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Can Friction do Positive WorK?

Xylem101Xylem101 Posts: 868Registered User Member
edited November 2006 in AP Tests Preparation
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?

  • ktotoktoto Posts: 743Registered User 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.
  • BoridiBoridi Posts: 172Registered User Junior Member
    Work is a scalar and doesn't have a direction... so all work should be positive, really.
  • Xylem101Xylem101 Posts: 868Registered User 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.
  • warblersrulewarblersrule Posts: 9,249Super Moderator Senior Member
    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.
  • idamayeridamayer Posts: 1,313Registered User 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
  • TopekaTopeka Posts: 80Registered User 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.
  • idamayeridamayer Posts: 1,313Registered User 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...
  • TopekaTopeka Posts: 80Registered User 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.
  • idamayeridamayer Posts: 1,313Registered User Senior Member
    Oh, you're right. I'm stupid. sorry.
  • mmkaymmkay Posts: 268Registered User Junior Member
  • MechRocketMechRocket Posts: 1,975- Senior Member
    the first few replies that i read are stupid. the answer is no.
  • skatearabiaskatearabia Posts: 124Registered User 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.
  • stupakstupak Posts: 1,084Registered User 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.
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