Rigid-body rotation worksheet with answers

Free printable Rigid-body rotation worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy
Or = 0.25 mL = 1.1 m1.35 m4.5 kg10 kg

A pendulum is made of a uniform slender rod (4.5 kg, 1.1 m long) pivoted at its top end O, with a thin solid disc of mass 10 kg and radius 0.25 m fixed to its lower end. The disc swings in its own plane and its centre is 1.35 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 4.5 kgL = 1.1 mmdisc = 10 kgr = 0.25 md = 1.35 m
IO = kg·m²
2.Easy
Or = 0.15 mL = 0.6 m0.75 m5 kg1 kg

A pendulum is made of a uniform slender rod (5 kg, 0.6 m long) pivoted at its top end O, with a thin solid disc of mass 1 kg and radius 0.15 m fixed to its lower end. The disc swings in its own plane and its centre is 0.75 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 5 kgL = 0.6 mmdisc = 1 kgr = 0.15 md = 0.75 m
IO = kg·m²
3.Medium
0 → 900 rpmI = 36 kg·m²

A motor brings a flywheel with a moment of inertia of 36 kg·m² from rest up to 900 rpm in 40 s. Ignoring losses, find the average power Pavg it delivers, in kW.

I = 36 kg·m²n = 900 rpmt = 40 s
Pavg = kW
4.Medium
L = 1.5 mm = 14 kgα

A uniform slender rod of mass 14 kg and length 1.5 m is pinned at one end and held horizontal. It is released from rest. Use g = 9.81 m/s². Find its angular acceleration α just after release.

m = 14 kgL = 1.5 mg = 9.81 m/s²
α = rad/s²
5.Hard
320 m/s15 g1.3 mL = 1.5 m2 kg

A uniform slender rod of mass 2 kg and length 1.5 m hangs at rest from a frictionless pivot at its top end. A 15 g bullet moving horizontally at 320 m/s hits the rod 1.3 m below the pivot and stays embedded. Find the angular velocity ω of the rod just after the impact.

M = 2 kgL = 1.5 mm = 15 gv = 320 m/sd = 1.3 m
ω = rad/s
6.Hard
θ = 15°vs = 10 m

A hollow sphere starts from rest and rolls without slipping down a slope inclined at 15° to the horizontal. Use g = 9.81 m/s². Find its speed v after it has rolled 10 m along the slope.

θ = 15°s = 10 mI = (2/3)·m·r²g = 9.81 m/s²
v = m/s

Answer key

  1. 1. IO = 20.35 kg·m²
  2. 2. IO = 1.174 kg·m²
  3. 3. Pavg = 3.997 kW
  4. 4. α = 9.81 rad/s²
  5. 5. ω = 4.091 rad/s
  6. 6. v = 5.52 m/s

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