1. Apr 28, 2013

### jklops686

1. The problem statement, all variables and given/known data

A particle with a mass of 65 g is moving with simple harmonic motion. At time t = 0, the particle is at its extreme positive displacement of 18.0 cm. The period of the motion is 0.600 s. Find the vecocity of the particle at t = 1.35 s

2. Relevant equations

(1). ω=2∏/T or
(2). ω=√k/m

(3). x=Acos(ω∏+δ)
(4). v=-Aωsin(ω∏+δ)

3. The attempt at a solution

First question: to find ω, why don't I get the same answer for both equations 1 and 2?

With equation 1 I get 10.47 (which I think is correct) and with eqn. 2 I get 7.45.

Second question: How do I find the phase change for this problem? I tried to set the displacement equation equal to zero at t=0 and im getting ∏/2 but it seems that the only way to get the correct answer for the problem is to have zero phase change.

2. Apr 28, 2013

### TSny

It will help if show your calculations. In particular, how did you determine the value of k to use in eqn. 2?

The problem states that the particle is at its extreme positive displacement at t = 0.

3. Apr 28, 2013

### SteamKing

Staff Emeritus
How do you know what k is?

4. Apr 28, 2013

### jklops686

I was thinking I could use hooke's law to find k.

Tsny...

I guess i'm just confused at finding phase changes.

5. Apr 28, 2013

### TSny

What does the equation x = Acos(ωt+δ) become for t = 0? Knowing that x is at it's maximum positive value at t = 0, what can you conclude about the value of δ?

6. Apr 28, 2013

### SteamKing

Staff Emeritus
But how did you get a value of k to calculate omega of 7.45?

7. Apr 28, 2013

### jklops686

Using F=kx

0.65N=k(.18)

k=3.61

ω=√3.61/.065=7.45

TSny... I think I see what you're saying. x would equal 18 at t=0.

so 18cm = Acos(δ) ?

Also, how do you quote two people in one reply?

8. Apr 28, 2013

### TSny

But you can't assume that the force acting on the particle is the weight of the particle. The simple harmonic motion might be due to a spring, and the force would be whatever the force of the spring happens to be for a given value of x.

What does "A" stand for in this equation? What is the value of A?

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