### Spring 2009 Physics 101 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 123. The exam period was 90 minutes; the mean score was 84.0; the median score was 87. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1*

This question and the following two concern the same physical situation.

From the top of a tower it takes 3.5 seconds for a ball to fall freely to the ground (see the figure).

From the top of the tower, you throw the ball vertically downward to the ground. The ball reaches the ground 2.5 seconds later. What is the initial speed of the ball?

(a)   7.4 m/s
(b)   9.6 m/s
(c)   10.7 m/s
(d)   11.8 m/s
(e)   12.9 m/s

### QUESTION 2*

If you perform the same experiment on the planet with the acceleration of gravity being 10.5 m/s2, how long does it take for the ball to fall freely to the ground (with the initial velocity being zero) and same initial height?

(a)   2.5 s
(b)   2.7 s
(c)   3.2 s
(d)   3.4 s
(e)   3.6 s

### QUESTION 3*

Let us return to the earth. From the top of the tower, as above, the ball is thrown with an initial velocity that makes an angle θ from the horizontal direction (see Figure). This time it takes 3.6 seconds for the ball to reach the horizontal ground. What can we say about the angle θ ?

(a)   θ is positive (upward).
(b)   θ is negative (downward).
(c)   not enough information

### QUESTION 4*

This question and the following two concern the same physical situation.

A person with body mass 55 kg is standing on the scale on an elevator.

The elevator starts to go up. What is the reading of the scale at this moment?

(a)   The reading of the scale is larger than 55 kg.
(b)   The reading of the scale is smaller than 55 kg.
(c)   The reading is 55 kg.

### QUESTION 5*

The reading of the scale is 57 kg. What can you say about the movement of the elevator?

(a)   The elevator is going up.
(b)   The elevator is going down.
(c)   There is not enough information.

### QUESTION 6**

The reading of the scale is 50 kg. What is the (magnitude of) acceleration of the elevator?

(a)   0.23 m/s2
(b)   0.45 m/s2
(c)   0.89 m/s2
(d)   0.98 m/s2
(e)   1.21 m/s2

### QUESTION 7*

This question and the following two concern the same physical situation.

There is a ramp that makes an angle of 35° with the horizontal ground. The top of the ramp is 3 m from the ground. There is a moat of width 26 m beyond the ramp as illustrated in the figure.

The speed of the ball reaching the top of the ramp is 5.0 m/s. What is the height of the highest point of the trajectory of the ball from the ground?

(a)   3.2 m
(b)   3.4 m
(c)   3.6 m
(d)   3.8 m
(e)   4.0 m

### QUESTION 8***

What is the speed of the ball when it reaches the ground level, if its speed is 5.0 m/s when it leaves the ramp?

(a)   5.0 m/s
(b)   6.1 m/s
(c)   7.3 m/s
(d)   8.1 m/s
(e)   9.2 m/s

### QUESTION 9***

We know the initial speed given above is insufficient for the ball to go beyond the moat. What is the minimum speed when the ball leaves the top of the ramp in order for the ball to go over the moat?

(a)   11.2 m/s
(b)   12.5 m/s
(c)   13.3 m/s
(d)   14.9 m/s
(e)   15.3 m/s

### QUESTION 10*

There is a river flowing with speed 7 km/h. A boy is in danger of drowning in the river 1.1 km from the shore. When you set out to rescue him, your boat is the closest to (i.e., 1.1 km from) the boy as illustrated in the figure.

The speed of your boat is 15 km/h relative to water. How long will it take for you to reach the boy?

(a)   2.2 min
(b)   3.3 min
(c)   4.4 min
(d)   5.5 min
(e)   6.6 min

### QUESTION 11**

Which one of the following is closest to the gravitational force between you (mass about 50 kg) and your neighbor (same mass) at a distance of about 1 meter?

(a)   10-11 N
(b)   10-9 N
(c)   10-7 N
(d)   10-5 N
(e)   10-3 N

### QUESTION 12**

This question and the following two concern the same physical situation.

Consider a one-dimensional motion of a 1.3 kg mass along the x-coordinate. The velocity v as a function of time t is graphed in the following figure.

There are moments when there is no total force acting on the mass. How many such moments are there before t = 7 sec?

(a)   1
(b)   2
(c)   3

### QUESTION 13*

What is the magnitude of the total force acting on the mass around t = 1/2 sec approximately?

(a)   6.5 N
(b)   8.5 N
(c)   13 N
(d)   21 N
(e)   26 N

### QUESTION 14**

Does the mass return to its starting point before t = 5 sec?

(b)   The mass returns to the starting point once.
(c)   The mass returns to the starting point twice.

