Why is the Work Done by Friction on a Ramp Uncertain?

[Lori]

The question that puzzled me during lecture! :

A block is pushed so that it moves distance L up a ramp (incline angle q ) at constant speed. If there is friction, the magnitude of the work done on the block by friction…(ramp is up and leftwards)
A. is mgsinqL.
B. is less than mg sinqL.
C. is greater than mg sinq L.
D. could be greater or less than mg sinqL.
E. is zero

(q is angle of ramp and L is displacent by hand)

So I thought that the answer would be A. If we are talking about the friction work's magnitude then the value is positive work. Since mgsinqL is negative work, wouldn't it be greater than? Why is the answer D?Thank you. Stressing out about physics late at night >.>..

 

[jbriggs444]

First, realize that you are not asked about how much work is done by whatever force is pushing the block up the slope. Nor are you asked about how much work is done by gravity. You are asked about friction alone.

What if the coefficient of friction is very low and the ramp is very steep?
What if the coefficient of friction is very high and the ramp is at a shallow angle?

 

[ChocolateScrub]

mg sinqL is work done by gravity, no? mg is the force, sinqL is the displacement. So, mgsinqL has nothing to do with the work done by friction. The WD by friction is calculated by F x L, which may be greater than or less than mgsinqL.
Also, the friction and the weight are not acting in opposite directions, so we can't say that one is positive and the other is negative when dealing with calculations involving the 2 forces.

 

[Lori]

First, realize that you are not asked about how much work is done by whatever force is pushing the block up the slope. Nor are you asked about how much work is done by gravity. You are asked about friction alone.

What if the coefficient of friction is very low and the ramp is very steep?
What if the coefficient of friction is very high and the ramp is at a shallow angle?

I always thought that friction force would balance out the applied force because the object is at constant speed.

 

[Lori]

I attached a image below of the ramp

 

[jbriggs444]

I always thought that friction force would balance out the applied force because the object is at constant speed.

Yes, the friction force will match the sum of all the other applied forces. What are all the other applied forces? Is gravity the only one?

 

[Lori]

Yes, the friction force will match the sum of all the other applied forces. What are all the other applied forces? Is gravity the only one?

The normal force would balance out the gravity force right? The x component of gravity force will balance friction force?

 

[I like Serena]

The normal force would balance out the gravity force right? The x component of gravity force will balance friction force?

Normally yes, but if that were the case here, the block would either be in equilibrium, or slide down the incline, wouldn't it?
Isn't there someone pushing the block up with a finger?

 

[Lori]

Normally yes, but if that were the case here, the block would either be in equilibrium, or slide down the incline, wouldn't it?
Isn't there someone pushing the block up with a finger?

I think I understand now. Friction and the x component are both forces that affect work. We just don't know if friction is greater or less than force of gravity in its x component.

 

[I like Serena]

I think I understand now. Friction and the x component are both forces that affect work. We just don't know if friction is greater or less than force of gravity in its x component. View attachment 213985

Indeed!