Physics 201 MWF10:20 Fall 2001 (Ford)

Exam III Chapts. 8-11 in Cutnell and Johnson 5th ed.

Multiple choice questions. Circle the correct answer. No work need be shown and no partial credit will be given.

(3 pts) 1. A uniform solid sphere and a hollow spherical shell have the same mass and radius. They are rolling on the floor with the same linear speed. Which statement is true:

a) the kinetic energy of the solid sphere is greater than the kinetic energy of the hollow sphere
b) the kinetic energy of the hollow sphere is greater than the kinetic energy of the solid sphere
c) the two spheres have the same kinetic energy


(3 pts) 2. An object is moving in simple harmonic motion on the end of a spring. The period of the motion is 0.6 s and the amplitude is 0.2 m. The time it takes the object to move from x =0.2 m to x = -0.2 m is

a) 0.6 s
b) 0.3 s
c) 0.15 s
d) 1.2 s


(3 pts) 3. A block of wood is floating in a pail of water with 70% of its volume submerged. The weight of the water displaced by the floating block is

a) equal to the weight of the block
b) less than the weight of the block
c) greater than the weight of the block


On the following problems show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.

(18 pts) 4. A large horizontal platform with radius 3 m rotates without friction about an axle through its center. The platform has moment of inertia I = 120 kg·m² and is initially rotating with an angular velocity of 6.0 rad/s. A bag of sand is then dropped onto the platform at its rim, a distance of 3 m from the axle. Assume that the bag of sand can be treated as a point mass. After the bag has come to rest relative to the platform, the tangential speed of the bag is 15 m/s. What is the mass of the bag of sand?

Ans. ________________


(20 pts) 5. A uniform solid disk of radius 0.2 m is free to rotate about an axis through its center. The disk has moment of inertia I = 80 kg·m² for rotation about this axis. The disk is initially at rest. At t = 0, a constant tangential force of 160 N is applied to the rim of the disk. After the force has been applied for 5.0 s, what is

a) the tangential acceleration of a point on the rim of the disk?

Ans. _______________

b) the tangential velocity of a point on the rim of the disk?

Ans. ______________


(18 pts) 6. A uniform board with length 10 m is leaning against a smooth vertical wall and makes an angle of 53° with the floor. There is no friction exerted on the board by the wall, but there is friction at the floor. The board weighs 80 N. What is the magnitude of the friction force exerted on the board by the floor?

Ans. ______________


(17 pts) 7. A block with mass 5.0 kg is attached to a horizontal spring and oscillates back and forth on a frictionless horizontal surface. The spring has spring constant 200 N/m and the amplitude of the motion is 0.30 m.

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

Ans. _______________

b) At the point where the block has its maximum speed it suddenly splits into two identical pieces and only one piece remains attached to the spring. Just after the block splits the speed of the piece that remains attached to the spring is the same as it was just before the block split. What is the amplitude of the simple harmonic motion that exists after the block splits?

Ans. _______________


(18 pts) 8. When a rock of uniform density is suspended in air by a string, the tension in the string is 60 N. When the rock is totally immersed in water (density 1000 kg/m³), the tension in the string is 52 N.

a) What is the density of the rock?

Ans. ___________________

b) When the rock is totally immersed in alcohol (density 800 kg/m³), what is the tension in the string?

Ans. __________________