Ch. 10 - Rotational Motion

Giancoli Douglas - Physics for Scientists and Engineers 5th edition

Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Not the one you use?Change textbook

Giancoli Douglas 5th edition

Ch. 10 - Rotational Motion

Problem 33b

Chapter 10, Problem 33b

Determine the net torque on the 2.0-m-long uniform beam shown in Fig. 10–56. All forces are shown. Calculate about point P at one end.

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Identify the forces acting on the beam and their respective distances from point P. Label these forces as F₁, F₂, etc., and their distances from point P as r₁, r₂, etc. (Note: The distances should be measured perpendicularly from the line of action of each force to point P).

Recall the formula for torque: τ = r × F × sin(θ), where r is the distance from the pivot point, F is the magnitude of the force, and θ is the angle between the force and the lever arm. For each force, determine the angle θ and calculate the torque contribution.

Assign a sign to each torque based on its direction. Use the convention that counterclockwise torques are positive and clockwise torques are negative.

Sum up all the individual torques to find the net torque about point P. Use the equation: τ_net = Στ = τ₁ + τ₂ + τ₃ + ...

Ensure that the units of torque are consistent (e.g., N·m) and verify that all forces and distances have been accounted for. The result will be the net torque about point P.

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Key Concepts

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

Torque

Torque is a measure of the rotational force applied to an object, calculated as the product of the force and the distance from the pivot point (lever arm). It is a vector quantity, meaning it has both magnitude and direction, and is influenced by the angle at which the force is applied. The formula for torque (τ) is τ = r × F × sin(θ), where r is the distance from the pivot, F is the force applied, and θ is the angle between the force vector and the lever arm.

Net Torque

Net torque is the sum of all individual torques acting on an object, taking into account their directions. Positive torque typically causes counterclockwise rotation, while negative torque causes clockwise rotation. To find the net torque, one must consider the contributions from all forces acting on the beam and their respective distances from the pivot point, ensuring to account for the sign of each torque based on its rotational direction.

Equilibrium

In physics, an object is in equilibrium when the net force and net torque acting on it are both zero. For a beam to be in rotational equilibrium, the sum of the torques about any point must equal zero. This principle is crucial when analyzing systems like beams, as it allows for the determination of unknown forces or torques by setting up equations based on the conditions of equilibrium.

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