## brain is stuck

• This has become our largest and most active forum because the physics GRE is just one aspect of getting accepted into a graduate physics program.
• There are applications, personal statements, letters of recommendation, visiting schools, anxiety of waiting for acceptances, deciding between schools, finding out where others are going, etc.

monker
Posts: 1
Joined: Thu Aug 28, 2008 6:19 pm

### brain is stuck

I am trying to figure out something that I thought would be very simple. I am trying to see how much psi it would take to get a ball to 300fps. What I have so far...
ball
m=3.201g
v= 300fps = 91.44m/s

I have calculated the Kintic energy that is required to push that object which is .00047 J. The par that i am stuck at is how much PSI would it take to make .00047 J. Or im I just looking at this thing all wrong?

WontonBurritoMeals
Posts: 103
Joined: Sat Feb 23, 2008 9:43 pm

### Re: brain is stuck

I wonder...

Does PSI make less sense in this problem as pounds per square inch or psi, the catch-all term that paranormal scientists use??

May the wind be always at your back,
-Wonton Burrito Meals

a bucket
Posts: 60
Joined: Fri Aug 08, 2008 9:02 am

### Re: brain is stuck

monker wrote:I am trying to see how much psi it would take to get a ball to 300fps.

whut?

monocles
Posts: 32
Joined: Thu Feb 21, 2008 2:08 pm

### Re: brain is stuck

What you're looking for is psi for a length of time. It could theoretically be done with any nonzero psi, given enough time.

WontonBurritoMeals
Posts: 103
Joined: Sat Feb 23, 2008 9:43 pm

### Re: brain is stuck

Depending on the surface area of the ball. The only thing I remember from analytical mechanics is that the cross-sectional surface area of a sphere is half the surface area.

May the wind be always at your back,
-Wonton Burrito Meals

quizivex
Posts: 1035
Joined: Tue Jan 09, 2007 6:13 am

### Re: brain is stuck

On this forum it's easy to forget that some problems in physics pertain to real physics, as opposed to admission committees lol.

monker wrote:I am trying to figure out something that I thought would be very simple. I am trying to see how much psi it would take to get a ball to 300fps. What I have so far...
ball
m=3.201g
v= 300fps = 91.44m/s
I have calculated the Kintic energy that is required to push that object which is .00047 J. The par that i am stuck at is how much PSI would it take to make .00047 J. Or im I just looking at this thing all wrong?

Huh?
I think you're leaving out information in this problem.

Is this what you meant to say?
"Suppose a ball of mass 3.201g is inflated to a pressure P. A small hole is made in the ball that causes the ball to release air which propels the ball forward. Assuming the ball moves in a straight line, is not affected by gravity and does not interact with the ground, what value of P would be necessary to assure that the final velocity of the ball is 91.44m/s?"
I think this problem could be solved by considering the conservation of energy and momentum of the ball and the air (treated as an ideal gas in the ball). But I'm not sure how to set up the equations. The problem you listed makes no sense as it stands.