Math Problem Statement

A thin rod of mass 101.56 Kg rotates in deep space about an axis perpendicular to its length, and through the rods center of mass, as shown:

When the spinning rod has kinetic energy 1.62 kJ, what is the linear velocity of the rod, that is, the linear speed of the rod at either end of the spining rod?

Assume that there is no translational velocity of interest in this problem, and that you are "floating at rest" for the purposes of the problem.

Please give your answer in m/s, and to two decimal places.

Solution

To find the linear velocity of the rod's ends, we can use the relationship between rotational kinetic energy and linear velocity.

Step 1: Set up the Known Information

  • Mass of the rod, m=101.56kgm = 101.56 \, \text{kg}
  • Rotational kinetic energy, KE=1.62kJ=1620JKE = 1.62 \, \text{kJ} = 1620 \, \text{J}
  • The rod rotates about its center.

Step 2: Rotational Kinetic Energy and Moment of Inertia

The formula for rotational kinetic energy is: KE=12Iω2KE = \frac{1}{2} I \omega^2 where:

  • II is the moment of inertia of the rod about its center.
  • ω\omega is the angular velocity.

For a thin rod rotating about its center, the moment of inertia II is: I=112mL2I = \frac{1}{12} m L^2 where LL is the length of the rod.

Step 3: Express ω\omega in Terms of Linear Velocity

The linear velocity vv at either end of the rod is related to ω\omega by: v=L2ωv = \frac{L}{2} \cdot \omega which rearranges to give: ω=2vL\omega = \frac{2v}{L}

Step 4: Substitute ω\omega into the Kinetic Energy Equation

Substituting ω=2vL\omega = \frac{2v}{L} into the kinetic energy equation: KE=12112mL2(2vL)2KE = \frac{1}{2} \cdot \frac{1}{12} m L^2 \cdot \left(\frac{2v}{L}\right)^2

This simplifies to: KE=12112mL24v2L2KE = \frac{1}{2} \cdot \frac{1}{12} m L^2 \cdot \frac{4v^2}{L^2} KE=16mv2KE = \frac{1}{6} m v^2

Now, solve for vv: v2=6KEmv^2 = \frac{6 \cdot KE}{m} v=6KEmv = \sqrt{\frac{6 \cdot KE}{m}}

Step 5: Substitute Values and Calculate

v=61620101.56v = \sqrt{\frac{6 \cdot 1620}{101.56}} v=9720101.56v = \sqrt{\frac{9720}{101.56}} v=95.73v = \sqrt{95.73} v9.78m/sv \approx 9.78 \, \text{m/s}

Final Answer

The linear velocity at either end of the spinning rod is approximately: 9.78m/s\boxed{9.78 \, \text{m/s}}

Would you like a deeper explanation on any part?


  1. What is the angular velocity of the rod?
  2. How would changing the mass of the rod affect the velocity?
  3. How is kinetic energy distributed in rotational systems?
  4. Why is the moment of inertia important in rotational motion?
  5. How would this problem change if the axis was at one end?

Tip: Rotational kinetic energy depends on both the moment of inertia and angular speed, showing the influence of distribution of mass and rotational rate.

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Math Problem Analysis

Mathematical Concepts

Rotational Kinematics
Energy Conservation
Physics of Motion

Formulas

KE = (1/2) I ω²
I = (1/12) m L²
v = (L/2) ω
v = sqrt((6 * KE) / m)

Theorems

Conservation of Energy
Moment of Inertia

Suitable Grade Level

Grades 11-12