Homework Help: Physics

Posted by Ryan on Tuesday, April 13, 2010 at 1:52am.

Holding a glass of water filled to a depth of 7.8 cm you step into an elevator. The elevator moves upward with constant acceleration, increasing its speed from zero to 2.2 m/s in 3.1 s.

Find the change in pressure exerted on the bottom of the glass during the time the elevator accelerates.

I understand that you use the P_2_=P_at_+ p_water_(g+a)h!

I find P_2_ and I attained a difference of pressures to be 820. I keep rechecking my answers and submiting new ones, I just cant seem to understand it!

PLEASE HELP!

Physics - drwls, Tuesday, April 13, 2010 at 2:34am
The pressure at the bottom of the glass changes from
rho(water)*g*h to rho(water)*(g+a)*g
when the elevator is accelerating

I have neglected the atmospheric pressure term, which does not change, and is the same on both sides of the bottom of the glass, anyway.

The pressure increase is rho(water)*a*h

h = 0.078 m
a = (2.2 m/s)/3.1s = 0.71 m/s^2
rho(water) = density = 1000 kg/m^2
Physics - Ryan, Tuesday, April 13, 2010 at 3:52am
THANK YOU!
Physics (correction) - drwls, Tuesday, April 13, 2010 at 10:13am
I meant to write
<<The pressure at the bottom of the glass changes from
rho(water)*g*h to rho(water)*(g+a)*h>>
but the last letter came out g.

The final answer remains correct. The pressure will be in Pascals, ths standard S.I. (Systeme International) unit.

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