Kinetic energy, Potential energy and change in water pressure

[Northbysouth]

Homework Statement

I'm not particularly confident with my calculations and I would appreciate any feed back.

A) What is the kinetic energy of an object with:
mass = 5lbm
velocity = 200 ft/sec

B) What is the potential energy of an object with:
mass = 5lbm
height = 200ft
g = 32.2 ft/sec²

C) What is the pressure difference between the top and bottom of a water column that is 15ft high and gravity is 32.2 ft/sec²

Homework Equations

KE = 1/2mv²/g_c

PE = mgh/g_c

The Attempt at a Solution

A)
KE = 1/2(5 lbm)(200 ft/sec)²/32.17 ft-lbm/lbf-sec²
KE = 3.11x10³ lbf-ft

B)
PE = (5 lbm)(32.2 ft/sec²)(200ft)/32.17 ft-lbm/lbf-sec²
PE = 1001 lbf-ft

C) I'm not quite sure where to start with this one and I would appreciate an guidance.

 

[tiny-tim]

Hi Northbysouth!

A)
KE = 1/2(5 lbm)(200 ft/sec)²/32.17 ft-lbm/lbf-sec²
KE = 3.11x10³ lbf-ft

B)
PE = (5 lbm)(32.2 ft/sec²)(200ft)/32.17 ft-lbm/lbf-sec²
PE = 1001 lbf-ft

yes, they look fine to me, except:

i] i don't understand the pound system , so i can;t confirm whether you need to divide by g !
ii] if the question asks you to take g as 32.2, then shouldn't you use the same figure instead of 32.17 ?

C) I'm not quite sure where to start with this one and I would appreciate an guidance.

you'll need to find the density of water (mass per cubic foot) …

pressure = weight/area = density*height*area/area = density*height

 

[HallsofIvy]

Hi Northbysouth!


yes, they look fine to me, except:

i don't understand the pound system , so i can;t confirm whether you need to divide by g !

Since the problem says that the mass (rather than weight) is "5 lbm", no, you shouldn't divide by g. "1 lbm" or "one pound mass" is the mass of an object that has, in standard gravity, a weight of 1 lb. The division by g has already been done.

[ii] if the if the question asks you to take g as 32.2, then shouldn't you use the same figure instead of 32.17 ?


you'll need to find the density of water (mass per cubic foot) …

pressure = weight/area = density*height*area/area = density*height