Alexia354
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Hi all new here, and wondering if you can help with one physics question that has been bothering me for hours, any help is appreciated!

“A water pump draws still water from a lake and pumps it through a pipe to a height of 4.5 m. The water emerges from the pipe at a speed of 2.4 m/s. The mass of water pumped per minute is 100 kg.
Calculate the useful output power of the water pump.”

Edit to add: the answer is 78W.
Last edited by Alexia354; 1 month ago
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Stonebridge
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(Original post by Alexia354)
Hi all new here, and wondering if you can help with one physics question that has been bothering me for hours, any help is appreciated!

“A water pump draws still water from a lake and pumps it through a pipe to a height of 4.5 m. The water emerges from the pipe at a speed of 2.4 m/s. The mass of water pumped per minute is 100 kg.
Calculate the useful output power of the water pump.”

Edit to add: the answer is 78W.
The answer to the question assumes that the 'useful' power is made up of
a) the potential energy per second given to the water and
b) the kinetic energy per second given to the water

So you have formulas for potential energy in terms of height and kinetic energy in terms of velocity - use the mass of water per second in those formulas to find the power.
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Alexia354
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(Original post by Stonebridge)
The answer to the question assumes that the 'useful' power is made up of
a) the potential energy per second given to the water and
b) the kinetic energy per second given to the water

So you have formulas for potential energy in terms of height and kinetic energy in terms of velocity - use the mass of water per second in those formulas to find the power.
Thank you! I got it.. I thought we had to use instantaneous motive power P=Fv and forgot about the kinetic energy change.
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