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Learning Introductory Physics with Activities

Section 14.8 Practice - Rotational Energy and Angular Momentum

Subsection Numerical Practice

Calculation 14.8.1. Olympic High Diver.

An Olympic high diver in midair pulls her legs inward toward her chest. Doing so changes which of these quantities?
  1. Angular momentum
  2. Rotational inertia about her center of mass
  3. Angular velocity
  4. Translational (linear) momentum
  5. Translational (linear) kinetic energy
  6. Rotational kinetic energy
Answer.
B., C., E.

Calculation 14.8.2. Helicopter I.

A typical small rescue helicopter has four blades. Each blade is \(4.0 \mathrm{~m}\) long and has a mass of \(50.0 \mathrm{~kg}\text{.}\) The blades can be approximated as thin rods that rotate about one end of an axis perpendicular to their length. The helicopter has a total loaded mass of \(1000 \mathrm{~kg}\text{.}\) Calculate the rotational kinetic energy in the blades when they rotate at \(300 \mathrm{~rpm}\text{.}\)
Answer.
\(5.26 \times 10^5 \mathrm{~J}\)

Calculation 14.8.3. Helicopter II.

A typical small rescue helicopter has four blades. Each blade is \(4.00 \mathrm{~m}\) long and has a mass of \(50.0 \mathrm{~kg}\text{.}\) The blades can be approximated as thin rods that rotate about one end of an axis perpendicular to their length. The helicopter has a total loaded mass of \(1000 \mathrm{~kg}\text{.}\) What is the ratio of translational kinetic energy of the helicopter over the rotational kinetic energy of its blades when it flies at \(20.0 \mathrm{~m/s}\text{?}\)
Answer.
0.38

Calculation 14.8.4. Helicopter III.

A typical small rescue helicopter has four blades. Each blade is \(4.00 \mathrm{~m}\) long and has a mass of \(50.0 \mathrm{~kg}\text{.}\) The blades can be approximated as thin rods that rotate about one end of an axis perpendicular to their length. The helicopter has a total loaded mass of \(1000 \mathrm{~kg}\text{.}\) To what height could the helicopter be raised if all of the rotational kinetic energy could be used to lift it?
Answer.
\(53.7 \mathrm{~m}\)

References References

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Numerical practice activities provided by BoxSand: https://boxsand.physics.oregonstate.edu/welcome.