# Homework Help: Pendulum with horizontal spring

1. Apr 26, 2010

### mountainbiker

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

A 5kg sphere is connected to a thin massless but rigid rod of length L=1.3 m to form a simple pendulum. The rod is connected to a nearby vertical wall by a spring with a spring constant k= 75N/m, connected to it at a distance h=1.1 m below the pivot point of the pendulum. What is the angular frequency (in rad/s) of the system for small amplitude oscillations.

2. Relevant equations

$$\alpha$$ = $$\omega$$2*$$\theta$$
I=m*r2
Torque=f*d*sin$$\theta$$

or small oscillations, sin$$\theta$$=$$\theta$$ and cos$$\theta$$=1

3. The attempt at a solution

sum the torque and set them equal to $$\alpha$$I

gravitational torque= Lmg$$\theta$$

if theta is the angle between the rod and its equilibrium position, the angle of the spring torque is (theta + $$\pi$$/2), so the spring torque is
f*d*sin(theta + $$\pi$$/2). The force is k*$$\Delta$$x, or h*sin(theta + $$\pi$$/2) and the distance is h, giving

torque from spring =k*h2*sin$$\theta$$*sin(theta + $$\pi$$/2)
or k*h2*$$\theta$$*(theta + $$\pi$$/2)

My final equation is

mL2$$\omega$$2tex]\theta[/tex] = Lmg$$\theta$$ + k*h2*$$\theta$$*(theta + $$\pi$$/2)

The problem is that this gives me a $$\omega$$ that is dependant on $$\theta$$, which isn't possible. It should be constant. Can someone please tell me where I'm going wrong?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Apr 27, 2010

### ehild

sin(theta+pi/2)= cos(theta). For small angles, cos(theta) =1.

ehild