Consider a dumbbell consisting of a light rod of length 2 m with a 5 kg monkey clinging to each end. It is rotated at 2 radians per second, about a vertical axis passing through its centre and at right angles to the rod. The tangential speed of the monkeys is 2m/s and the angular momentum of the dumbbell 20 kg(m²/s). The monkeys now crawl inwards along the rod, until they are 0.5 m from the axis. The angular speed of the dumbell as a function of the position of the monkeys (assuming there is no external torque) is 4 rad/s. a) What is the kinetic energy of the monkeys in their new position? b) What is the centrifugal force felt by the monkeys as they crawl? c) What is the work done by the monkeys as they crawl between their initital and fi nal position?
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.
Consider a dumbbell consisting of a light rod of length 2 m with a 5 kg monkey clinging to each end. It is rotated at 2 radians per second, about a vertical axis passing through its centre and at right angles to the rod. The tangential speed of the monkeys is 2m/s and the
The monkeys now crawl inwards along the rod, until they are 0.5 m from the axis. The angular speed of the dumbell as a function of the position of the monkeys (assuming there is no external torque) is 4 rad/s.
a) What is the kinetic energy of the monkeys in their new position?
b) What is the centrifugal force felt by the monkeys as they crawl?
c) What is the work done by the monkeys as they crawl between their initital and fi nal position?
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