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 = 0.05 mL = 1.3 m1.35 m4.5 kg9.5 kg

A pendulum is made of a uniform slender rod (4.5 kg, 1.3 m long) pivoted at its top end O, with a solid sphere of mass 9.5 kg and radius 0.05 m fixed to its lower end. The centre of the sphere is 1.35 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 4.5 kgL = 1.3 mmsphere = 9.5 kgr = 0.05 md = 1.35 m
IO = kg·m²
2.Easy
L = 1.2 mGOd = 0.6 mm = 3 kg

A uniform slender rod of mass 3 kg and length 1.2 m rotates about an axis O perpendicular to the rod, 0.6 m from its centre G. Find its mass moment of inertia IO about that axis.

m = 3 kgL = 1.2 md = 0.6 m
IO = kg·m²
3.Medium
L = 0.5 mm = 11 kgα

A uniform slender rod of mass 11 kg and length 0.5 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 = 11 kgL = 0.5 mg = 9.81 m/s²
R = N
4.Medium
L = 1.7 mm = 11.5 kgα

A uniform slender rod of mass 11.5 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 acceleration atip of the free end just after release.

m = 11.5 kgL = 1.7 mg = 9.81 m/s²
atip = m/s²
5.Hard
disc: 31 kgring: 6 kgn1​ = 900 rpm

A uniform solid disc (mass 31 kg, radius 0.2 m) spins freely at 900 rpm about a vertical frictionless shaft. A thin ring of mass 6 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 = 31 kgMring = 6 kgR = 0.2 mn1 = 900 rpm
n2 = rpm
6.Hard
θ = 30°a

A hollow sphere rolls without slipping down a slope inclined at 30° to the horizontal. Use g = 9.81 m/s². Find the smallest coefficient of static friction μs that prevents it from slipping.

θ = 30°I = (2/3)·m·r²g = 9.81 m/s²
μs =

Answer key

  1. 1. IO = 19.86 kg·m²
  2. 2. IO = 1.44 kg·m²
  3. 3. R = 26.98 N
  4. 4. atip = 14.71 m/s²
  5. 5. n2 = 648.8 rpm
  6. 6. μs = 0.2309

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