- #1

charon_9

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- Homework Statement:
- Newton's second law on a compound pendulum

- Relevant Equations:
- I*α = ΣM, F=-kx, G=mg

I need to write an equation for Newton's second law for the above system, where k1=k2 (both springs are the same). The red line represents a bar with m=2kg, l=2m.

I know that I*α = M1 + M2 + M3

If we displace the bar by

**x**, we get the angle of displacement theta.

M1=M2=-k*x

I know that, judging by the picture, tan(Θ)=x/l, thus x=tan(Θ)*l.

However, my workbook states that M1=-k*tan(Θ)*l*

**l*cos(Θ**). Where did the

**l*cos(Θ**) come from?