Rigid Bodies and Torque Concepts
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Questions and Answers

What defines a rigid body in the context of rotational motion?

  • An object where distances between particles are fixed (correct)
  • An object that primarily consists of fluids
  • An object that can easily deform under stress
  • An object with flexible distances between particles

What is the unit of torque in the SI system?

  • Newton (N)
  • Joule (J)
  • Newton-meter (N·m) (correct)
  • Watt (W)

Which scenario would yield zero torque?

  • Using a larger moment arm with the same force
  • Applying force at a perpendicular distance from the axis of rotation
  • Applying force directly through the axis of rotation (correct)
  • Applying equal force in the opposite direction

If a force acts through a longer moment arm, which statement is true?

<p>It produces more torque than the same force through a shorter moment arm (D)</p> Signup and view all the answers

What type of motion is described when all particles of an object have the same instantaneous angular velocity?

<p>Rotational motion (A)</p> Signup and view all the answers

In the context of torque, what does the angle ϕ represent?

<p>The angle between the force vector and the moment arm (A)</p> Signup and view all the answers

What is the relationship between torque and the direction of rotation?

<p>Clockwise torque is negative and counterclockwise torque is positive (C)</p> Signup and view all the answers

Which of the following correctly defines translational motion?

<p>Every particle in the object has the same instantaneous velocity (B)</p> Signup and view all the answers

What is the condition for an object to be in translational equilibrium?

<p>The sum of the forces on the body is zero. (A)</p> Signup and view all the answers

Which equation represents rotational equilibrium?

<p>$\Sigma \tau i = 0$ (A)</p> Signup and view all the answers

What must be true for mechanical equilibrium to occur?

<p>Both translational and rotational conditions must be satisfied. (B)</p> Signup and view all the answers

In static equilibrium, how is torque related to the forces acting on the object?

<p>The sum of torques must equal zero. (A)</p> Signup and view all the answers

For a wrench applying a force at an angle of 35° to the wrench, which of the following best describes the required force compared to a perpendicular application?

<p>More force is required due to the cosine of the angle. (A)</p> Signup and view all the answers

If a board weighing 40 N supports two children weighing 500 N and 350 N, where is the position of the lighter child if the heavier child is placed 1.5 m from the center?

<p>2 m from the center. (C)</p> Signup and view all the answers

What describes static equilibrium in terms of motion?

<p>The object remains at rest. (D)</p> Signup and view all the answers

What determines if a body is in rotational equilibrium?

<p>The sum of the torques must be zero. (A)</p> Signup and view all the answers

What is the center of gravity (CG) for an object with a symmetrical shape?

<p>At the geometric center (D)</p> Signup and view all the answers

What happens to an object when its center of gravity (CG) extends outside the area of support?

<p>It will topple (D)</p> Signup and view all the answers

How does the length of a wrench's handle affect the amount of force needed to apply torque?

<p>Longer handle requires less force (B)</p> Signup and view all the answers

What defines a stable equilibrium for an object?

<p>Movement increases its height (A)</p> Signup and view all the answers

In rotational dynamics, what is the relationship between torque, force, and distance?

<p>Torque is the product of force and distance (C)</p> Signup and view all the answers

Which condition indicates an unstable equilibrium?

<p>Any movement lowers its center of gravity (A)</p> Signup and view all the answers

Why is it easier to open a cabinet door when the doorknob is at the end?

<p>Less force is required due to increased distance (A)</p> Signup and view all the answers

What is the effect of the center of mass on an object's stability?

<p>Lower center of mass increases stability (A)</p> Signup and view all the answers

What is the primary reason a big doorknob is easier to turn than a small one?

<p>It allows for a greater torque with the same force. (A)</p> Signup and view all the answers

If a solid cylinder has a mass of 10 kg and is pivoted about a frictionless axis, what is the moment of inertia of the cylinder (I) for rotation about its center?

<p>$5 kg , m^2$ (D)</p> Signup and view all the answers

In the context of rotational motion, which formula represents the work done (W) when torque (τ) is applied?

<p>$W = τθ$ (D)</p> Signup and view all the answers

How is the total kinetic energy (K) of a rolling body expressed?

<p>K = ½ ICM ω^2 + ½ mvCM^2 (B)</p> Signup and view all the answers

If a cylindrical hoop is accelerated from rest to an angular speed of 20 rad/s in 0.40 s, what is likely the main energy expenditure during this time?

<p>Rotational kinetic energy needs to be calculated. (A)</p> Signup and view all the answers

What is the form of energy transfer during the work-energy theorem for rotational motion?

<p>$W = ΔK = ½ I ω^2 - ½ I ω_0^2$ (B)</p> Signup and view all the answers

What is the power (P) related to in rotational dynamics?

<p>Torque multiplied by angular velocity (A)</p> Signup and view all the answers

What happens to the potential energy of a solid sphere when it rolls up an inclined plane?

<p>It is transformed into rotational energy. (D)</p> Signup and view all the answers

What is the SI unit of rotational work derived from the formula W = ?

<p>N·m (D)</p> Signup and view all the answers

Which type of kinetic energy does a bowling ball have while rolling without slipping?

<p>both rotational and translational kinetic energies (A)</p> Signup and view all the answers

What units are used to express angular momentum?

<p>kg·m²/s (B)</p> Signup and view all the answers

How does net torque affect angular momentum in a rotating rigid body?

<p>It causes a change in angular momentum. (C)</p> Signup and view all the answers

What effect does increasing the moment of inertia have on the rotational speed of Earth if the polar ice caps melt?

