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I) Multiple choice (9 questions, 5 points each)

Use the following situation for the next two problems.

A block of mass m1 is attached to a cord of length L1, which is fixed at one end. The block moves in a horizontal circle on a frictionless tabletop with a period T. A second block of mass m2is attached to the first by a cord of length L2 and also moves in a circle with the same period on the same frictionless tabletop, as shown to the right. The tension in the cord of length L2 is F2. /
  1. What is the tension in the cord of length L1?

A)

B)

C)

D)

E)

  1. What is the work done by F2on the block of mass m2?

A)0

B)πF2L2

C)πF2(L1+L2)

D)2πF2L2

E)2πF2(L1+L2)

  1. While a7.6-kg box is lifted vertically at a constant speed of 3.0m/s. What is the power delivered by the person lifting the box?

A)0 W

B)2.5 W

C)23 W

D)149 W

E)224 W

A skier of mass 50 kg is moving at speed 10 m/s at point P1 down a ski slope with negligible friction. /
  1. What is the skier’s kinetic energy when she is at point P2, 20m below P1?

A)2500 J

B)9800 J

C)12300 J

D)13100 J

E)15000 J

Use the following situation for the next three problems.

A man of mass 80kg is standing on a sled, which is sliding on ice. Assume that the surface between the sled and the ice is frictionless. The mass of the sled is 200kg. At time to the center-of-mass of the man is , and the center-of-mass of the sled is . At to the man-sled system is moving to the right with a constant velocity of 3.0m/s. /
  1. What is the center-of-mass of the man-sled system at time to?

A)

B)

C)

D)

E)

  1. At time t1 the man is walking slowly to the right, and at timet2 the man is running to the left. Which relationship best describes the relative magnitude of the acceleration of the center-of-mass of the man-sled system, aCM(t)?

A)aCM(t2) >aCM(t1) > 0

B)aCM(t1) >aCM(t2) > 0

C)aCM(t2) 0 aCM(t1)

D)aCM(t2) =aCM(t1) > 0

E)aCM(t2) =aCM(t1) = 0

  1. At time t3 the man jumps off the sled to the right with a velocity of 10.0 m/s to the right relative to ground. What is the velocity of the sled after he jumps off?

A)7.0 m/s (to the left)

B)7.0 m/s (to the right)

C)0.20 m/s (to the right)

D)1.4 m/s (to the right)

E)1.4 m/s (to the left)

Use the following situation for the next two problems.

A stream of elastic glass beads, each with a mass of 0.45 g, hit a wall with a velocity of 3.3m/s to the right, and bounce back with the same speed to the left. The rate of beads hitting the wall is 94 per second.

  1. What is the average force (calculated over 1s) of the beads on the wall?

A)0.14 N

B)0.0029 N

C)0.0059 N

D)0.28 N

E)0.0044 N

  1. In case 1 the beads directly hit the wall. In this case the magnitude of the average and maximum forces, respectively, that the beads exert on the wall areFave,1 and Fmax,1. In case 2 the beads hit a thin spring attached to the wall. In this case the magnitude of the average and maximum forces, respectively, that the beads exert on the wall are Fave,2 and Fmax,2. The incoming and outgoing velocities are the same in both cases. How do the magnitudes of the average forces and maximum forces compare in the two cases?

A)Fave,1Fave,2 and Fmax,1Fmax,2

B)Fave,1Fave,2 and Fmax,1=Fmax,2

C)Fave,1=Fave,2 and Fmax,1=Fmax,2

D)Fave,1=Fave,2 and Fmax,1Fmax,2

E)Fave,1Fave,2 and Fmax,1Fmax,2

III) Lecture written response (25 points)

Show all work and explain in enough detail so the grader understands your thinking and methodology!

Use the following situation for the next three problems.

A racecar of mass m goes around a circular racetrack with radius r. The racetrack is banked at an angle of θ. The coefficient of static friction between the tires and the track is µs. The racecar driver goes around the track at the maximum speed possible without sliding off the track. /
  1. (5 points) Draw a free body diagram of the forces on the car. Write down what each force represents.
  1. (5 points) Whatis the size of the normal force in terms of µs, m, g, and θ?
  1. (5 points) What is the maximum speed in terms of µs, m, r, N(normal force), and θ?

Use the following situation for the next two problems.

A girl with mass 23kg slides down a slide that is 3.3m high. Her speed is 1.3m/s when she reaches the bottom of the slide. The bottom of the ramp is frictionless and there is a spring as shown in the diagram on the right. /
  1. (5 points) How much energy is dissipated by friction?
  1. (5 points) After she hits the spring the maximum compression of the spring is 0.2m. What is the spring constant of the spring?

PHYS 121Midterm 2Feb 14, 2013