Homework Help: Equations & Solutions

[yecko]

Homework Statement

http://i.imgur.com/wSvUEU2.png

Homework Equations

(written in the photo)

The Attempt at a Solution

(written in the photo)
Thanks

 

[Cutter Ketch]

It all seems to be there, so I'm not sure what you are missing.

The area is clear from the 10cm cylinder. If you know the area then the pressure is clear from the force of gravity on the weight divided by the area (and + 1 atmosphere for the external pressure on the cylinder.) n is given, the Ts are given. What else do you need?

 

[TSny]

As Cutter Ketch noted, the pressure of the gas is not atmospheric pressure. The weight of the piston must be taken into account.

 

[yecko]

Do I really need to consider the weight of the piston? because it seems to be a constant factor... no change in the case...

If you know the area then the pressure is clear from the force of gravity on the weight divided by the area (and + 1 atmosphere for the external pressure on the cylinder.)

do you mean "F=PA=mg" to be simple? however, what's going on with it?

 

[Cutter Ketch]

Oh, I see. You aren't getting the right answer. So I take it the hand written answers are correct and the red is what you got?

 

[yecko]

You aren't getting the right answer. So I take it the hand written answers are correct and the red is what you got?

Words in red are written by me
and the circle of the option is the right answer
i got c, while the right answer is a...
thanks

 

[Cutter Ketch]

Do I really need to consider the weight of the piston? because it seems to be a constant factor... no change in the case...

do you mean "F=PA=mg" to be simple? however, what's going on with it?

You don't need it because it changes. You need it because you can't define the initial condition without it. Since h1 isn't given you have to figure it out. Your formula for h1 is correct, and in that formula is pressure. You need the absolute pressure to determine h1, and that pressure includes holding up the piston.

 

[yecko]

so you mean i should eliminate the factor for h1 first in order to find out the pressure if piston is not present?
how should i relate PV=nRT to force?

 

[TSny]

Your calculation in red is correct except for the value of the pressure P₁. P₁ does not equal atmospheric pressure.

To find the correct pressure, switch your attention to the piston and consider all the forces acting on the piston.

 

[yecko]

P1 does not equal atmospheric pressure

by F=PA=mg, P should be equal to mg/A=50*9.81/(0.05^2*pi)=62452Pa?
I still can't get the written answer...

 

[TSny]

by F=PA=mg, P should be equal to mg/A=50*9.81/(0.02^2*pi)=62452Pa?

That's not the pressure of the gas inside the container.

How many forces act on the piston? Describe each force.

 

[yecko]

Describe each force

weight and pressure by gas
am i right?

 

[TSny]

weight and pressure by gas
am i right?

There's one more force. The problem states that the top of the piston is "open to the air".

 

[yecko]

pressure of the air?
mg+P(a)A=P(g)A
P(g)=163689Pa ?

 

[TSny]

That's it. Good!

 

[yecko]

I see
thank you very much
do you mind also to visit my another post about amplitude of oscillation?
it is still haven't been solved
your advice are REALLY useful

the link:
https://www.physicsforums.com/threads/amplitude-of-oscillation.896853/#post-5641605

 

[TSny]

I'm off to bed. Will check back tomorrow.

 

[yecko]

Thanks you~
Then I will come back here at night of my local time~
because i am going to have exam tomorrow, hope all problems in the exercises can be settled by today:)