Go Back   Physics Help Forum > High School and Pre-University Physics Help > Energy and Work

Energy and Work Energy and Work Physics Help Forum

Reply
 
LinkBack Thread Tools Display Modes
Old Oct 1st 2015, 10:58 AM   #1
Daz
Junior Member
 
Join Date: Oct 2015
Posts: 2
total mecanical energy of the river

just need some help understanding one term in my equation I have taken a screen shot of the answers and question and posted the link to it below and highlighted the term I donít understand the answers says you can ignore the kinetic and potential energy of the flowing river of height 90m and take the free surface of the lake as if it was of height 90m ok with that but then it says ke+pe=gh +0.5v^2 which is just the formulas for ke and pe energy without the mass if it wasnít trying to add on the velocity of the river then I would have no problem with the answers because I know that to work out the energy content of the water it would be pressure/density=gh=m2s2=KJ/KG please can someone please help me out trying to self study this before university next year thanx in advance

https://onedrive.live.com/redir?resi...nt=photo%2cPNG
Daz is offline   Reply With Quote
Old Oct 1st 2015, 11:53 AM   #2
Senior Member
 
Join Date: Apr 2015
Location: Somerset, England
Posts: 708
Does it not say the energy in kJ per kilogramme?

Thus mass will not enter the equation because it is per kilogramme.
studiot is offline   Reply With Quote
Old Oct 1st 2015, 12:07 PM   #3
Physics Team
 
ChipB's Avatar
 
Join Date: Jun 2010
Location: Naperville, IL USA
Posts: 2,271
First I suggest you have your computer repaired, as it appears that none of your punctuation marks is working.

The units of gh + (1/2)v^2 is m^2/s^2, which is equivalent to J/kg. They want energy per unit mass, so this is the correct units. Then they divide by 1000 to get the answer in KJ/Kg. Perhaps your confusion is that they tell you the density of water is 1000 KG/m^3, but then don't actually use that fact in the solution. The conversion that they use fro J to KJ is a bit counter-intuitive:

1 = 1000 J/KJ = 1000 (Kg-m^2/s^2) /KJ = 1000 (m^2/s^2)/(Kj/Kg)
ChipB is offline   Reply With Quote
Old Oct 2nd 2015, 09:52 AM   #4
Daz
Junior Member
 
Join Date: Oct 2015
Posts: 2
Thank you for replying to me question I am still confused though I have 2 questions I would like to ask 1st if we are ignoring the flow energy which is made up of kinetic and potential energy then why are we using the formula for potential and kinetic energy if we are ignoring it


2nd if we are using the formulas for kinetic and potential energy then how are we just ignoring the mass example ke=m*g*h itís as if we are just dropping the mass out of the formula to make the units we wonít I am lost pleases explain as much as possible
Daz is offline   Reply With Quote
Old Oct 2nd 2015, 10:20 AM   #5
MBW
Senior Member
 
MBW's Avatar
 
Join Date: Apr 2008
Location: Bedford, England
Posts: 668
The initial post indicates that the "total" energy of a river with a drop of 90 metres is the same as a lake which maintains the same 90m height throughout (but otherwise has the same dimensions as the river).

What this implies is that the additional Potential Energy that the water in the lake has (by virtue of always being raised by 90m) is exactly the same as the additional Kinetic Energy that the River has by virtue of the water flowing down from a height of 90m.

This is sensible, the Potential Energy of the lake is converted into Kinetic Energy of the river, but the Total Energy remains the same.
__________________
You have GOT to Laugh !
MBW is offline   Reply With Quote
Old Oct 2nd 2015, 12:44 PM   #6
Physics Team
 
ChipB's Avatar
 
Join Date: Jun 2010
Location: Naperville, IL USA
Posts: 2,271
I think the issue here is that the explanation is poorly worded. I think what they mean to say is that since the mechanical energy is constant, and is equal to the potential plus flow energy at h= 90 m, you can ignore the change in flow energy as the water flows down the hill.
ChipB is offline   Reply With Quote
Old Oct 3rd 2015, 01:20 PM   #7
Senior Member
 
Join Date: Jun 2010
Location: NC
Posts: 366
Niagara Power

Hello Daz,

I solved a problem similar to yours. It might be of interest.

http://www.thermospokenhere.com/wp/0...a/niagara.html

Also of possible interest is:

http://www.thermospokenhere.com/wp/0...orricelli.html

Good luck with your studies. Jim
THERMO Spoken Here is offline   Reply With Quote
Reply

  Physics Help Forum > High School and Pre-University Physics Help > Energy and Work

Tags
energy, mecanical, river, total



Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Total energy of the flowing fluid? ling233 Advanced Mechanics 6 Jan 15th 2016 06:46 PM
Total energy, with some distractions Devlan Special and General Relativity 2 Apr 11th 2014 03:36 PM
Total energy after emitting photon Devlan Special and General Relativity 1 Apr 11th 2014 01:48 PM
total energy of an airplane, work done against air resistance furor celtica Kinematics and Dynamics 2 Oct 28th 2011 08:47 AM
tough total heat energy problem galactus Thermodynamics and Fluid Mechanics 0 Dec 11th 2008 03:50 PM


Facebook Twitter Google+ RSS Feed