Physics 201 MWF10:20 Fall 1993 (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 disk with radius R=0.50 m is pivoted on frictionless bearings about an axis through its center. A rope is wrapped around the disk, and a constant pull of 50 N is exerted on the rope. The disk starts from rest and turns through 30 revolutions in 5 seconds.

a) What is the angular acceleration of the disk?

Ans. ___15.0 rad/s²___

b) What is the moment of inertia I of the disk?

Ans. ___1.67 kg·m²___

c) How much work does the tension in the rope do during these 5 seconds?

Ans. ___4700 J___


(20 pts) 2. A target in a shooting gallery consists of a vertical square board whose sides are each 0.30 m long. The board is pivoted about a horizontal frictionless axis along its top edge; the moment of inertia of the board about this axis is 0.40 kg·m². The board is initially at rest and is struck at its center by a bullet of mass 5.00×10^-3 kg that is traveling at 300 m/s. The bullet remains embedded in the board.

a) Before the collision, what is the angular momentum of the bullet about the axis that is along the top edge of the board?

Ans. ___0.225 kg·m²/s___

b) What is the angular velocity of the board just after being struck by the bullet?

Ans. ___0.562 rad/s___

c) What is the kinetic energy of the board (with the bullet embedded in it) just after being struck by the bullet?

Ans. ___0.0632 J___


(22 pts) 3. Consider the system shown in the sketch. A uniform bar 4.0 m long and weighing 80 N is attached at one end to a wall by a hinge. The bar is held in the horizontal position by a rope that goes from the other end of the bar to the wall. This rope makes an angle of 53° with the bar. A block weighing 160 N is suspended from the bar, 1.0 m from the end. Calculate

a) the tension in the rope;

Ans. ___200 N___

b) the horizontal and vertical components of the force exerted by the hinge on the bar.

Ans. horizontal ___120 N___

vertical ___80 N___


(20 pts) 4. A rock is suspended by a string. When the rock is in air the tension in the string is 9.8 N, when the rock is immersed in water (density 1000 kg/m³) the tension in the string is 7.2 N, and when the rock is immersed in an unknown liquid the tension in the string is 4.0 N. Calculate:

a) the volume of the rock;

Ans. ___2.65×10^-4 m³___

b) the density of the unknown liquid.

Ans. ___2230 kg/m³___


(18 pts) 5. A block with mass m = 0.20 kg is on a horizontal frictionless surface and is attached to a horizontal spring. The block is displaced 0.30 m from equilibrium and released. As the block moves back and forth in simple harmonic motion its maximum speed is 4.0 m/s.

a) How much potential energy is stored in the spring when the block has its maximum displacement from equilibrium?

Ans. ___1.6 J___

b) What is the magnitude of the force that the spring exerts on the block when the block is 0.10 m from its equilibrium position?

Ans. ___3.56 N___

c) How much time does it take the block to move from x = 0.30 m to x = 0?

Ans. ___0.118 s___