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
k = 0.35 m300 → 1800 rpm170 kg

A flywheel of mass 170 kg with a radius of gyration of 0.35 m is accelerated uniformly from 300 rpm to 1800 rpm. This takes 18 s. Ignoring friction, find the constant torque T needed.

m = 170 kgk = 0.35 mn1 = 300 rpmn2 = 1800 rpmt = 18 s
T = N·m
2.Easy
L = 2.6 mGOd = 0.45 mm = 6.5 kg

A uniform slender rod of mass 6.5 kg and length 2.6 m rotates about an axis O perpendicular to the rod, 0.45 m from its centre G. Find its mass moment of inertia IO about that axis.

m = 6.5 kgL = 2.6 md = 0.45 m
IO = kg·m²
3.Medium
L = 2.3 mm = 2.5 kgα

A uniform slender rod of mass 2.5 kg and length 2.3 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 = 2.5 kgL = 2.3 mg = 9.81 m/s²
atip = m/s²
4.Medium
n = 2900 rpmTb​ = 60 N·mI = 17 kg·m²

A flywheel with a moment of inertia of 17 kg·m² spins at 2900 rpm. A brake applies a constant friction torque of 60 N·m. How long t does it take to stop?

I = 17 kg·m²n = 2900 rpmTb = 60 N·m
t = s
5.Hard
460 m/s20 g0.52 mL = 1.3 m4.5 kgφ

A uniform slender rod of mass 4.5 kg and length 1.3 m hangs at rest from a frictionless pivot at its top end. A 20 g bullet moving horizontally at 460 m/s hits the rod 0.52 m below the pivot and stays embedded. Use g = 9.81 m/s². Find the largest angle φ from the vertical through which the rod swings.

M = 4.5 kgL = 1.3 mm = 20 gv = 460 m/sd = 0.52 mg = 9.81 m/s²
φ = °
6.Hard
disc: 33 kgring: 12 kgn1​ = 1050 rpm

A uniform solid disc (mass 33 kg, radius 0.15 m) spins freely at 1050 rpm about a vertical frictionless shaft. A thin ring of mass 12 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 = 33 kgMring = 12 kgR = 0.15 mn1 = 1050 rpm
n2 = rpm

Answer key

  1. 1. T = 181.7 N·m
  2. 2. IO = 4.978 kg·m²
  3. 3. atip = 14.71 m/s²
  4. 4. t = 86.04 s
  5. 5. φ = 32.48 °
  6. 6. n2 = 607.9 rpm

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