### Spring 2009 Physics 211 Hour Exam 1(25 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 ***.

True-false questions are worth 2 points each, three-choice multiple choice questions are worth 3 points each, five-choice multiple choice questions are worth 6 points each. The maximum possible score is 112. The exam period was 90 minute; the mean score was 97.6; the median score was 103. Click here to see the formula sheet that came with the exam.

Unless otherwise stated, ignore air resistance and assume the acceleration of gravity is g = 9.81 m/s2 vertically downward.

### QUESTION 1*

This question and the next three are about the following situation.

Two stones are released from rest at a height of 600 m, one 3 s after the other, and fall vertically downward.

During the time interval that both stones are in flight, the difference in their speeds will

(a)   increase.
(b)   decrease.
(c)   stay the same.

### QUESTION 2**

During the time interval that both stones are in flight, the difference in their heights will

(a)   increase.
(b)   decrease.
(c)   stay the same.

### QUESTION 3*

How long after the first stone hits the ground will the second stone hit the ground?

(a)   Δt < 3.0 s
(b)   Δt = 3.0 s
(c)   Δt > 3.0 s

### QUESTION 4*

What will be the height separation of the two stones 2.0 s after the second stone has been released?

(a)   18.7 m
(b)   31.4 m
(c)   53.8 m
(d)   103 m
(e)   165 m

### QUESTION 5*

This and the next question are about the following situation.

A block of mass M = 7.3 kg is supported on a frictionless ramp by a spring having a constant k = 106 N/m. When the ramp is horizontal, as in view a) below, the equilibrium position of the mass is at x = 0. The angle of the ramp is then changed to 20°, as in view b) below.

What is the new equilibrium position of the block, x1?

(a)   0.11 m
(b)   0.13 m
(c)   0.20 m
(d)   0.23 m
(e)   0.28 m

### QUESTION 6*

If the block is pulled further down the ramp, to a position x2 = 0.6 m, and released from rest, what is the magnitude of the net force acting on the block immediately after its release?

(a)   17.6 N
(b)   25.4 N
(c)   33.7 N
(d)   39.1 N
(e)   47.6 N

### QUESTION 7*

This question and the next two are about the following situation.

A space telescope of mass mt = 30,000 kg is in a stable orbit above the Earth at an altitude h = 2320 km. The radius and mass of the Earth are RE = 6370 km and ME = 6 × 1024 kg, respectively. Newton's gravitational constant is GN = 6.672 × 10-11 N m2/kg2.

Once in orbit, what is the force of gravitational attraction on the space telescope due to the Earth?

(a)   103,000 N
(b)   127,000 N
(c)   159,000 N
(d)   177,000 N
(e)   196,000 N

### QUESTION 8*

If the force of gravitational attraction on the space telescope due to the Earth is denoted F, then which one of these expresses the tangential speed v of the telescope in its orbit?

(a)
(b)
(c)

### QUESTION 9*

If the space telescope is moved to an orbit with h = 1300 km, the new period of revolution would be

(a)   shorter.
(b)   longer.
(c)   the same.

### QUESTION 10*

A boat is traveling directly across a river (as seen by an observer standing on the shore) that flows at a uniform rate of vr,g = 8 ft/s, as shown in the figure. To compensate for the flow of the river, the boat must head upstream as it travels. The speed of the boat is 22 ft/s with respect to the water. What is the angle (in degrees) between the direction the boat points and the direction it is traveling with respect to shore?

(a)   θ = 17.6°
(b)   θ = 21.3°
(c)   θ = 27.8°
(d)   θ = 31.2°
(e)   θ = 34.5°

### QUESTION 11*

This and the next question are about the following situation.

A physics 211 student is trying to pull a box of books of mass m = 80 kg across the floor as shown in the following figure. The coefficient of static friction is μS.

If the angle 0 < θ < 90°, the normal force, N is:

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

### QUESTION 12**

If the angle θ = 36.9° and the coefficient of friction μS = 0.4, what is the minimum tension T required to move the box?

