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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 37a

A 2.00-kg rock has a horizontal velocity of magnitude 12.0 m/s when it is at point P in Fig. E10.35. At this instant, what are the magnitude and direction of its angular momentum relative to point O?

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Identify the given values: mass of the rock (m) is 2.00 kg, velocity (v) is 12.0 m/s, distance from point O to point P (r) is 8.00 m, and the angle between the line OP and the horizontal is 36.9 degrees.
Recall the formula for angular momentum (L) relative to a point: L = r * m * v * sin(θ), where θ is the angle between the position vector and the velocity vector.
Determine the angle θ between the position vector and the velocity vector. Since the velocity is horizontal and the position vector makes an angle of 36.9 degrees with the horizontal, θ = 90 degrees - 36.9 degrees.
Substitute the known values into the angular momentum formula: L = 8.00 m * 2.00 kg * 12.0 m/s * sin(53.1 degrees).
Calculate the sine of 53.1 degrees and multiply all the values to find the magnitude of the angular momentum. The direction of the angular momentum is perpendicular to the plane formed by the position vector and the velocity vector, following the right-hand rule.

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Key Concepts

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

Angular Momentum

Angular momentum is a measure of the rotational motion of an object and is given by the cross product of the position vector and the linear momentum vector. For a point mass, it is calculated as L = r × p, where r is the position vector from the point of rotation to the object, and p is the linear momentum (mass times velocity).
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Cross Product

The cross product is a vector operation used to find a vector perpendicular to two given vectors in three-dimensional space. It is essential in calculating angular momentum, as it determines the direction of the angular momentum vector. The magnitude of the cross product is given by |A × B| = |A||B|sin(θ), where θ is the angle between vectors A and B.
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Conservation of Angular Momentum

The conservation of angular momentum states that if no external torque acts on a system, the total angular momentum of the system remains constant. This principle is crucial in analyzing systems where rotational motion is involved, as it allows for the prediction of future states of the system based on initial conditions.
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Related Practice
Textbook Question

A 2.00-kg rock has a horizontal velocity of magnitude 12.0 m/s when it is at point P in Fig. E10.35. If the only force acting on the rock is its weight, what is the rate of change (magnitude and direction) of its angular momentum at this instant?

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Textbook Question

A 2.80-kg grinding wheel is in the form of a solid cylinder of radius 0.100 m. What constant torque will bring it from rest to an angular speed of 1200 rev/min in 2.5 s?

Textbook Question

A woman with mass 50 kg is standing on the rim of a large disk that is rotating at 0.80 rev/s about an axis through its center. The disk has mass 110 kg and radius 4.0 m. Calculate the magnitude of the total angular momentum of the woman–disk system. (Assume that you can treat the woman as a point.)

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Textbook Question

An airplane propeller is 2.08 m in length (from tip to tip) and has a mass of 117 kg. When the airplane's engine is first started, it applies a constant torque of 1950 Nm to the propeller, which starts from rest. What is the instantaneous power output of the motor at the instant that the propeller has turned through 5.00 revolutions?

Textbook Question

An electric motor consumes 9.00 kJ of electrical energy in 1.00 min. If one-third of this energy goes into heat and other forms of internal energy of the motor, with the rest going to the motor output, how much torque will this engine develop if you run it at 2500 rpm?

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Textbook Question

A hollow, thin-walled sphere of mass 12.0kg12.0\(\operatorname{kg}\) and diameter 48.0 cm48.0\(\text{ cm}\) is rotating about an axle through its center. The angle (in radians) through which it turns as a function of time (in seconds) is given by θ(t)=At2+Bt4θ(t) = At^2 + Bt^4, where A has numerical value 1.501.50 and B has numerical value 1.101.10. What are the units of the constants A and B?

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