A ball rolling on a circular track, starting from rest, has angular acceleration . Find an expression, in terms of , for the time at which the ball's acceleration vector a is away from a radial line toward the center of the circle.
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12. Rotational Kinematics
Rolling Motion (Free Wheels)
Problem 96a
Textbook Question
A solid uniform disk of mass 21.0 kg and radius 85.0 cm is at rest flat on a frictionless surface. Figure 10–76 shows a view from above. A string is wrapped around the rim of the disk and a constant force of 35.0 N is applied to the string. The string does not slip on the rim. In what direction does the cm move?


1
Step 1: Identify the forces acting on the disk. The force applied to the string is 35.0 N, and it is tangential to the rim of the disk. Since the surface is frictionless, there is no opposing force to the motion of the disk's center of mass (cm).
Step 2: Recognize that the applied force causes both translational motion of the center of mass and rotational motion of the disk. The translational motion is due to the net force acting on the disk, while the rotational motion is due to the torque generated by the force applied at the rim.
Step 3: Calculate the torque (τ) about the center of the disk using the formula τ = r × F, where r is the radius of the disk and F is the applied force. Here, r = 85.0 cm = 0.85 m and F = 35.0 N. The torque will cause the disk to rotate in the clockwise direction.
Step 4: Determine the direction of the center of mass (cm) motion. Since the force is applied tangentially to the rim and there is no friction to resist motion, the center of mass will move in the direction of the applied force, which is horizontally to the right as shown in the figure.
Step 5: Note that the rotational motion does not affect the direction of the center of mass motion. The center of mass moves purely due to the net external force applied, which is directed to the right.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Newton's Second Law of Motion
Newton's Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This relationship is expressed by the equation F = ma, where F is the net force, m is the mass, and a is the acceleration. In this scenario, the applied force will cause the disk to accelerate in the direction of the force.
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Intro to Forces & Newton's Second Law
Torque and Rotational Motion
Torque is the rotational equivalent of linear force and is calculated as the product of the force applied and the distance from the pivot point (radius in this case). For the disk, the force applied to the string creates a torque that causes the disk to rotate. The direction of the torque will influence the rotational motion and the resulting linear motion of the center of mass.
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Guided course
Torque & Acceleration (Rotational Dynamics)
Center of Mass
The center of mass of an object is the point where its mass is evenly distributed in all directions. For a uniform disk, the center of mass is at its geometric center. When a force is applied to the rim, the center of mass will move in the direction of the applied force, which is crucial for determining the overall motion of the disk on a frictionless surface.
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Intro to Center of Mass
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