Spring 2012
Physics 111-106
Exam 2A
April 04
Name _________________________
1. A daring downhill skier is sliding down a steep slope that makes an angle 
370 with the horizontal direction.
The mass of the fully equipped skier is m = 80 kg, the coefficient of kinetic friction for the waxed skis on the
snowy slope is = 0.15.
1. Sketch a Free Body Diagram indicating all forces acting on the skier as he accelerates down the slope. (5 pts.)
2. Calculate the magnitude of Normal Force (5 pts.)
3. Calculate the friction force (5 pts.)
4. Calculate the magnitude of his acceleration (5 pts.)
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2. A 0.1-kg steel ball is suspended from the ceiling by a light string 0.4 m long as shown in fig. A physics
instructor moves the ball to the side by an angle  with vertical and then releases it so that the ball passes through
the lowest point in the arc with a speed of 0.8 m/s.
1. Draw a Free Body Diagram of the ball when it is at the lowest point in its arc. (5 pts.)
2. Calculate the tension in the string when the ball is passing through the lowest point in its arc. (5 pts.)
3. Calculate at what height was the ball released. (5 pts.)
4. Calculate the angle  with vertical at which the ball was released. (5 pts.)
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3. A spring is held vertically with a 100-kg mass attached to one end. An additional force of magnitude 2.5.103 N
downward is able to hold the spring a distance of 10-cm from its equilibrium position.
1. Calculate the spring constant k of the spring. (5 pts.)
2. Calculate the force of the spring on mass m. (5 pts.)
3. Calculate the elastic energy in the spring at the location of 10-cm. (5 pts.)
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4. Blocks A and B in the figure each have a mass of 2 kg. The slope is at the angle  with horizontal. The
slope is frictionless.
1. Draw a Free Body Diagram for blocks A and B. (5 pts.)
2. Calculate the acceleration of block A. (5 pts.)
3. Find the change of potential energy of block B if block A falls down by 0.5 m. (5 pts.)
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5. A quarterback of mass 100 kg, running at speed 10 m/s runs directly into the arms of a 150 kg tackler moving in
the opposite direction at 5 m/s. The impact occurs in one dimension in midair and lasts for 0.2 s, after which the
players are locked together.
1. Calculate the linear momentum of each individual player. (5 pts.)
2. Calculate the velocity of the composite mass after the collision. (5 pts.)
3. Calculate the impulse imparted to the quarterback by the tackler. (5 pts.)
4. Calculate the magnitude of the average force exerted by the tackler on the quarterback. (5 pts.)
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6. A man pushes an m=20-kg box with a force of F=1000 N at an angle of 370 below horizontal. Coefficient of
friction k = 0.5. The box moves distance d = 10 m across the floor.
1. Calculate the Normal Force (5 pts)
2. Calculate the Friction force. (5 pts.)
3. Calculate the work done by friction force on the box. (5 pts.)
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7. Two identical 1.2 kg balls A and B collide head-on with each other. Velocity of ball A before the collision is 3i
and the velocity of ball B before collision is -7i . After the collision ball A has the velocity -5i.
1. Calculate the total linear momentum of the system before collision. (5 pts.)
2. Calculate the total linear momentum of the system after collision. (5 pts.)
3. Calculate the change of linear momentum of ball A during collision. (5 pts.)
4. Calculate the change of kinetic energy of the system as a result of the collision. (5 pts.)
5. Is the collision elastic? Explain. (5 pts.)
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