Physics 201 MWF10:20 Fall 1997 (Ford)
Exam III Chapts. 9,10,11,13 in SZY College Physics

Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(20 pts) 1. A solid wooden sphere with mass 200 kg and density = 600 kg/m³ is held beneath the surface of a lake by a rope attached to the bottom of the lake. The density of the water in the lake is 1000 kg/m³. What is the tension in the rope?

Ans. ___1307 N___


(20 pts) 2. A uniform board 8.0 m long and weighing 90 N rests on sawhorses at A and B, as shown in the sketch. Point A is at the left-hand end of the board and B is 7.0 m to the right of A. A small boy weighing 140 N stands on the right-hand end of the board.

a) What is the upward normal force exerted on the board by the sawhorse at A?

Ans. ___18.6 N___

b) What is the upward normal force exerted on the board by the sawhorse at B?

Ans. ___211 N___


(18 pts) 3. A block on a horizontal frictionless surface is attached to a spring and set into simple harmonic motion with amplitude 0.20 m. The maximum acceleration of the block during its motion is 6.0 m/s². Let x be the displacement of the block from its equilibrium position.

a) What is the magnitude of the maximum velocity of the block during its motion?

Ans. ___1.10 m/s___

b) What is the value of x when the velocity has its maximum value?

Ans. ___x = 0___

c) How much time does it take the block to travel from x = 0.20 m to x = 0?

Ans. ___0.287 s___


(18 pts) 4. A man is standing on a platform that is free to rotate without friction. He is holding a 1.5 kg mass in each hand. The man is holding his arms outstretched and is rotating at an angular velocity of = 8 rad/s. In this position each of the 1.5 kg masses is 0.6 m from the rotation axis. The man then pulls his arms in to his sides so that each 1.5 kg mass is now 0.2 m from the rotation axis. Assume that the moment of inertia of the man without the 1.5 kg masses) plus the platform is equal to 3.4 kg·m², and remains constant.

a) What is the initial moment of inertia of the system (man plus platform plus 1.5 kg masses), before he pulls the masses in?

Ans. ___4.48 kg·m²___

b) What is the final moment of inertia of the system, after he pulls the masses in?

Ans. ___3.52 kg·m²___

c) What is the final angular velocity of rotation after the man has pulled in his arms?

Ans. ___10.2 rad/s___

d) How much work is done by the force the man applies when he pulls in the masses?

Ans. ___40 J___


(24 pts) 5. A cylinder with radius R = 0.20 m is pivoted on frictionless bearings about an axis through its center. A rope is wrapped around the cylinder, and a block with mass m = 5.0 kg is suspended from the free end of the rope. The system is released from rest and the block descends. While the block is moving the tension T in the rope is 12 N.

a) What is the angular acceleration of the cylinder?

Ans. ___37 rad/s²___

b) What is the mass M of the cylinder?

Ans. ___3.24 kg___

c) When the block has kinetic energy 16 J, what is the kinetic energy of the cylinder?

Ans. ___5.19 J___