Is the work done by friction on a moving car's wheel positive or negative?

In summary, friction does work in the sense that it drains energy from a sliding system, but this work is always frame dependent.
  • #1
songoku
2,287
322
Homework Statement
The work done of friction is always:
a. positive
b. negative
c. zero
d. any of these
Relevant Equations
W = F.d
The answer key is (b) but in my opinion it should be (d) because I think about work done by friction on wheel of car when the car moves. Let say the car moves to the right, then the wheel will rotate clockwise and the direction of friction acting on the wheel by the road will be to the right and since the displacement is also to the right the work done by friction on the wheel is positive.

Am I missing something? Thanks
 
Physics news on Phys.org
  • #2
I agree. And note that the work done by friction can also be zero, e.g. a car parked using its brakes on a hill.

I think the question is badly written and should include a phrase such as “when sliding occurs”. [Edited.]
 
  • Like
Likes Delta2, songoku, sophiecentaur and 1 other person
  • #3
Even when sliding occurs, the work done by friction on a particular object can be positive.

Take, for example, a child who is running rearward in the aisle on an airliner and who then slides to a stop next to the rear lavatory. The force of kinetic friction from the floor on the child's feet acts forward -- in the direction of the child's net motion, increasing the child's net velocity and ground-relative kinetic energy and doing positive work [in the ground frame]. Both energy and work are frame-relative numbers.

What is true is that if one counts both third law partner forces in the interaction, sliding friction always drains mechanical energy from the system. The net is negative work done. Or zero in the case of static friction. The net work done is an invariant quantity. It does not change depending on the chosen frame of reference.
 
  • Like
Likes songoku
  • #4
I think friction's positive work is called pseudo-work, I wonder why.
 
  • Like
Likes songoku
  • #5
It all boils down to the force being frame independent but the displacement being frame dependent. There is no such thing as ”work done by friction” without reference to a frame. You can always choose that frame such that the work done by friction is positive, negative, or zero.
 
  • Like
Likes songoku and jbriggs444
  • #8
Thank you very much for the explanation Steve4Physics, jbriggs444, Delta2, Orodriun, robphy
 
  • Like
  • Love
Likes Steve4Physics, Delta2 and Lnewqban

What is work done by friction?

Work done by friction is the energy expended when two surfaces rub against each other, resulting in the conversion of kinetic energy into thermal energy.

How is work done by friction calculated?

The work done by friction can be calculated by multiplying the coefficient of friction between the two surfaces by the normal force and the distance traveled.

What factors affect the amount of work done by friction?

The amount of work done by friction is affected by the type of surfaces in contact, the force applied, and the distance traveled.

What are the different types of friction?

The three main types of friction are static friction, which occurs when two surfaces are at rest, kinetic friction, which occurs when two surfaces are in motion, and rolling friction, which occurs when one surface is rolling over another surface.

How does work done by friction affect the efficiency of a machine?

Work done by friction can decrease the efficiency of a machine by converting some of the input energy into thermal energy, resulting in energy loss. This can be minimized by reducing the coefficient of friction or by using lubricants.

Similar threads

  • Introductory Physics Homework Help
Replies
5
Views
859
  • Introductory Physics Homework Help
Replies
18
Views
1K
  • Introductory Physics Homework Help
Replies
19
Views
1K
  • Introductory Physics Homework Help
Replies
13
Views
1K
  • Introductory Physics Homework Help
Replies
7
Views
1K
Replies
29
Views
1K
  • Introductory Physics Homework Help
Replies
7
Views
3K
  • Introductory Physics Homework Help
Replies
2
Views
200
  • Mechanics
Replies
10
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
918
Back
Top