Skip to main content
Ch 10: Dynamics of Rotational Motion
Young & Freedman Calc - University Physics 15th Edition
Young & Freedman Calc15th EditionUniversity PhysicsISBN: 9780135159552Not the one you use?Change textbook
Chapter 10, Problem 50a

A thin uniform rod has a length of 0.500 m0.500\(\text{ m}\) and is rotating in a circle on a frictionless table. The axis of rotation is perpendicular to the length of the rod at one end and is stationary. The rod has an angular velocity of 0.400 rad/s0.400\(\text{ rad/s}\) and a moment of inertia about the axis of 3.00×103kg/m23.00\(\times\)10^{-3}\(\text{kg/m}\)^2. A bug initially standing on the rod at the axis of rotation decides to crawl out to the other end of the rod. When the bug has reached the end of the rod and sits there, its tangential speed is 0.160 m/s0.160\(\text{ m/s}\). The bug can be treated as a point mass. What is the mass of the rod.

Verified step by step guidance
1
Start by understanding the conservation of angular momentum. Since there are no external torques acting on the system, the initial angular momentum of the system (rod + bug) must equal the final angular momentum.
The initial angular momentum (L_initial) is given by the product of the moment of inertia of the rod (I_rod) and its angular velocity (ω_initial). Use the formula: L_initial = I_rod * ω_initial.
When the bug reaches the end of the rod, the system's moment of inertia changes. The final moment of inertia (I_final) is the sum of the moment of inertia of the rod and the moment of inertia of the bug treated as a point mass at the end of the rod. Use the formula: I_final = I_rod + m_bug * L^2, where L is the length of the rod.
The final angular momentum (L_final) is the product of the final moment of inertia (I_final) and the final angular velocity (ω_final). Since angular momentum is conserved, set L_initial equal to L_final: I_rod * ω_initial = (I_rod + m_bug * L^2) * ω_final.
Solve for the mass of the rod (m_rod) using the known values and the relationship between the initial and final conditions. Note that the tangential speed of the bug (v_bug) is related to the final angular velocity by the equation: v_bug = ω_final * L. Use this to find ω_final and substitute back into the angular momentum conservation equation to solve for m_rod.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
4m
Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Moment of Inertia

Moment of inertia is a measure of an object's resistance to changes in its rotation. It depends on the mass distribution relative to the axis of rotation. For a rod rotating about an axis at one end, the moment of inertia is calculated using the formula I = (1/3) * m * L^2, where m is the mass and L is the length of the rod.
Recommended video:
Guided course
11:47
Intro to Moment of Inertia

Angular Velocity

Angular velocity is the rate of change of angular position of a rotating object, typically measured in radians per second. It describes how fast the object is rotating around a fixed axis. In this scenario, the rod's angular velocity is given as 0.400 rad/s, which helps determine the rotational dynamics of the system.
Recommended video:
Guided course
06:18
Intro to Angular Momentum

Conservation of Angular Momentum

Conservation of angular momentum states that if no external torque acts on a system, its angular momentum remains constant. As the bug moves along the rod, the system's angular momentum is conserved, allowing us to relate the initial and final states to find unknown quantities like the mass of the rod.
Recommended video:
Guided course
12:12
Conservation of Angular Momentum
Related Practice
Textbook Question

A uniform, 4.5-kg, square, solid wooden gate 1.5 m on each side hangs vertically from a frictionless pivot at the center of its upper edge. A 1.1-kg raven flying horizontally at 5.0 m/s flies into this door at its center and bounces back at 2.0 m/s in the opposite direction. During the collision, why is the angular momentum conserved but not the linear momentum?

3
views
Textbook Question

A certain gyroscope precesses at a rate of 0.50 rad/s when used on earth. If it were taken to a lunar base, where the acceleration due to gravity is 0.165g, what would be its precession rate?

Textbook Question

A uniform, 4.5-kg, square, solid wooden gate 1.5 m on each side hangs vertically from a frictionless pivot at the center of its upper edge. A 1.1-kg raven flying horizontally at 5.0 m/s flies into this door at its center and bounces back at 2.0 m/s in the opposite direction. What is the angular speed of the gate just after it is struck by the unfortunate raven?

17
views
Textbook Question

A large wooden turntable in the shape of a flat uniform disk has a radius of 2.00 m and a total mass of 120 kg. The turntable is initially rotating at 3.00 rad/s about a vertical axis through its center. Suddenly, a 70.0-kg parachutist makes a soft landing on the turntable at a point near the outer edge. Compute the kinetic energy of the system before and after the parachutist lands. Why are these kinetic energies not equal?

1
views
Textbook Question

A small 10.0-g bug stands at one end of a thin uniform bar that is initially at rest on a smooth horizontal table. The other end of the bar pivots about a nail driven into the table and can rotate freely, without friction. The bar has mass 50.0 g and is 100 cm in length. The bug jumps off in the horizontal direction, perpendicular to the bar, with a speed of 20.0 cm/s relative to the table. What is the angular speed of the bar just after the frisky insect leaps?

Textbook Question

Suppose that an asteroid traveling straight toward the center of the earth were to collide with our planet at the equator and bury itself just below the surface. What would have to be the mass of this asteroid, in terms of the earth's mass M, for the day to become 25.0% longer than it presently is as a result of the collision? Assume that the asteroid is very small compared to the earth and that the earth is uniform throughout.

1
views