asked to give an example of friction doing +ve Work, how is this possible and Give me an example if it is...
Post edited by Xylem101 on

## Replies to: Can Friction do Positive WorK?

• Registered User Posts: 743 Member
If I remember correcly, work is relative. It can change signs depending on the reference points, so I guess friction can do positive work.
• Registered User Posts: 172 Junior Member
Work is a scalar and doesn't have a direction... so all work should be positive, really.
• Registered User Posts: 868 Member
Yes work IS a scalar But it DOES have a sign, The fact dthat it doesnt have direction doesnt mena that it doesnt have a sign lol.
• Super Moderator Posts: 9,406 Super Moderator
Think about a small block resting on a larger block. If I move the large block forward, the static friction causes the small block to move in the same direction as the displacement, so the work is positive. I can't think of an example of kinetic friction doing positive work, though.
• Registered User Posts: 1,313 Senior Member
warblersrule, i think that is more inertia than friction... work of friction < = (coefficient of friction) * force of the normal... when there is no force acting on the smaller block, friction does no work...

xylem: force of friction is in the opposite direction of the motion... so if you move a block to the right (+) friction does - work... if you move the block to the left (-) the friction does + work
• Registered User Posts: 80 Junior Member
Friction does negative work. Work=friction*r*costheta
when you sum the forces, friction is a negative force because postitive forces are in direction of motion. theta is the angle between friction and r, your displacement. Since friction is in the direction opposite displacement, theta= 180 and work is thus negative. Therefore, friction always does negative work.
• Registered User Posts: 1,313 Senior Member
topeka... but if the force on the object that makes it move is in the negative direction (i.e. left), the force of friction is in the positive direction, and therefore does positive work...
• Registered User Posts: 80 Junior Member
I see what you're saying, but the work is still negative even though the friction is positive. Work=r*friction. Friction and r are always in opposite directions. So if r is in the negative direction, then friction is in the positive direction. So work is still neagive because a negative *postive is still negative.

Another way of looking at it is with the equation Work=magnitude of friction*magnitude of r*cos180. In this equation, you only look at the MAGNITUDES of friction and r, which are always positive. In this case, cos 180 gives you the negative sign and Work done by friction is still negative.

The cos 180 tells you that friction is always opposite the direction of displacement.
• Registered User Posts: 1,313 Senior Member
Oh, you're right. I'm stupid. sorry.
• Registered User Posts: 268 Junior Member
mmmmmhhhhmmmmmm
• - Posts: 1,975 Senior Member
the first few replies that i read are stupid. the answer is no.
• Registered User Posts: 124 Junior Member
What about a moving car? The only force acting on the wheels is the static frictional force between them and the road...it's acting in the same direction as the displacement, and thus is doing positive work.
• Registered User Posts: 1,084 Senior Member
no static force is working against displacement- that's why it's stuck there for a bit until you put enough force; if it went with displacement, the car would go super accelerated.
This discussion has been closed.