Write the equations of motion of a pendulum consisting of a thin, 2 kg stick of length suspended from a pivot. How long should the rod be in order for the period to be exactly 1 sec

3g/(8π²) ≈ 0.372 m

Explanation:

Draw a free body diagram.  There is a weight force at the center of the pendulum.

Sum of the torques about the pivot:

∑τ = Iα

mg (½ L sin θ) = (⅓ mL²) α

3g sin θ = 2L α

α = 3g/(2L) sin θ

For small θ, sin θ ≈ θ.

α = 3g/(2L) θ

θ" = 3g/(2L) θ

The solution of this differential equation is:

θ = θ₀ cos(√(3g/(2L)) t)

So the period is:

T = 2π / √(3g/(2L))

If the period is 1 second:

1 = 2π / √(3g/(2L))

√(3g/(2L)) = 2π

3g/(2L) = 4π²

L = 3g/(8π²)

L ≈ 0.372 m