Work Done by Gravity

[member232]

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The question:
A person pushes a block up a ramp, which makes a 30 degree angle to the ground. Assume the block starts and ends at rest. Neglect friction, let d be the length of the ramp and h be the vertical displacement of the block. What is the work done by gravity on the block, after the block is moved up the incline?

A. mgh
B. -mgdsin(30)
C. mgd
D. Gravity does no work on the block.

The answer is B, which I would understand if the block were moving up the incline. But the question is asking for the work done by gravity after the block is moved up the incline, which is when the block is at rest. Isn't the work done by gravity at this point zero? If the object is at rest, there is no displacement, and thus gravity is not doing any work.

Someone please clarify? Isn't the work done by gravity zero if the object is at rest?

 

[MedPR]

The question:
A person pushes a block up a ramp, which makes a 30 degree angle to the ground. Assume the block starts and ends at rest. Neglect friction, let d be the length of the ramp and h be the vertical displacement of the block. What is the work done by gravity on the block, after the block is moved up the incline?

A. mgh
B. -mgdsin(30)
C. mgd
D. Gravity does no work on the block.

The answer is B, which I would understand if the block were moving up the incline. But the question is asking for the work done by gravity after the block is moved up the incline, which is when the block is at rest. Isn't the work done by gravity at this point zero? If the object is at rest, there is no displacement, and thus gravity is not doing any work.

Someone please clarify? Isn't the work done by gravity zero if the object is at rest?

I think this is one of those questions where you have to know (guess) that they are asking about the block during it's movement up the incline.

 

[member232]

Allright that makes more sense, but for peace of mind, am I correct to think that if an object is just stationary, the work done by gravity is zero?

 

[milski]

The total work on the block is zero. The work done by gravity is as they correctly state -mgh or -mgd.sin(30). It's a single conservative force, the standard force*displacement is what you need.

And not, you are not correct to think that stationary object is the same as zero work done by gravity.

no change in speed of the object -> no change in KE -> no total work done on it

To say that the work done by gravity on the object is 0, you need to have 0 displacement in the direction in which the gravity force is pointed. For small objects close to the surface of Earth, that would be vertical displacement.

 

[member232]

I'm sorry, milski, you just confused me a bit... If an object is stationary and resting on a surface, then is the work done by gravity at that moment zero?

There is no vertical displacement if the object is resting on a flat surface and no external forces are applied...

Thanks in advance

 

[milski]

I'm sorry, milski, you just confused me a bit... If an object is stationary and resting on a surface, then is the work done by gravity at that moment zero?

There is no vertical displacement if the object is resting on a flat surface and no external forces are applied...

Thanks in advance

At that moment no, but rarely do we talk about work done at a specific moment. The question is asking you about the work done from the moment that the person started pushing to after the moment the block was pushed all the way to the top.

What you're thinking of is the answer of "what work is being done after it has moved up." Yes, I know that sounds like the grammar police, but in general you're not going to be asked for work in a specific moment - the term does not really make sense unless you define the length of that moment.

 

[member232]

Thanks so much 🙂