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 = 0.5 m0.75 m2.5 kg2.5 kg

A pendulum is made of a uniform slender rod (2.5 kg, 0.5 m long) pivoted at its top end O, with a solid sphere of mass 2.5 kg and radius 0.25 m fixed to its lower end. The centre of the sphere is 0.75 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 2.5 kgL = 0.5 mmsphere = 2.5 kgr = 0.25 md = 0.75 m
IO = kg·m²
2.Easy
Or = 0.15 mL = 0.7 m0.85 m1 kg1.5 kg

A pendulum is made of a uniform slender rod (1 kg, 0.7 m long) pivoted at its top end O, with a solid sphere of mass 1.5 kg and radius 0.15 m fixed to its lower end. The centre of the sphere is 0.85 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 1 kgL = 0.7 mmsphere = 1.5 kgr = 0.15 md = 0.85 m
IO = kg·m²
3.Medium
L = 1.7 mm = 19 kgα

A uniform slender rod of mass 19 kg and length 1.7 m is pinned at one end and held horizontal. It is released from rest. Use g = 9.81 m/s². Find the vertical force R that the pin exerts on the rod just after release.

m = 19 kgL = 1.7 mg = 9.81 m/s²
R = N
4.Medium
n = 1200 rpmTb​ = 200 N·mI = 5.5 kg·m²

A flywheel with a moment of inertia of 5.5 kg·m² spins at 1200 rpm. A brake applies a constant friction torque of 200 N·m. How long t does it take to stop?

I = 5.5 kg·m²n = 1200 rpmTb = 200 N·m
t = s
5.Hard
disc: 25 kgring: 4 kgn1​ = 200 rpm

A uniform solid disc (mass 25 kg, radius 0.3 m) spins freely at 200 rpm about a vertical frictionless shaft. A thin ring of mass 4 kg and the same radius is dropped onto it from rest, centred on the shaft, and the two quickly spin together. Find their common speed n2 in rpm.

Mdisc = 25 kgMring = 4 kgR = 0.3 mn1 = 200 rpm
n2 = rpm
6.Hard
5.5 m/sω

A solid sphere of mass 4.5 kg rolls without slipping at 5.5 m/s. What percentage p of its total kinetic energy is rotational?

m = 4.5 kgv = 5.5 m/sI = (2/5)·m·r²
p = %

Answer key

  1. 1. IO = 1.677 kg·m²
  2. 2. IO = 1.261 kg·m²
  3. 3. R = 46.6 N
  4. 4. t = 3.456 s
  5. 5. n2 = 151.5 rpm
  6. 6. p = 28.57 %

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