# Homework Help: Oscillator problem

1. Sep 2, 2007

### lostinphysics44

1. The problem statement, all variables and given/known data
An air-track glider attached to a spring oscillates with a period of 1.50 s. At t=0s the glider is 4.50 cm left of the equilibrium position and moving to the right at 32.6 cm/s.find the phase constant.

2. Relevant equations
w=2pi/t=2pi/1.5=4.189
x=Acos(wt+phi)
v=-awsin(wt+phi)

3. The attempt at a solution
i tried a systems of equations approach with time equals 0, so the w part drops out, and i keep getting the wrong answer. is this the right approach?

Last edited: Sep 2, 2007
2. Sep 2, 2007

### learningphysics

What did you get for a and phi?

3. Sep 2, 2007

### lostinphysics44

i plugged in and got 32.9=-4.189(4.5/cos(phi))(sin(phi)) and phi=-1.049, which was incorrect

4. Sep 2, 2007

### lostinphysics44

sorry should say 32.6, but either way it is still wrong

5. Sep 2, 2007

### rootX

I get 1.5535, is that correct?

6. Sep 2, 2007

### rootX

i dun have any 4.5 in my equation though

7. Sep 2, 2007

### Irid

Your answer seems correct to me. Maybe the equation for motion is

$$x=A \sin \omega t$$

instead. Then $$\Delta \phi = 0.522$$. All is a matter of convention.

8. Sep 2, 2007

### lostinphysics44

no that is wrong too, i only have 2 chances left to get this right.

9. Sep 2, 2007

### rootX

I simply used this eq:
d = (a).sin(m[t] + p)
v = (a.m).cos(m[t]+p)

and didn't give consideration to the signs

10. Sep 2, 2007

### Irid

Also you may consider the fact that the glider is in the right, and moving to the left, so velocity might be negative, but it depends on axis orientation.

11. Sep 2, 2007

### rootX

oops, I used 450 for 4.5
so now i get phi = 0.5242

12. Sep 2, 2007

### lostinphysics44

i used the same equations but the one i recieved has cosine in the distance formula and sine in the velocity since velocity is the derivative of the distance function

13. Sep 2, 2007

### lostinphysics44

.52 is also incorrect

14. Sep 2, 2007

### learningphysics

You should have used -4.5 in your calculation...

15. Sep 2, 2007

### learningphysics

Do they want the phase in degrees?

16. Sep 2, 2007

### rootX

wait for someone else, I will try to find the mistake.

maybe, that's why! lol

17. Sep 2, 2007

### lostinphysics44

18. Sep 2, 2007

### rootX

I think that's the only option left,
but why its -4.5?

yes, because d(cos t) dt = -sin t~!~!! :D

mine weren't correct because that value was for sine functions.

19. Sep 2, 2007

### learningphysics

OK... I'm getting phi = 1.0465 and A = -8.98966

Are you sure the distance is in the form of Acos(wt + phi) and not Asin(wt + phi) ?

20. Sep 2, 2007

### rootX

yes, I also got that too for phi