What is Potential Energy of Mass m?

[senseandsanity]

The problem is:
A block of mass m slides at a speed v along a horizontal, smooth table. It next slides down a smooth ramp, descending a height h , and then slides along a horizontal rough floor, stopping eventually.
If the zero level is a distance (2h/3) above the floor, what is the potential energy U of the block on the floor?

I'm confused about the zero level.

 

[Doc Al]

I'm confused about the zero level.

To specify the potential energy, you need a reference point--a place where U = zero. For example, if the ground were the zero point, then the gravitational PE of an object at a height h would be U = mgh; but if 2h/3 were the reference point, U = mgh/3.

 

[Ytsehoos]

Old thread but I happen to have the exact same homework question. I hate MasteringPhysics.com.

Anyway, I understand U = mgh
But I really don't get what 2h/3 is trying to tell me.

"If the zero level is a distance above the floor, what is the potential energy of the block on the floor?" & "Suppose the potential energy of the block at the table is given by . This implies that the chosen zero level of potential energy is __________."

I have no idea what this is asking me.

 

[Doc Al]

Anyway, I understand U = mgh
But I really don't get what 2h/3 is trying to tell me.

With U = mgh, where do you measure h from?

You measure it from the zero level, which is completely arbitrary. For example, if a mass is on the floor, and I choose the floor as the zero level, then h = 0 and U = 0. But what if the zero level were chosen to be 10 meters below the floor (in the basement, perhaps)? Then h = 10m.

Another way to write the potential energy formula is: U = mgy, where y is the vertical position measured from the zero level, which is where y = 0.

The point is: You can put the zero level anywhere.

 

[jcarreon0807]

I too am confused on this which is how I actually came across this forum.

Anyway, if the U=mgh/3 on the table and zero point is 2h/3 then wouldn't that mean that the zero point is above the table since 2/3 is bigger than 1/3. This lead me to believe that the floor would in turn be 3h/3 (or just h) but when I put the answer of mgh in it was wrong. Any insight would be appreciated.

 

[Doc Al]

Anyway, if the U=mgh/3 on the table and zero point is 2h/3 then wouldn't that mean that the zero point is above the table since 2/3 is bigger than 1/3.

No. That would be a contradiction, since you just said that the zero point is at 2h/3.

The point is to choose the zero point (in this case it's given to you), and measure the displacement from that point. If the table top is h/3 above the zero point, where is the floor measured from that same zero point?

 

[jcarreon0807]

Ok now I get what it is saying. Thank you for clearing it up!