Work and Forces -- Not understanding this basic friction on a ramp type of problem

In summary, the question is asking about the work done by friction alone on the block as it is pushed up an incline at constant speed. The magnitude of this work can be greater or less than the work done by gravity, depending on the angle of the incline and the coefficient of friction. We cannot determine the exact magnitude without more information.
  • #1
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 >.>..
 
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  • #2
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?
 
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  • #3
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.
 
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  • #4
jbriggs444 said:
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.
 
  • #5
I attached a image below of the ramp
Screenshot_20171029-111435.jpg
 

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  • #6
Lori said:
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?
 
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  • #7
jbriggs444 said:
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?
 
  • #8
Lori said:
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?
 
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  • #9
I like Serena said:
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.
20171029_142330.jpg
 

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  • #10
Lori said:
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!
 

Related to Work and Forces -- Not understanding this basic friction on a ramp type of problem

1. What is friction and how does it affect objects on a ramp?

Friction is the force that resists the motion of an object when it comes into contact with another surface. It can either slow down or stop the motion of an object. On a ramp, friction is important because it can either help or hinder the movement of an object depending on its direction and the angle of the ramp.

2. How do you calculate the amount of friction present on a ramp?

To calculate the amount of friction on a ramp, you need to know the coefficient of friction between the two surfaces in contact and the normal force acting on the object. The coefficient of friction is a measure of how rough or smooth the surfaces are and the normal force is the force perpendicular to the surface that the object is resting on. Multiply these two values together to find the amount of friction present.

3. Why does increasing the angle of the ramp increase the amount of friction?

As the angle of the ramp increases, the normal force acting on the object also increases. This means that there is more force pressing the two surfaces together, resulting in a stronger force of friction. This is why it can be more difficult to move objects on steeper ramps.

4. How does the weight of an object affect its movement on a ramp?

The weight of an object, or the force of gravity acting on it, can greatly affect its movement on a ramp. Heavier objects will have a greater normal force and therefore more friction, making it harder to move them up the ramp. On the other hand, lighter objects may not have enough weight to overcome the force of friction, causing them to slide down the ramp.

5. Can you reduce the amount of friction on a ramp?

Yes, there are a few ways to reduce the amount of friction on a ramp. One way is to make the surfaces smoother by polishing or lubricating them. This reduces the roughness and allows the surfaces to slide against each other more easily. Another way is to decrease the angle of the ramp, which decreases the normal force and therefore reduces the force of friction. Additionally, using objects with lower coefficients of friction, such as plastic or Teflon, can also help reduce friction on a ramp.

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