about power

Aug 2008
17
0
Singapore
Water flows over a dam at rate of 750kg/s,takes 1.8 s to fall vertically into the river below.accerleration of free fall is 10m/s ².

Qn:calculate (a)
(i)power of the water flowing over the dam in 10.0s
(ii)work done ont the weight of water when it hits the river
(iii)power of the falling water at the instant it hits the river

i've got ans for(i) which is 75000N (ii)121500j

but the ans for (iii) is 121500w according to the ans sheet. however, i cannot get the ans. is there somehting wrong with the ans sheet or is it me?
 

arbolis

PHF Hall of Fame
Apr 2008
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I didn't do any calculus. Generally a physics course starts by saying to take care of dimensional analysis, in other words we have to take care about units.
A speed will quite likely be in distance/time. (for example m/s).
For the (i) you got that a power is measured in N. But it seems it should be measured in watts (I make the supposition because while looking at the answer of the (iii), it is so.) Power is measure by energy/time. Could be Joule/second which is a watt. So check out your units.
 

topsquark

Forum Staff
Apr 2008
3,115
661
On the dance floor, baby!
Water flows over a dam at rate of 750kg/s,takes 1.8 s to fall vertically into the river below.accerleration of free fall is 10m/s ².

Qn:calculate (a)
(i)power of the water flowing over the dam in 10.0s
(ii)work done ont the weight of water when it hits the river
(iii)power of the falling water at the instant it hits the river

i've got ans for(i) which is 75000N (ii)121500j

but the ans for (iii) is 121500w according to the ans sheet. however, i cannot get the ans. is there somehting wrong with the ans sheet or is it me?
Hints:
i) Let's consider a time span of 1 s. How much water goes over the fall in one second? How much force (due to its weight) is on that amount of water? How much work is done by gravity in moving that water down the fall? And finally, we now know how much work is done in 1 s. What is the power output of the falls in 10 s?

I'm not quite sure about ii). How much water is involved? Unless I'm not seeing something the amount would seem to matter.

And I'm confused on the wording on iii). The power output of the falls is constant as long as the rate of water going over the falls is constant. So why specify the "instant" it hits the water? Anyway, you've already got that in your solution to i). That's the power output of the falls due to the work done on the water in 1 s.

-Dan
 

topsquark

Forum Staff
Apr 2008
3,115
661
On the dance floor, baby!
A note about the units here.

The answer to i) is a power, so as arbolis mentioned, the unit that helloying gave is incorrect. However arbolis isn't quite right either. The unit for power is Watts, not watts. Also the unit of energy/work is Joules, not joules.

Why do we care? These units are named for people. So as in English they must be capitolized. Thus
N = Newton
J = Joules
W = Watt
are the major three you see in Introductory Physics.

-Dan
 
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