1. Name: __________________________ Physics 231 Midterm I 2054
Circle the letter of the most correct answer. Each questions is worth 3 points.
1. The average speed of a moving object during a given interval of
time is always: 7. A heavy ball is suspended as shown. A quick jerk on the lower
A) the magnitude of its average velocity over the interval string will break that string but a slow pull on the lower string will
B) the distance covered during the time interval divided by the break the upper string. The first result occurs because:
time interval
C) one-half its speed at the end of the interval A) the force is too small to move the ball
D) the magnitude of its acceleration multiplied by the time B) action and reaction is operating
interval C) the ball has inertia
E) one-half the magnitude of its acceleration multiplied by the D) air friction holds the ball back
time interval. E) the ball has too much energy
2. A car starts from Hither, goes 50 km in a straight line to Yon, 8. A car travels east at constant velocity. The net force on the car is:
immediately turns around, and returns to Hither. The time for this A) east
round trip is 2 hours. The magnitude of the average velocity of the B) west
car for this round trip is: C) up
A) 0 D) down
B) 50 km/hr E) zero
C) 100 km/hr
D) 200 km/hr 9. The “reaction” force does not cancel the “action” force because:
E) cannot be calculated without knowing the acceleration A) the action force is greater than the reaction force
B) they are on different bodies
3. Of the following situations, which one is impossible? C) they are in the same direction
A) A body having velocity east and acceleration east D) the reaction force exists only after the action force is
B) A body having velocity east and acceleration west removed
C) A body having zero velocity and non-zero acceleration E) the reaction force is greater than the action force
D) A body having constant acceleration and variable velocity
E) A body having constant velocity and variable acceleration 10. An object is shot vertically upward. While it is rising:
A) its velocity and acceleration are both upward
B) its velocity is upward and its acceleration is downward
4. The diagram shows a C) its velocity and acceleration are both downward
velocity-time graph for a car D) its velocity is downward and its acceleration is upward
moving in a straight line. At E) its velocity and acceleration are both decreasing
point P the car must be:
11. A vector of magnitude 3 CANNOT be added to a vector of
A) moving with zero magnitude 4 so that the magnitude of the resultant is:
acceleration A) zero
B) climbing the hill B) 1
C) accelerating C) 3
D) stationary D) 5
E) moving at about 45º with respect to the x axis E) 7
5. Acceleration is always in the direction: r r
A) of the displacement 12.
r r A and B
Vectors lie in the xy plane. We can deduce that
B) of the initial velocity
C) of the final velocity
A = B if:
D) of the net force A) Ax2 + Ay2 = Bx2 + By2
E) opposite to the frictional force B) Ax + Ay = Bx + By
C) Ax = Bx and Ay = By
D) Ay/Ax = By/Bx
6. Which of the following E) Ax = Ay and Bx = By
five acceleration
versus time graphs is r
correct for an object
moving in a straight
13. The vector − A is: r
line at a constant A) greater than Ainmagnitude
velocity of 20 m/s? r
B) less than Ainmagnitude
A) A r
B) B C) in the same direction as A
C) C r
D) D D) in the direction opposite to A
E) E r
E) perpendicular to A

2. r r 18. Two bodies are falling with negligible air resistance, side by side,
14. A vector S of magnitude 6 and another vector T have a sum of above a horizontal plane. If one of the bodies is given an
r additional horizontal acceleration during its descent, it:
magnitude 12. The vector T : A) strikes the plane at the same time as the other body
A) must have a magnitude of at least 6 but no more than 18 B) strikes the plane earlier than the other body
B) may have a magnitude of 20 C) has the vertical component of its velocity altered
C) cannot have a magnitude greater than 12
r D) has the vertical component of its acceleration altered
D) must be perpendicular to S E) follows a straight line path along the resultant acceleration
vector
E) must be perpendicular to the vector sum
15. Mass differs from weight in that:
A) all objects have weight but some lack mass 19. Which of the curves on the graph
B) weight is a force and mass is not below best represents the vertical
C) the mass of an object is always more than its weight component vy of the velocity
D) mass can be expressed only in the metric system versus the time t for a projectile
E) there is no difference fired at an angle of 45o above the
horizontal?
16. Which of the following is a scalar quantity?
A) Speed A) OC
B) Velocity B) DE
C) Displacement C) AB
D) Acceleration D) AE
E) None of these E) AF
17. Which of the following is NOT an example of accelerated
motion? 20. An object moves in a circle. If the radius is doubled keeping the
A) Vertical component of projectile motion speed the same then the magnitude of the centripetal force must
B) Circular motion at constant speed be:
C) A swinging pendulum A) twice as great
D) Earth's motion about sun B) half as great
E) Horizontal component of projectile motion C) four times as great
D) one-fourth as great
E) the same
You must show your work to receive credit for the following problems. Each problem is worth 10 points
21. At time t = 0 a car has a velocity of (16 m/s) î. It slows down with an acceleration given by (– 0.50 m/s3)t î. How much time
t does it take to stop the car?
22. At a stop light, a truck traveling at 15 m/s passes a car as it starts from rest. The truck travels at constant velocity and the car
accelerates at 3 m/s2. How much time will it take for the car to catch up to the truck?
23. An object is thrown vertically upward at 35 m/s. Taking the magnitude of the local gravitational strength to be g = 10 N/kg,
the velocity of the object 5 s later is:

3. 24. Three books (X, Y, and Z) rest on a table. The gravitational force on each book is indicated. The force of book Z on book Y
is:
25. A 5-kg concrete block is lowered with a downward acceleration of 2.8 m/s2 by means of a rope. The force of the block on
the rope is (magnitude and direction)?
26. When a certain force is applied to the standard kilogram its acceleration is 5.0 m/s2. When the same force is applied to
another object its acceleration is one-fifth as much. The mass of the object is:
r r r r
27. If A = (2 m) ˆ − (3m) ˆ and B = (1m) ˆ − (2 m) ˆ , then A − 2 B =
i j i j
r
28. The angle between A = (25 m) ˆ + (45 m) ˆ and the positive x axis is:
i j

4. 29. A vector has a magnitude of 12. When its tail is at the origin it lies between the positive x axis and the negative y axis and
makes an angle of 30º with the x axis. Its y component is:
30. A particle goes from x = –2 m, y = 3 m, z = 1 m to x = 3 m, y = –1 m, z = 4 m. Its displacement is:
31. One end of a 1.0-m long string is fixed, the other end is attached to a 2.0-kg stone. The stone swings in a vertical circle,
passing the bottom point at 4.0 m/s. The force of the string on the stone at this point is about:
32. The airplane shown is in level flight at an altitude of 0.50 km and a speed of 150 km/h. At what distance d should it release a
heavy package to hit the target X? Take the magnitude of the local gravitational strength to be 10 N/kg2.

5. Answer Key
1. B
2. A
3. E
4. C
5. D
6. E
7. C
8. E
9. B
10. B
11. A
12. C
13. D
14. A
15. B
16. A
17. E
18. A
19. D
20. B