(a)   T = 187 N
(b)   T = 212 N
(c)   T = 243 N
(d)   T = 288 N
(e)   T = 302 N

### QUESTION 13*

A flight from Chicago to Munich, Germany takes 8 hrs and 15 minutes while the return flight is 10 hrs and 45 minutes. The distance between Chicago and Munich is 7318 km. What is the average speed of the tail wind experienced by the plane on the flight to Munich? (Assume that the wind is blowing the same speed, from west to east, on both flights, and that the plane travels directly eastward to Munich and directly westward to Chicago).

(a)   66 km/hr
(b)   73 km/hr
(c)   89 km/hr
(d)   97 km/hr
(e)   103 km/hr

### QUESTION 14*

This and the next question are about the following situation.

Ms. Stick hangs a picture of her family on the wall. The picture has mass m = 1.50 kg and hangs from two massless strings which are fixed to a common point on the wall. The picture hangs so that θ1 = θ2 = 20°.

What is the tension T1?

(a)   21.5 N
(b)   26.4 N
(c)   29.8 N
(d)   32.5 N
(e)   34.0 N

### QUESTION 15*

The next time Ms. Stick hangs a picture with the same mass, she decides to decrease the angles. What happens to the tension in the strings when θ1 and θ2 are reduced from 20° to 10°?

(a)   The tension decreases.
(b)   The tension stays the same.
(c)   The tension increases.

### QUESTION 16**

This question and the next three are about the following situation.

Two boxes are pulled across a surface by a falling weight as shown below. The coefficients of static and kinetic friction between the boxes and the surface are μS and μK, respectively. Neglect the masses of the ropes and the pulley.

For the frictionless case (μS = μK = 0) what is the relationship between T1 and T2?

(a)   T1 > T2
(b)   T1 = T2
(c)   T1 < T2

### QUESTION 17**

What is the acceleration, a, of the falling weight in the frictionless case (μS = μK = 0)?

(a)   a = g / 6
(b)   a = g / 5
(c)   a = g / 4
(d)   a = g / 3
(e)   a = g / 2

### QUESTION 18*

If the blocks are sliding, but μK = 0.2, what is the new acceleration af relative to the acceleration a in the frictionless case?

(a)   af > a
(b)   af = a
(c)   af < a

### QUESTION 19*

Suppose the blocks are stationary. What is the minimum coefficient of static friction, μS, needed to keep them from moving?

(a)   μS = 0.10
(b)   μS = 0.20
(c)   μS = 0.30
(d)   μS = 0.40
(e)   μS = 0.50

### QUESTION 20*

This question and the next two are about the following situation.

A tennis ball of weight W = 0.3 N is attached to a rope and swung in a vertical circle. The rope is L = 1.4 m in length. When the ball is at its highest point, the tension in the rope is measured to be zero.

What is the tangential velocity of the ball at its highest point?

(a)   1.9 m/s
(b)   2.3 m/s
(c)   3.7 m/s
(d)   4.6 m/s
(e)   5.8 m/s

### QUESTION 21*

Which one of the following is a valid expression for the tension in the string when the ball is at the lowest point (where v is the speed at the lowest point)?

(a)   W
(b)   W (1+v2/gL)
(c)   W (1-v2/gL)

### QUESTION 22*

If the string breaks, the only acceleration the ball will experience is downward with magnitude g.

(T)   True
(F)   False

### QUESTION 23*

This question and the next two are about the following situation.

Tiger Woods hit a "hole-in-one" on the first hole of the PGA Open golf tournament (held in Flatlands, Illinois). This golf course is perfectly flat. Tiger hit the golf ball such that it initially made a 45° angle with respect to the horizontal. There was no wind during the shot. The golf ball fell directly into the hole, a distance of 187.7 m away! In the following two questions, neglect the effect(s) of air resistance.

What was the golf ball's initial speed?

(a)   28.7 m/s
(b)   36.4 m/s
(c)   42.9 m/s
(d)   55.3 m/s
(e)   61.9 m/s

### QUESTION 24*

What was the maximum height attained by Tiger Wood's "hole-in-one" golf ball?

(a)   46.9 m
(b)   53.9 m
(c)   58.0 m
(d)   63.2 m
(e)   69.7 m

### QUESTION 25*

For the same initial speed, if the angle were decreased to 40°, the golf ball would travel further before landing.

(T)   True
(F)   False

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.