(a) Find the angular speed of a CD of radius 6.45 cm when the read head is at r = 2.2 cm and again at r = 5.8 cm. (b) An old-fashioned record player rotates at a constant angular speed, so the linear speed of the record groove moving under the detector (the stylus) changes. Find the linear speed of a 45.0-rpm record at points 2.3 cm and 5.9 cm from the center. (c) In both the CDs and phonograph records, information is recorded in a continuous spiral track. Calculate the total length of the track for a CD designed to play for 1 hour and 15 minutes.
Angular Momentum
The momentum of an object is given by multiplying its mass and velocity. Momentum is a property of any object that moves with mass. The only difference between angular momentum and linear momentum is that angular momentum deals with moving or spinning objects. A moving particle's linear momentum can be thought of as a measure of its linear motion. The force is proportional to the rate of change of linear momentum. Angular momentum is always directly proportional to mass. In rotational motion, the concept of angular momentum is often used. Since it is a conserved quantity—the total angular momentum of a closed system remains constant—it is a significant quantity in physics. To understand the concept of angular momentum first we need to understand a rigid body and its movement, a position vector that is used to specify the position of particles in space. A rigid body possesses motion it may be linear or rotational. Rotational motion plays important role in angular momentum.
Moment of a Force
The idea of moments is an important concept in physics. It arises from the fact that distance often plays an important part in the interaction of, or in determining the impact of forces on bodies. Moments are often described by their order [first, second, or higher order] based on the power to which the distance has to be raised to understand the phenomenon. Of particular note are the second-order moment of mass (Moment of Inertia) and moments of force.
In a compact disc (CD) player, as the read head moves out from the center of the
disc, the angular speed of the disc changes so that the linear speed at the position of
the head remains at a constant value of about 1.2 m/s.
(a) Find the angular speed of
a CD of radius 6.45 cm when the read head is at r = 2.2 cm and again at r = 5.8 cm.
(b) An old-fashioned record player rotates at a constant angular speed, so the linear
speed of the record groove moving under the detector (the stylus) changes. Find the
linear speed of a 45.0-rpm record at points 2.3 cm and 5.9 cm from the center.
(c) In both the CDs and phonograph records, information is recorded in a continuous spiral track. Calculate the total length of the track for a CD designed to play for 1 hour and
15 minutes.
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