friction and rolling resistance, and work done queries

it's simply torque = moment of inertia times angular acceleration …

the torque has to be in the same direction as the angular acceleration

if you push the cart, then looking from the right-hand side, the wheels will be accelerating clockwise,

and the only external forces on the wheel are the weight (irrelevant), some friction from the axle (negligible), and the friction from the road

so the friction from the road has to be clockwise, which means it's backwards

hi sgstudent!


the engine exerts a torque on the back axle, which rotates the back wheels

the static friction from the road makes the car move forward

opposing this are the friction in the bearings, and the rolling resistance, which isn't friction but is basically the energy lost in the continual deforming of the tyre


the free body diagram of the back wheel has a torque at the axle, and a forward friction force from the road, combining to give a forward acceleration

(we don't normally put the rolling resistance or the friction in the bearings onto the free body diagram … the assumption is that they are already subtracted from the engine torque to give a net torque, which does go on the diagram)


the work done against gravity is the same,

thw work done against friction is different


the work energy equation always works

I don't quite understand the net work done=final KE-initial KE.

i don't think i said that

i said that in this case the work done against gravity is the same …

so in the general case the (difference in) gravitational energy would have to be included

Oh, so will I have to include GPE when using the work energy theorem? Doc Al was trying to explain why not to here: http://www.physicsforums.com/showthread.php?t=92470

If you include potential energy along with the kinetic energy, then you must exclude from the net force, the force that is associated with the potential energy. In your situation, if you take "net force" to mean the sum of all forces except gravity, then the work done by that net force equals the change in the sum of kinetic and (gravitational) potential energy.

The purpose of gravitational potential energy is to account for the effect of gravity, so if you consider the object's weight as an external force then you don't also include potential energy--you'd be counting it twice!

… also when I use the work energy theorem, must the forces be constant force, or can it be a situation like I kick a ball so it starts from rest to rest so no net work done?

Oh so if an object moves up an incline with constant speed, is the network done 0 or is it the gain in GPE? Thanks for the help!

exam questions are usually about the work done by a particular force

they usually aren't interested in the net work done (since that will usually be zero, except in the case of friction, when net work done will equal loss of mechanical energy, ie gain in thermal energy)


an exam question wouldn't leave it ambiguous like that, it would specify whether the work done by gravity was to be included (usually by asking what is the work done by gravity, or what is the work done by the applied force)

hi tiny tim, i was thinking about the problem with the wheels again so for the non-driving wheels shouldn't the friction be forwards also? Since by torque they are both having the same direction ( as in turning direction)

also, when principle of moments occurs where there is no net moment where clockwise moment=anticlockwise moment can there be rotation? Like even constant speed rotation or will there be always no net moment?

Hi tiny Tim! thanks for the help all this while so for the driving wheel is positive work being done? Doc Al said no here: http://www.physicsforums.com/showthread.php?t=92895 but in your previous post it was otherwise..

Also I came across a weird moments question. They asked me to find the moment about a point to lift a trap door. I'm not sure if I can use principles of moment here cos lifting it requires a net moment but then again constant speed moment can also occur. So I'm sure how does this work..