### Spring 1999 Physics 101 Hour Exam 2(24 questions)

The grading button and a description of the scoring criteria are at the bottom of this page. Basic questions are marked by a single star *. More difficult questions are marked by two stars **. The most challenging questions are marked by three stars ***.

This exam consists of true-false questions worth 2 points each, traditional multiple-choice questions worth 3 points each, and enhanced multiple-choice questions worth 6 points each. The maximum possible score is 93. When the exam was given, the minimum "A" score was 82; the minimum "B" was 70; the minimum "C" was 55; the minimum "D" was 36. The mean was 65.1; the median was 68. Click here to see the formula sheet that came with the exam.

### QUESTION 1***

The driver of an empty speeding truck slams on the brakes and skids to a stop in a distance D. If the truck were carrying a load that doubled its mass, what would its stopping distance have been? (Assume the road is straight and horizontal and that the coefficient of kinetic friction is constant).

(a)   D
(b)   2D
(c)   D/2

### QUESTION 2**

A car of mass M is parked (at rest) on a hill that makes an angle of 30 degrees with the horizontal. Which one of the following statements correctly describes the magnitude f of the frictional force acting on the car.

(a)   f < Mg
(b)   f = Mg
(c)   f > Mg

### QUESTION 3**

A box of mass m = 25 kg slides down a 30° ramp (inclined plane) with an acceleration a1 = 1.4 m/s2. Determine the coefficient of kinetic friction µk between the box and the ramp.

(a)   µk = 0.17
(b)   µk = 0.22
(c)   µk = 0.29
(d)   µk = 0.34
(e)   µk = 0.41

### QUESTION 4**

If the mass of the box in the above problem were halved, which one of the following statements would best describe the magnitude of the new acceleration a2 of the box as it slid down the ramp.

(a)   a2 < a1
(b)   a2 = a1
(c)   a2 > a1

### QUESTION 5*

The change in kinetic energy of an object is always equal to the net work done on that object by all acting forces.

(T)   True
(F)   False

### QUESTION 6*

Two identical blocks, A and B, slide down two frictionless ramps that make angles of 30° and 60° respectively with the horizontal. Assuming the blocks start from rest at the same vertical height above the floor, compare the speeds of the blocks, VA and VB, when they reach the floor.

(a)   VA < VB
(b)   VA = VB
(c)   VA > VB

### QUESTION 7**

A force F = 2000 N is used to push a box of mass m = 5 kg mass up a q=30° frictionless ramp that is a length L = 12 m long. Suppose the force acts parallel to the surface of the ramp, and that the box starts from rest at ground level. The force stops pushing just as the box leaves the top of the ramp, after which the box is acted on only by gravity. What is the speed of the box VG when it hits the ground? (Hint: remember that Wnet = DKE).

(a)   VG = 98 m/s
(b)   VG = 147 m/s
(c)   VG = 139 m/s
(d)   VG = 203 m/s
(e)   VG = 188 m/s

### QUESTION 8**

Let the answer to the above problem be VG. If the both the length of the ramp L and the magnitude of the force F in the above problem were doubled, what would the new speed VG,NEW of the box be as it hit the ground ?

(a)   VG,NEW = 2VG
(b)   VG,NEW = 4VG
(c)   VG,NEW = 8VG

### QUESTION 9*

Gravity does positive work on a downward moving object.

(T)   True
(F)   False

### QUESTION 10*

A vertical spring having a spring constant of
k = 500 N/m is used to launch a 2 kg box straight up. If the compression of the spring from its equilibrium length is d = 50 cm prior to launch, what is the maximum height H above its initial position that the box will reach? (Assume that the spring is massless and returns to its equilibrium length after the launch).

(a)   1.15 m
(b)   1.87 m
(c)   2.38 m
(d)   2.73 m
(e)   3.19 m

### QUESTION 11**

In the above problem, what is the change in total potential energy DPE of the box-spring system in going from the "pre-launch" position to the "maximum height" position shown.

(a)   DPE = 1/2kd2
(b)   DPE = mgH
(c)   DPE = 0

### QUESTION 12*

By what factor should the spring constant in the above problem be increased if we want the same height H to be reached by the box but we want the initial compression d of the spring to be 5 cm rather than 50 cm?

(a)   sqrt(10)
(b)   10
(c)   100

### QUESTION 13**

A student is initially standing still on a frictionless ice rink. Her friend throws a Frisbee directly toward her. After which of the following cases will the student be sliding on the ice with the greatest speed?

