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Old Oct 25th 2009, 02:07 PM   #1
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Water mass, density and gravitational energy question

I have a question that gives an example of a waterfall that is 48m high, 780m wide. It says the water is moving at 9m/s and has depth of 0.80m.

The first question is asking for the mass of the water that goes over the fall every second if the density is 1000 kg/m^3.

I'm seriously stumped because I reviewed my textbook chapter and nothing helped. But I think you're supposed to find the volume of the waterfall? Or at least how much water is displaces? Yeah, I really don't know, I'm trying to work on it right now.

The second questions ask for the change in gravitational energy of this mass of water. What does that mean?
9.8 m/s^2 to 9.8N kg/m? I definitely need some more explanations for this questions please.
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Old Oct 25th 2009, 03:40 PM   #2
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Originally Posted by omgwtfitsp View Post
I have a question that gives an example of a waterfall that is 48m high, 780m wide. It says the water is moving at 9m/s and has depth of 0.80m.

The first question is asking for the mass of the water that goes over the fall every second if the density is 1000 kg/m^3.

I'm seriously stumped because I reviewed my textbook chapter and nothing helped. But I think you're supposed to find the volume of the waterfall? Or at least how much water is displaces? Yeah, I really don't know, I'm trying to work on it right now.

The second questions ask for the change in gravitational energy of this mass of water. What does that mean?
9.8 m/s^2 to 9.8N kg/m? I definitely need some more explanations for this questions please.
Hi and welcome to the forum!
For the first question, I suggest you to find out how many m^3 of water flow by second. Then multiply by the density of water and you have the mass of water flowing by second as required.
How would you work out the m^3 of water flowing by second?

For the second question, I'd set the 0J of potential energy at the bottom of the waterfall. Let me ask you a question : what is the potential energy of a 1 kg mass situated 78 meters over the ground? (I'm asking the potential energy of the mass with respect to the ground level)
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Old Oct 25th 2009, 04:23 PM   #3
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Okay, so for the first question, would this be correct:

Volume of the waterfall: LxWxH = (9m)(780m)(0.8m)
V = 5616 m^3

D = m / v

1000 kg/m^3 = m / 5616m^3

m = 5,616,000 kg or 5.616 x 10^6 kg

So that would be the mass of water going over the fall every second right?

Lemme work on the second part now. Please tell me if I did the first question right.
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Old Oct 25th 2009, 05:15 PM   #4
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Originally Posted by omgwtfitsp View Post
Okay, so for the first question, would this be correct:

Volume of the waterfall: LxWxH = (9m)(780m)(0.8m)
V = 5616 m^3

D = m / v

1000 kg/m^3 = m / 5616m^3

m = 5,616,000 kg or 5.616 x 10^6 kg

So that would be the mass of water going over the fall every second right?

Lemme work on the second part now. Please tell me if I did the first question right.
Looks good to me.
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