Finding the differential equation for a oscillating system

JustinLiang · Dec 8, 2012

Homework Statement

Diagram of a bent rod pivoted at O with a block A at the top end, a 0.6 m horizontal segment, a 1.2 m vertical segment, a spring of k = 15 N/m attached below, and a motor at B with angular velocity ω.

The Attempt at a Solution

I don't think this is the correct answer because for some reason I have a constant mg term. Usually I get mgsinθ and from small angle approximations it becomes mgθ, but this time I am getting mgcosθ and from small angle approximations it becomes mg.

Handwritten free-body diagram of a rod pivoting at O with a block of mass m hanging from a point, forces mg, Rx, Ry, Fs, and angle theta marked, followed by handwritten moment equations including -mg cos theta and 0.1 cos 15t.

What am I doing wrong? Or is this correct?

haruspex · Dec 8, 2012

I don't see the distances in your expressions for the moments.

theBEAST · Dec 8, 2012

haruspex said:

I don't see the distances in your expressions for the moments.

I just tried this question and did it with the distances, but I also don't understand why there is a constant term. That term is throwing me off.

haruspex · Dec 8, 2012

theBEAST said:

I just tried this question and did it with the distances, but I also don't understand why there is a constant term. That term is throwing me off.

Please post your working if you'd like further assistance.

Finding the differential equation for a oscillating system

Homework Statement

The Attempt at a Solution

Discussion

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