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Help with 2 problems! they seem so simple but I can't get it

Posted: Mon Apr 16, 2007 6:41 am
by melissa1234

I am stuck with this 2 problems from an undergrad book. They seem so easy but I can't find the answer (and this book only give answers to some questions...

Can u give me a tip in any one of those PLEASE?????

1. Water flows over a section of niagara falls at a rate of 1.20 kg/s and falls 5.0 m. What is the power dissipated by the waterfall?

2. A 2 kg ball has zero kinematic and potential energy. You drop this ball into a 10. m-deep well. Just before the ball hits the bottom, the sum of its potential and kinetic energy is? How about after the ball comes to a stop?

thank you sooooo much for any help!!!!


Posted: Mon Apr 23, 2007 11:58 am
by twistor
1) Power = energy / time. So how much energy is dissipated by this waterfall in 1 sec? You know that after 1 second 1.20 kg of water have fallen over the hill. What energy does that correspond to?

2) In any conservative system (i.e. a gravitational field) the sum of kinetic and potential energies is constant. So how much energy does it have before it strikes?. After the ball stops it has no more kinetic energy. I would suggest drawing a picture where you clearly indicate that at the top of the well the ball has 0 potential energy. Hint: The potential energy should be negative when the ball is in the well.


Posted: Thu Jul 12, 2007 2:58 am
by salehajaweid
I am getting 58.8 for the first question ... is it correct?

ans 2

Posted: Thu Jul 12, 2007 3:10 am
by salehajaweid
well after it stops energy is 0

in the well i got 196 when something is dropped its potential energy is greatest at the greatest height and as it falls that converts to kinetic energy ... if i remember my high schools physics :) but i do love it so!

ans 1

Posted: Thu Jul 26, 2007 10:56 am
by dunecastle
Energy per second= mgh= 1.2kg/s*9.8*5m=58.8 W