Physics 201 MWF10:20 Fall 2000 (Ford)

Exam III Chapts. 8-11 in Cutnell and Johnson 4th ed.

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

(18 pts) 1. A rock of uniform density has a volume of 0.0040 m³. When the rock is suspended from a string and totally immersed in water (density = 1000 kg/m³) the tension in the string is 30 N. If the rock is taken out of the water and suspended from the string, what is the tension in the string?

Ans. _______________


(20 pts) 2. A thin uniform rod has a length of 0.50 m and is rotating in a circle on a frictionless tabletop. The axis of rotation is perpendicular to the length of the rod at one end. The rod has an angular velocity of 0.40 rad/s and a moment of inertia about the axis of 3.0 × 10^-3 kg·m². A bug initially standing on the axis decides to crawl out to the other end of the rod. When the bug reaches the end of the rod and sits at that end its tangential speed is 0.16 m/s.

a) What is the mass of the rod?

Ans. ______________

b) What is the mass of the bug?

Ans. ______________


(20 pts) 3. A grindstone in the shape of a uniform solid disk with a radius of 0.40 m and a mass of 50.0 kg is rotating freely at a constant angular speed of 2.0 rad/s about an axis through its center. You then press an ax blade against the rim of the grindstone with a normal force of 15 N and the grindstone comes to rest in 8.0 s.

a) What is the initial kinetic energy of the grindstone, before it starts to slow down?

Ans. _______________

b) What is the angular acceleration of the grindstone as it slows down?

Ans. ______________

c) What is the coefficient of kinetic friction between the ax and the grindstone?

Ans._____________


(22 pts) 4. A uniform metal rod with mass 40 kg and length 5.0 m is attached to the wall by a frictionless hinge. A 20 kg fish is suspended by a rope attached 1.0 m from the upper end of the rod. A horizontal wire is attached between the end of the rod and the wall and holds the rod so that it makes an angle of 53° with the wall.

a) What is the tension in the wire?

Ans. _________________

b) What is the magnitude of the force the hinge exerts on the rod?

Ans. ________________


(20 pts) 5. A block of 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 120 N/m. The maximum speed of the block is 4.0 m/s.

a) What is the amplitude of the motion?

Ans. ________________

Then a 2.0 kg piece of the block breaks off when the block is at its maximum displacement from equilibrium and only the remaining 3.0 kg piece remains attached to the spring. The 2.0 kg piece that breaks off sits at rest on the surface. After the block splits,

a) What is the amplitude of the motion?

Ans. ___________________

c) What is the maximum speed of the remaining 3.0 kg piece?

Ans. ______________