(a)   The student catches and holds on to the Frisbee.
(b)   The student catches the Frisbee and then throws it back to her friend.
(c)   The student catches the Frisbee and then drops it.

### QUESTION 14*

A bullet having an initial velocity of 300 m/s in the +x direction penetrates an initially stationary pop can of mass 100 gm and emerges in the other side with a final velocity of 200 m/s in the +x direction. The velocity of the pop can after the collision is 5 m/s, also in the +x direction. Assume the pop can slides on a horizontal frictionless surface. What is the mass of the bullet?

(a)   2 gm
(b)   5 gm
(c)   12 gm
(d)   21 gm
(e)   25 gm

### QUESTION 15**

Which one of the following statements describing the above collision is true:

(a)   The momentum of the system in the x direction is conserved but the total mechanical energy of the system is not.
(b)   The total mechanical energy of the system is conserved but the momentum of the system in the x direction is not.
(c)   Both the momentum of the system in the x direction and the total mechanical energy of the system are conserved.

### QUESTION 16**

Two objects have the same momentum, but have different masses. If KEH and KEL are the kinetic energies of the heavier and the lighter object respectively then:

(a)   KEH < KEL
(b)   KEH = KEL
(c)   KEH > KEL

### QUESTION 17**

An open railroad car is rolling along a horizontal straight frictionless track during a vertically falling rain. The mass of the car increases as the rain accumulates, causing the speed of the car to:

(a)   increase
(b)   decrease
(c)   not change

### QUESTION 18**

A bomb is initially at rest when it suddenly explodes into three pieces. A 5-kg piece goes north at 6 m/s, and a 2-kg piece goes east at 20 m/s. The speed of the third piece is 25 m/s. What is the mass of the third piece?

(a)   1.2 kg
(b)   2.5 kg
(c)   3.6 kg
(d)   2.0 kg
(e)   4.8 kg

### QUESTION 19**

To test an aircraft window, a cannon is used to shoot dead chickens at it to simulate mid-air collisions with birds. All chickens have the same mass and are shot with the same velocity, but since they come from a local supermarket some chickens are frozen (hard) and some chickens are thawed (soft). Which chickens are more likely to break the window? (In other words, which chickens will exert the greatest average force on the window during the collision, assuming DP is the same for both kinds?).

(a)   Frozen chickens are more likely to break the window.
(b)   Thawed chickens are more likely to break the window.
(c)   Frozen and thawed chickens are equally likely to break the window.

### QUESTION 20**

A kid ties a rock of mass m to the end of a string and spins it in a circular path in the vertical plane. The speed of the rock is constant. When the rock is at the bottom of the circle, which of the following is true about the tension T in the string.

(a)   T = mg
(b)   T < mg
(c)   T > mg

### QUESTION 21*

Suppose the mass of the rock is 0.65 kg, the speed of the rock is 4 m/s, and the radius of the circle is 0.5 m. What is the tension in the string when the rock is at the bottom of the circle?

(a)   T = 19.8 N
(b)   T = 12.1 N
(c)   T = 27.2 N
(d)   T = 6.17 N
(e)   T = 4.21 N

### QUESTION 22*

What is the angular velocity of the rock in the above problem?

### QUESTION 23***

Suppose a car accelerates from 0 to 60 miles per hour along a straight level road heading toward the north, and suppose also that this is done without the wheels slipping on the road. During this acceleration there is a static frictional force by the road on the car that points north.

(T)   True
(F)   False

### QUESTION 24*

A car drives around a flat circular track that has a radius of R = 100 m. If the coefficient of static friction between the wheels and the surface of the road is µs = 0.15, what is the maximum constant speed vmax that the car can have without starting to slip?

(a)   vmax = 9.81m/s
(b)   vmax = 12.1 m/s
(c)   vmax = 16.8 m/s
(d)   vmax = 21.2 m/s
(e)   vmax = 27.3 m/s

Scoring of True-False (T,F) Questions:
• If you mark the correct answer, you get 2 points.
• If you mark the wrong answer or if you mark neither answer, you get 0 points.
Scoring of Multiple Choice I (a,b,c) Questions:
• If you mark the correct answer, you get 3 points.
• If you mark a wrong answer or if you mark none of the answers, you get 0 points.
Scoring of Multiple Choice II (a,b,c,d,e) Questions:
• If you mark one answer and it is the correct answer, you get 6 points.
• If you mark two answers and one of them is the correct answer, you get 3 points.
• If you mark three answers and one of them is the correct answer, you get 2 points.
• If you do not mark the correct answer, or if you mark more than three answers, or if you mark none of the answers, you get 0 points.