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 = 0.1 mL = 0.8 m0.9 m1 kg9 kg

A pendulum is made of a uniform slender rod (1 kg, 0.8 m long) pivoted at its top end O, with a thin solid disc of mass 9 kg and radius 0.1 m fixed to its lower end. The disc swings in its own plane and its centre is 0.9 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 1 kgL = 0.8 mmdisc = 9 kgr = 0.1 md = 0.9 m
IO = kg·m²
2.Easy
Or = 0.2 mL = 0.7 m0.9 m0.5 kg7.5 kg

A pendulum is made of a uniform slender rod (0.5 kg, 0.7 m long) pivoted at its top end O, with a solid sphere of mass 7.5 kg and radius 0.2 m fixed to its lower end. The centre of the sphere is 0.9 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 0.5 kgL = 0.7 mmsphere = 7.5 kgr = 0.2 md = 0.9 m
IO = kg·m²
3.Medium
8 kgaαM = 12.5 kgR = 0.15 m

An 8 kg block hangs from a light cord wrapped around a pulley, a uniform solid disc of mass 12.5 kg and radius 0.15 m that turns on a frictionless horizontal axle. The block is released from rest. Use g = 9.81 m/s². Find the acceleration a of the block.

m = 8 kgM = 12.5 kgR = 0.15 mg = 9.81 m/s²
a = m/s²
4.Medium
n = 2900 rpmTb​ = 125 N·mI = 16 kg·m²

A flywheel with a moment of inertia of 16 kg·m² spins at 2900 rpm. A brake applies a constant friction torque of 125 N·m. How long t does it take to stop?

I = 16 kg·m²n = 2900 rpmTb = 125 N·m
t = s
5.Hard
θ = 10°a

A solid sphere rolls without slipping down a slope inclined at 10° to the horizontal. Use g = 9.81 m/s². Find the smallest coefficient of static friction μs that prevents it from slipping.

θ = 10°I = (2/5)·m·r²g = 9.81 m/s²
μs =
6.Hard
600 m/s45 g0.28 mL = 0.7 m4.5 kg

A uniform slender rod of mass 4.5 kg and length 0.7 m hangs at rest from a frictionless pivot at its top end. A 45 g bullet moving horizontally at 600 m/s hits the rod 0.28 m below the pivot and stays embedded. Find the angular velocity ω of the rod just after the impact.

M = 4.5 kgL = 0.7 mm = 45 gv = 600 m/sd = 0.28 m
ω = rad/s

Answer key

  1. 1. IO = 7.548 kg·m²
  2. 2. IO = 6.277 kg·m²
  3. 3. a = 5.507 m/s²
  4. 4. t = 38.87 s
  5. 5. μs = 0.05038
  6. 6. ω = 10.24 rad/s

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