Deflated tyres

  • #1

Homework Statement



Why does a car uses more fuel with lesser air in tyres?

Homework Equations



Work done =F.d

The Attempt at a Solution



According to me Friction does no work in rolling if the tyre is purely circular. But when tyes are deflated then they are not circular. Due to this friction does negative work on the wheels. Due to this kinetic energy of the car decreases and hence same velocity needs more fuel. Is that correct? Also is there anything else to be considered.
 

Answers and Replies

  • #2
Andrew Mason
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Homework Statement



Why does a car uses more fuel with lesser air in tyres?

Homework Equations



Work done =F.d

The Attempt at a Solution



According to me Friction does no work in rolling if the tyre is purely circular. But when tyes are deflated then they are not circular. Due to this friction does negative work on the wheels. Due to this kinetic energy of the car decreases and hence same velocity needs more fuel. Is that correct? Also is there anything else to be considered.
I don't follow your reasoning. Why would friction do negative work on the tires simply because the tires are not round?

Think of the tire wall. What happens to the tire during a rotation when it is properly inflated? Does the tire wall change shape? How about when it is under-inflated? Does it take energy to change the shape of the tire?

AM
 
  • #3
My point is: When considering a rolling circular disc, friction acts on the lowermost point. Let the displacement be dx and force be F. Then work done will be F.dx . After that the particle will not be in contact and hence friction force will not act. So work will be zero.

For a deflated tyre the particle will remain in contact for finite x and hence friction will do work on it.
 
  • #4
PhanthomJay
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My point is: When considering a rolling circular disc, friction acts on the lowermost point. Let the displacement be dx and force be F. Then work done will be F.dx . After that the particle will not be in contact and hence friction force will not act. So work will be zero.

For a deflated tyre the particle will remain in contact for finite x and hence friction will do work on it.
The resistance to motion of a deflated tyre that is not slipping with respect to the ground is neither due to static nor dynamic (sliding) friction, but rather, it is due to rolling friction, which is primarily caused by energy losses mostly in the form of heat when the tyre deforms and rebounds to its original shape. This is the energy loss referred to by Mr. Mason in the above post. This deformation is much greater than it would be for a properly inflated tyre, and hence, more energy losses.
 

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