# Homework Help: Frequency of Oscillation of a Block & Spring

1. Nov 17, 2014

### QuantumKnight

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

An oscillator consists of a block attached to a spring (k=400 N/m). At some time t, the position, velocity, and accelleration of the block are x = 0.100m, v = -13.6 m/s, a - 123 m/s2

A) Calculate the frequency of the oscillation for the system
B)What is the mass of the block
C) What is the amplitue of the block motion?

2. Relevant equations
x(t) = Acos (wt + psi)

3. The attempt at a solution
I have tried to use the equation for Simple Harmonic Motion for position and then take the derivaties to find velocity and acceleration. I am still not understanding the phase shifts. I am not worried about B as I can find the mass thanks to the assistance earlier. Just a little guiadance as to what i need to look for in setting up this problem

2. Nov 17, 2014

### Orodruin

Staff Emeritus
Hint: You do not need to worry about the phase shift in order to solve (A) - assuming that x is measured relative to the equilibrium position.

3. Nov 17, 2014

### QuantumKnight

For some reason the only equation I can think of to find frequency is 1/T. But this can't be right because I need to know the mass of the block. I can't use T = 2pi/w (w being omeaga) because I wasn't given an angular velocity. I know this is simple but I can't wrap my head around what I am missing.

4. Nov 17, 2014

### Orodruin

Staff Emeritus
How are acceleration, velocity, and displacement related? What expressions do you obtain for them?

5. Nov 17, 2014

### lep11

Hello. Draw a free body diagram of the block at time t. Then you will easily figure out the net force and you already know the acceleration so actually you could begin by solving the mass of the block using Newtons 2nd law. You know how to calculate the frequency when you know m and k.However it is not said in the problem statement whether the block is oscillating vertically or horizontally. I think the other way would be to derive formulas for velocity and acceleration and you would end up having three equations and three unknows.

Last edited: Nov 17, 2014
6. Nov 17, 2014

### Orodruin

Staff Emeritus
This is actually the long way around. In order to solve (A), it does not matter what the spring constant is (assuming the mass is chosen such that the data are fulfilled).

7. Nov 17, 2014

### lep11

Which one is the longer way?

8. Nov 17, 2014

### Orodruin

Staff Emeritus
To start by solving for the mass when you are a priori interested in the frequency. You do not need to include the information of the spring constant in order to find it, it is strictly only relevant for finding the mass in part (B).

9. Nov 17, 2014

### lep11

Okay. Then I suggest solving those three equations in a group. It wont hurt to think the problem more deeply or from a different point of view but using NII formula without further information would have been incorrect I think.

10. Nov 17, 2014

### Orodruin

Staff Emeritus
You could solve it by that method by simply assuming a spring constant k. The resulting mass will simply be proportional to it, which will cancel out in the end. However, just using the kinematics of the problem provides a far more elegant solution.

11. Nov 17, 2014

### lep11

But you dont have to assume the value k since it is given...

12. Nov 17, 2014

### Orodruin

Staff Emeritus
What I am saying is that you do not have to use it and that the answer is independent of the value provided.

13. Nov 17, 2014

### lep11

Yes it cancels out sorry for my laggy brain