# Zero level help

1. Dec 10, 2004

### 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.

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2. Dec 10, 2004

### Staff: Mentor

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.

3. Feb 10, 2012

### 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.

4. Feb 11, 2012

### Staff: Mentor

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.

5. Mar 4, 2012

### 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 wouldnt 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.

6. Mar 4, 2012

### Staff: Mentor

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?

7. Mar 4, 2012

### jcarreon0807

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