EXAM #2 (70 min) PHYS 2053 SEC 0002 Fall 2008
Name PID
THE EXAM CONSISTS OF A MULTIPLE CHOICE PART AND 4 PROBLEMS.
Your grade will be calculated as the sum of points in the multiple choice part plus the highest 3 grades out of the 4 grades from the problems. ALWAYS SHOW YOUR WORK.
MULTIPLE CHOICES (40 pts): Choose the one alternative that best completes the statements or answers the question.
1. (5pts) Three books are at rest on a horizontal table, as shown in the figure. The net force on the middle book is:
A. 10.0 N upward
B. 5.0 N downward
C. 0.0N
D. 15.0 N upwards
2. (5pts) An artist wearing spiked shoes pushes two crates across her frictionless horizontal studio floor. If she exerts a horizontal 53.0 N force on the smaller crate, then the smaller crate exerts a force on the larger crate that is closest to:
A. 44 N
B. 53 N
C. 260 N
D. 120 N
3. (5pts) The system shown in the figure is released from rest, there is no friction between B and the tabletop, and all of the objects move together. What must be true about the friction force on A ?
A. It is zero
B. It acts to the left on A
C. It acts to the right on A
D. We can not tell
4. (5pts) A frictional force f provides the centripetal force as a car goes around an unbanked curve of radius R at speed V. Later, the car encounters a similar curve, except of radius 2R, and the driver continues around this curve at the same speed V. In order to make this second curve, the frictional force on the car must be equal to:
A) (1/4)f B) f C)(1/2)f D) 2f
5. (5pts) Two massless bags contain identical bricks, each brick having a mass M. Initially, each bag contains four bricks, and the bags mutually exert a gravitational attraction F1 on each other. You now take two bricks from one bag and add them to the other bag, causing the bags to attract each other with a force F2. What is the closest expression for F2 in terms of F1?
A) F2 = 3F1 B) F2 = F1 C) F2 = (3/4)F1 D) F2 = (1/4) F1
6. (5pts). Two 1.00 kg point masses a distance D apart each exert a gravitational attraction F on the other one. If 3.00 kg is now added to each mass, the gravitational attraction exerted by each would be
A) 4F B) 16F C)15F D) 9F
7. (5pts) Which of the following statements about work is or are true?
A. Negative net work done on an object always reduces the object's kinetic energy.
B. If the work done on an object by a force is zero, then either the force or the displacement must have zero magnitude.
C. If a force acts downward, it does negative work.
8. (5pts) For each of two objects with different masses, the gravitational potential energy is 85.0 J. They are released from rest and fall to the ground. Which of the following statements is or are true?
A. Both objects are released from the same height.
B. Both objects will have the same kinetic energy when they reach the ground.
C. Both objects will have the same speed when they reach the ground.
D. Both objects will reach the ground at the same time.
9. (5 EXTRA POINTS) A weight W is swung in a vertical circle at constant speed at the end of a rigid bar that pivots about the opposite end (like the spoke of a wheel). If the tension in this bar is equal to 4.00 W when the weight is at its lowest point, then the tension when the weight is at the highest point will be:
A) W B) 2W C) 3W D) 5W
Problem 1 (20 pts).
A box with mass 54 kg is at rest when a force Fa (17N) making an angle of 31o with the inclined plane. The inclined plane makes an angle of 29o with the horizontal plane. When the box is moving on the inclined plane, there is a friction force Ff of 3.8 N opposing the motion. The box was originally at a height h from the ground (h=3.4 m). Use the x axis along the incline and y axis perpendicular to the incline.
1) (5pts). Draw a free body diagram for the box
2) (5pts) Determine the x component of each force acting on the box
3) (5pts) Determine the y component of each force acting on the box
4) (5pts) What is the acceleration on the box
Problem 2 (20pts).
A 1.2 kg picture is suspended by two cables, as shown in the figure below.
1) (5pts) Draw a free body diagram of the picture
2) (5pts) Write the equilibrium condition (sum of the forces) for the x-direction (horizontal).
3) (5pts)Write the equilibrium condition (sum of the forces) for the y-direction (vertical).
4) (5pts) determine the tension on each cable
Problem 3 (20pts).
The "Giant Swing" at a county fair consists of a vertical central shaft with a number of horizontal arms attached at its upper end. Each arm supports a seat suspended from a 5 m long cable, the upper end of which is fastened to the arm at a point 3m from the central shaft.
1) (5pts) Draw a free body diagram on the (person+seat)
2) (5pts) Find the radial acceleration of the (person+seat)
3) (5pts) Find the time of one revolution of the swing if the rod supporting the seat makes an angle of 30.0° with the vertical.
4) (5pts) If the mass of the (person+seat) is 82kg, find the tension in the arm
Problem 4 (20pts).
A 1.30 kg box moves back and forth on a horizontal frictionless surface between two different springs, as shown in the accompanying figure. The box is initially pressed against the stronger spring (left spring in the figure), compressing it 3.40 cm, and then is released from rest.
1) (5pts) Find the potential energy stored initially in the strong spring
2) (5pts) Find the maximum speed of the box
3) (5pts) By how much will the box compress the weaker spring?
4) (5pts) What would the maximum speed be if initially the box is pressed against the weaker spring (right spring in the figure), compressing it 3.40 cm, and then is released from rest.