1. A person holds a 1.42-N baseball in his hand, a distance of 2 L = 30.2 cm fro
ID: 1774579 • Letter: 1
Question
1. A person holds a 1.42-N baseball in his hand, a distance of 2L = 30.2 cm from the elbow joint, as shown in the figure. The biceps, attached at a distance of d = 2.43 cm from the elbow, exert an upward force of 13.5 N on the forearm. Consider the forearm and hand to be a uniform rod with a mass of 1.37 kg.
(a) Calculate the magnitude of the net torque acting on the forearm and hand. Use the elbow joint as the axis of rotation.
(b) If the net torque obtained in part (a) is nonzero, in which direction will the forearm and hand rotate?
2.You pull downward with a force of 24 N on a rope that passes over a disk-shaped pulley of mass 1.2 kg and radius 0.075 m. The other end of the rope is attached to a 0.63-kg mass.
(a) Is the tension in the rope the same on both sides of the pulley?
(b) Find the tension in the rope on both sides of the pulley. (two answers)
3.A circular saw blade accelerates from rest to an angular speed of 3650 rpm in 6.38 revolutions.
(a) Find the torque exerted on the saw blade, assuming it is a disk of radius 15.2 cm and mass 0.756 kg.
(b) Is the angular speed of the saw blade after 3.19 revolutions greater than, less than, or equal to 1825 rpm?
(c) Find the angular speed of the saw blade after 3.19 revolutions.
Explanation / Answer
the CM of the arm is at "L" from the elbow, and the ball is at 2L. Then
net = 1.42N * 30.2cm + 1.37kg * 9.81m/s² * 30.2cm/2 - 13.5N*2.43 cm
= 42.884 N.cm + 202.94 N.cm - 32.81 N.cm
= 213 N·cm
= 2.13 N·m
b)
Since the net torque is positive, it means the direction is clockwise (as per the figure).
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