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 m1.25 m5 kg7.5 kg

A pendulum is made of a uniform slender rod (5 kg, 1 m long) pivoted at its top end O, with a thin solid disc of mass 7.5 kg and radius 0.25 m fixed to its lower end. The disc swings in its own plane and its centre is 1.25 m below O. Find the mass moment of inertia IO of the pendulum about O.

mrod = 5 kgL = 1 mmdisc = 7.5 kgr = 0.25 md = 1.25 m
IO = kg·m²
2.Easy
L = 0.4 mGOd = 0.15 mm = 6.5 kg

A uniform slender rod of mass 6.5 kg and length 0.4 m rotates about an axis O perpendicular to the rod, 0.15 m from its centre G. Find its mass moment of inertia IO about that axis.

m = 6.5 kgL = 0.4 md = 0.15 m
IO = kg·m²
3.Medium
12.5 kgaαM = 17.5 kgR = 0.2 m

A 12.5 kg block hangs from a light cord wrapped around a pulley, a uniform solid disc of mass 17.5 kg and radius 0.2 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 = 12.5 kgM = 17.5 kgR = 0.2 mg = 9.81 m/s²
a = m/s²
4.Medium
L = 2.5 mm = 4.5 kgα

A uniform slender rod of mass 4.5 kg and length 2.5 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 = 4.5 kgL = 2.5 mg = 9.81 m/s²
α = rad/s²
5.Hard
160 m/s10 g1.72 mL = 1.8 m4.5 kg

A uniform slender rod of mass 4.5 kg and length 1.8 m hangs at rest from a frictionless pivot at its top end. A 10 g bullet moving horizontally at 160 m/s hits the rod 1.72 m below the pivot and stays embedded. Find the angular velocity ω of the rod just after the impact.

M = 4.5 kgL = 1.8 mm = 10 gv = 160 m/sd = 1.72 m
ω = rad/s
6.Hard
θ = 10°as = 5 m

A solid sphere rolls without slipping down a slope inclined at 10° to the horizontal. Use g = 9.81 m/s². It starts from rest. How long t does it take to roll 5 m down the slope?

θ = 10°I = (2/5)·m·r²g = 9.81 m/s²s = 5 m
t = s

Answer key

  1. 1. IO = 13.62 kg·m²
  2. 2. IO = 0.2329 kg·m²
  3. 3. a = 5.771 m/s²
  4. 4. α = 5.886 rad/s²
  5. 5. ω = 0.5628 rad/s
  6. 6. t = 2.867 s

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