<p>Decreases the rotational speed. (B)</p> Signup and view all the answers

Which of the following examples illustrates the conservation of angular momentum?

<p>A figure skater pulling in her arms. (B)</p> Signup and view all the answers

What happens to the direction of angular momentum based on the right-hand rule?

<p>It is directed along the axis of rotation. (C)</p> Signup and view all the answers

What is the relation between torque and angular momentum in a rotating body?

<p>Net torque causes a change in angular momentum over time. (D)</p> Signup and view all the answers

Flashcards

Rigid Body

An object where the distance between any two particles remains constant.

Translational Motion

Motion of a rigid body where every particle has the same velocity.

Rotational Motion

Motion of a rigid body around a fixed axis.

Torque

Rotational equivalent of force.

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Torque Formula

τ = rFsinϕ

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Moment Arm

Perpendicular distance from axis to force line.

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Equilibrium

A state of balance.

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Translational Equilibrium

Sum of forces on a body is zero.

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Rotational Equilibrium

Sum of torques on a body is zero.

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Center of Gravity

Average position of weight of an object.

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Center of Mass

Average position of all mass particles of object

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Stable Equilibrium

Movement raises the center of gravity.

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Unstable Equilibrium

Any movement lowers the center of gravity.

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Neutral Equilibrium

Movement does not change the center of gravity.

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Rotational Inertia (Moment of Inertia)

Resistance to changes in rotational motion.

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Moment of Inertia Formula

I = Σmr²

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Rotational Acceleration

Rate of change of angular velocity.

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Torque-Moment of Inertia-Rotation Acceleration

τ = Iα

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Rotational Work

Work done by a torque.

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Rotational Work Formula

W = τθ

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Rotational Kinetic Energy

Energy of rotational motion.

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Rotational Kinetic Energy Formula

K = ½ Iω²

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Angular Momentum

Measure of rotational inertia.

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Angular Momentum Formula

L = Iω

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Law of Conservation of Angular Momentum

If net external torque is zero, total angular momentum remains constant.

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Study Notes

Rigid Bodies, Translation, and Rotation

  • A rigid body is an object where the distance between particles remains constant.
  • Translational motion is when every particle in a rigid body has the same velocity.
  • Rotational motion is when a rigid body moves around a fixed axis.
  • Rigid body motion is a combination of translational and rotational motion.

Torque and Equilibrium

  • Torque is the rotational equivalent of force.
  • Torque is calculated with the formula τ = rFsinϕ, where τ is torque, F is force, r is the distance from the axis of rotation to the point where the force is applied, and ϕ is the angle between the force and the lever arm.
  • The perpendicular distance from the axis of rotation to the line of action of a force is called the moment arm (or lever arm).
  • Torque is positive if it rotates the body counterclockwise, and negative if it rotates the body clockwise.
  • Equilibrium is a state of balance.
  • Translational Equilibrium occurs when the sum of forces on a body is zero.
  • Rotational Equilibrium occurs when the sum of torques on a body is zero.
  • Mechanical Equilibrium is achieved when both translational and rotational equilibrium are present.
  • Static Equilibrium occurs when a body is at rest and in both translational and rotational equilibrium.

Stability and Center of Gravity

  • The center of Gravity (CG) is the average position of weight for an object.
  • The center of Mass (CM) is the average position of all particles of mass in an object.
  • For symmetrical objects, the CG is at the geometric center.
  • For irregular objects, the CG is at the heavier end.
  • For objects with varying densities, the CG is at the heaviest part.
  • If the CG of an object is above the area of support, the object will remain upright.
  • If the CG extends outside the area of support, the object will topple.
  • Unstable equilibrium occurs when any movement lowers the CG.
  • Stable equilibrium occurs when any movement raises the CG,
  • Neutral equilibrium occurs when movement does not cause a change in CG.

Rotational Dynamics

  • Rotational inertia (I) is the resistance of an object to changes in its rotational motion, also called the Moment of Inertia.
  • I = Σmr2 (where I is the Moment of Inertia, m is mass, and r is the distance from the axis of rotation).
  • Rotational acceleration (α) is the rate of change of angular velocity.
  • The relationship between torque, moment of inertia, and rotational acceleration is given by the equation: τ = Iα.
  • Rotational work (W) is done when a torque acts through an angle.
  • Rotational work can be calculated using the equation: W = τθ.
  • Rotational kinetic energy is the energy of rotational motion.
  • Rotational kinetic energy can be calculated using the equation: K = ½ Iω2.
  • Rotational Power (P) is the rate at which rotational work is done.
  • Rotational power can be calculated using the equation: P = τω.

Angular Momentum

  • Angular Momentum (L) is a measure of the rotational inertia of a body.
  • It is calculated using the equation: L = Iω (where I is the moment of inertia and ω is the angular velocity).
  • The direction of angular momentum is determined using the right-hand rule.
  • The net torque on a rotating rigid body is equal to the time rate of change of angular momentum: τ = ΔL/Δt.
  • The Conservation of Angular Momentum states that if the net external torque on a system is zero, the total angular momentum of the system remains constant.
  • The conservation of angular momentum explains phenomena like the increased spin of a figure skater when they pull their arms in, and the rotation of a spinning top.

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AP Physics Rotational Motion and Equilibrium PDF

Description

Explore the fundamental principles of rigid body motion, including translation and rotation. Understand the concept of torque, its calculation, and the conditions for equilibrium. This quiz delves into the key aspects of how forces interact with rigid bodies in motion.

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