### QUESTION 15**

This and the following question concern the same physical situation.

There is a vertical hoop of radius 1.2 m fixed on the ground. A small block of mass 0.7 kg is sliding along the frictionless inside surface of the hoop. Its speed is not sufficiently large, so the mass cannot run along the hoop to the highest point as illustrate blow.

Since the block falls off before reaching the top, its speed when it falls off the hoop cannot be larger than a certain value. Choose the correct answer from the following.

(a)   The speed cannot exceed 1.2 m/s.
(b)   The speed can be larger than 1.2 m/s but cannot exceed 2.5 m/s.
(c)   The speed can be larger than 2.5 m/s but cannot exceed 3.2 m/s.
(d)   The speed can be larger than 3.2 m/s but cannot exceed 3.5 m/s.
(e)   The speed can be larger than 3.5 m/s.

### QUESTION 16*

If the mass of the block is doubled but if its speed is the same as above, what happens? Choose an appropriate answer.

(a)   The block still falls off the hoop.
(b)   The block can complete the rotation along the hoop.
(c)   Insufficient information is supplied to answer this question.

### QUESTION 17*

This question and the following two concern the same physical situation.

A block A of mass MA = 3 kg rests on a table and is attached by a string that runs over a frictionless, massless pulley, to a second block B of mass MB = 0.75 kg (see figure). The blocks are at rest. The coefficient of static friction between block A and the table is μs = 0.3 and the coefficient of kinetic friction is μk = 0.2 .

What is the tension T in the string?

(a)   0.75 N
(b)   2.5 N
(c)   3 N
(d)   5.75 N
(e)   7.35 N

### QUESTION 18***

What is the friction force on block A?

(a)   μs MA g
(b)   μk MA g
(c)   MB g

### QUESTION 19**

The mass of block B is doubled, so that mB is now 1.5 kg. The blocks now accelerate from rest. What is the acceleration of block B?

(a)   ay = 1.44 m/s2
(b)   ay = 0.24 m/s2
(c)   ay = -0.17 m/s2
(d)   ay = -2.0 m/s2
(e)   ay = -9.8 m/s2

### QUESTION 20***

An astronaut carries a box of equipment with her from the Earth to the Moon, where the gravitational acceleration is 1/6 that on Earth. On Earth she has to push on the box with a force Fstart to get the box moving. Once the box starts moving she must apply a force Fstop to stop it over a distance of one meter. How do the forces necessary to start the box moving and to stop it compare on Earth and Moon? Assume μk = 0 and μs = 0.3 on both Earth and Moon.

(a)   Fstart(Moon)  =  Fstart(Earth)    and    Fstop(Moon)  =  Fstop(Earth)
(b)   Fstart(Moon)  =  Fstart(Earth)    and    Fstop(Moon)  <  Fstop(Earth)
(c)   Fstart(Moon)  <  Fstart(Earth)    and    Fstop(Moon)  =  Fstop(Earth)
(d)   Fstart(Moon)  <  Fstart(Earth)    and    Fstop(Moon)  <  Fstop(Earth)
(e)   Fstart(Moon)  >  Fstart(Earth)    and    Fstop(Moon)  <  Fstop(Earth)

### QUESTION 21*

This question and the following two concern the same physical situation.

Your solar powered car can manage a constant acceleration of 1 m/s2. How long will it take you to cross a two-lane road (width 10 m), starting from rest (treat the car like a point, not an extended object)?

(a)   10 s
(b)   7 s
(c)   5 s
(d)   4.5 s
(e)   3.1 s

### QUESTION 22*

Suppose the answer to the last question is T. How long would it take you to cross an eight lane road?

(a)   T
(b)   2 T
(c)   4 T

### QUESTION 23*

What is your speed when you reach the other side of the two lane road?

(a)   9.8 m/s
(b)   5 m/s
(c)   4.7 m/s
(d)   4.5 m/s
(e)   3.1 m/s

### QUESTION 24*

This and the following question concern the same physical situation.

A man pulls a group of three, rigidly connected, identical carts forward in a straight line using a rope attached to the last car (A) in the group:

Using a tension TA in the rope produces an acceleration a of the carts. If he instead attached the rope to cart C and produced the same acceleration a, then the tension in the rope TC obeys:

(a)   TA < TC
(b)   TA = TC
(c)   TA > TC

### QUESTION 25**

Given the arrangement of the rope shown in the figure, which of the following is largest in magnitude?

(a)   TA
(b)   normal force of A on B
(c)   normal force of B on A
(d)   normal force of B on C
(e)   normal force of C on B

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.