# Homework Help: Cylinder fitted with piston and spring im lost

1. Dec 16, 2003

### Rival

A cylinder is fitted with a piston, beneath which is a spring, as in the drawing. The cylinder is open at the top. Friction is absent. The spring constant of the spring is 2900 N/m. The piston has a negligible mass and a radius of 0.023 m.

(a) When air beneath the piston is completely pumped out, how much does the atmospheric pressure cause the spring to compress?

(b) How much work does the atmospheric pressure do in compressing the spring?

i am lost...I have no idea as to what equation to use and where to start this problem. any help would be greatly appreciated

Kevin

2. Dec 16, 2003

### turin

You probably want to post stuff like this in the HW section.

The basic concepts in part a are balance of force, definition of pressure/relationship of pressure to force, and Hooke's law. The initial state has two forces pushing the piston out and one force pushing the pistion in. The final state has one force pushing the piston out and one force pushing the piston in. I won't tell you any more, but I will give you a hint in the form of a question that you should think about: what force is missing in the final state.

The basic concept in part b is work done by a fluid that undergoes a change in volume at constant pressure. Hint: there's probably an equation in the book.

3. Dec 16, 2003

### chroot

Staff Emeritus
Step 1) To begin with, note that atmospheric pressure is about 14.7 pounds per square inch, or about 101,325 newtons per square meter. Think about what that means: the atmosphere pushes on any object (including your cylinder-piston system) with 101 kilonewtons of force per square meter of surface area.

Step 2) You know the area of the cylinder, so you know how much downward force is applied to it by the atmosphere. Use the "result" of Step 1.

Step 3) You know the spring constant, so you know how much force the spring exerts for a given compression. You know how much force the spring needs to provide to stop the piston from moving -- it needs to push up just as hard as the atmosphere pushes down. Use the result of Step 2.

Step 4) You also know how to calculate the work done by any force acting over a distance:

$$W = \int F dx$$

Can you take it from here? Let me know if you don't understand part of this discussion.

- Warren

4. Dec 16, 2003

### Staff: Mentor

Here's another tip: learn about the properties of springs. How much force they exert and how much energy they store when compressed.

5. Dec 16, 2003

### Rival

well if i draw a free body diagram of the piston, i will get a force pushing down which would be Atmospheric pressure multiplied by area which is just (pi)(r^2) or in newtons 168.4. now i sitll have a force of 2900 newtons pushing up from the spring, correct? or am i wrong there
kevin

6. Dec 16, 2003

### chroot

Staff Emeritus
The spring only pushes with 2900 N of force if you compress it one whole meter. If you only compress it half a meter, it only pushes with 1450 N of force.

- Warren

7. Dec 16, 2003

### Rival

Ah ha!!!

i have F=kx as my equation...F=force pushing down on cylinder and spring

F=168.4N
k=2900N

F/k=x

x=.058m

ok first part done but still stuck on second part

kevin...
P.S. thanks everyone and especially Warren

Last edited: Dec 16, 2003
8. Dec 16, 2003

### chroot

Staff Emeritus
You're correct, and you're welcome.

For the second part, you need to consider the definition of work:

$$W = \int F\, dx$$

The force exerted by the spring (against the atmosphere) is just kx, so the expression becomes

$$W = \int k x\, dx$$

Do you know calculus? Can you evaluate this integral?

- Warren

9. Dec 16, 2003

### Staff: Mentor

Now consider how much work it took to compress the spring. Which is another way of asking how much energy is stored in the compressed spring.

10. Dec 16, 2003

### Rival

so is it just the kinetic energy in the spring? KE=1/2Mv^2?

Kevin

11. Dec 16, 2003

### chroot

Staff Emeritus
Spring don't have kinetic energy, unless they're flying across the room. Springs store potential energy. Can you evaluate the integral I posted above that relates W and x for a spring?

- Warren

12. Dec 16, 2003

### Rival

why yes i can evaluate that integral, or at least i better be able to. seeing how i have finished diff E.Q. last quarter... anyways the equation is just

1/2kx^2

so i have

.5(2900)(.058)^2= 4.8778J

thanks again...definitly a confidence booster right there =)

Kevin

Last edited: Dec 16, 2003
13. Dec 16, 2003

### chroot

Staff Emeritus