Chapter Test B Continues an Object Slowing Down Has a What Accerleration
One-Dimensional Kinematics Review
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Part B: Multiple Choice
8. If an object has an acceleration of 0 m/s2, then one can be sure that the object is not ____.
| a. moving | b. changing position | c. changing velocity |
Answer: C
The object could be moving and could be at rest; however, whether moving or not, it must not have a changing velocity.
9. If car A passes car B, then car A must be ____.
- accelerating.
- accelerating at a greater rate than car B.
- moving faster than car B and accelerating more than car B.
- moving faster than car B, but not necessarily accelerating.
Answer: D
All that is necessary is that car A has a greater speed (is moving faster). If so, it will eventually catch up and pass car B. Acceleration is not necessary to overcome car B; a car going 60 mi/hr at a constant speed will eventually pass a car going 50 mi/hr at a constant speed. Surely you have witnessed that while driving down a local highway.
10. Which one of the following is NOT consistent with a car which is accelerating?
- A car is moving with an increasing speed.
- A car is moving with a decreasing speed.
- A car is moving with a high speed.
- A car is changing direction.
Answer: C
An accelerating object must be changing its velocity by either slowing down, speeding up or changing direction. Moving fast merely means that the velocity is high; it says nothing about the acceleration.
11. A fullback is running down the football field in a straight line. He starts at the 0-yard line at 0 seconds. At 1 second, he is on the 10-yard line; at 2 seconds, he is on the 20-yard line; at 3 seconds, he is on the 30-yard line; and at 4 seconds, he is on the 40-yard line. This is evidence that
- he is accelerating
- he is covering a greater distance in each consecutive second.
- he is moving with a constant speed (on average).
Answer: C
The fullback is moving 10 yards every second. He has a constant speed and thus covers the same distance (10 yd) in each consecutive second. He is not accelerating.
12. A fullback is running down the football field in a straight line. He starts at the 0-yard line at 0 seconds. At 1 second, he is on the 10-yard line; at 2 seconds, he is on the 20-yard line; at 3 seconds, he is on the 30-yard line; and at 4 seconds, he is on the 40-yard line. What is the player's acceleration?
Answer: 0 m/s/s
The fullback is moving 10 yards every second. He has a constant speed. He also is running in a straight line, so he is not changing direction. Thus, his acceleration is 0 m/s/s. Only objects with changing velocity have a nonzero acceleration.
13. Olympic gold medalist Michael Johnson runs one time around the track - 400 meters - in 38 seconds. What is his displacement? ___________ What is his average velocity? ___________
Answer: d = 0 m and v = 0 m/s
Michael finishes where he started, so he is not "out of place." His displacement is 0 meters. Since average velocity is displacement over time, his average velocity is also 0 m/s.
14. If an object is moving eastward and slowing down, then the direction of its velocity vector is ____.
| a. eastward | b. westward | c. neither | d. not enough info to tell |
Answer: A
The direction of the velocity vector is always in the same direction as the direction which the object moves.
15. If an object is moving eastward and slowing down, then the direction of its acceleration vector is ____.
| a. eastward | b. westward | c. neither | d. not enough info to tell |
Answer: B
If an object is slowing down, then the direction of the acceleration vector is in the opposite direction as the direction which the object moves. (If the object were speeding up, the acceleration would be eastward.)
16. Which one of the following quantities is NOT a vector?
| a. 10 mi/hr, east | b. 10 mi/hr/sec, west | c. 35 m/s, north | d. 20 m/s |
Answer: D
A vector has both magnitude and direction. Only choice d does not show a direction; it must be a scalar.
17. Which one of the following quantities is NOT a speed?
| a. 10 mi/hr | b. 10 mi/hr/sec | c. 35 m/s | d. 20 m/s |
Answer: B
You can often tell a quantity by its units. 10 mi/hr/sec is an acceleration since there are two time units involved. In fact, the units are velocity change units (mi/hr) per time units (seconds). The quantity speed has units of distance/time.
18. Which one of the following statements is NOT true of a free-falling object? An object in a state of free fall ____.
- falls with a constant speed of -10 m/s.
- falls with a acceleration of -10 m/s/s.
- falls under the sole influence of gravity.
- falls with downward acceleration which has a constant magnitude.
Answer: A
A free-falling object is an object upon which the only force is gravity. As it falls, it accelerates at a rate of approx. 10 m/s/s. This acceleration value is constant during the entire trajectory of the motion. Since this is the case, the speed can not be constant.
19. The average speed of an object which moves 10 kilometers (km) in 30 minutes is ____.
| a. 10 km/hr | b. 20 km/hr | c. 30 km/hr | d. more than 30 km/hr |
Answer: B
The average speed is distance/time. In this case the distance is 10 km and the time is 0.5 hr (30 minutes). Thus
20. What is the acceleration of a car that maintains a constant velocity of 55 mi/hr for 10.0 seconds?
| a. 0 | b. 5.5 mi /hr/s | c. 5.5 mi /s/s | d. 550 mi/hr/s |
Answer: A
If the velocity is constant, then there is no acceleration. That is, the value of the acceleration is 0.
21. As an object freely falls, its ____.
| a. speed increases | b. acceleration increases |
| c. both of these | d. none of these |
Answer: A
As an object falls, it accelerates; this means that the speed will be changing. While falling, the speed increases by 10 m/s every second. The acceleration is a constant value of 10 m/s/s; thus, choice b should not be chosen.
22. A speedometer is placed upon a free-falling object in order to measure its instantaneous speed during the course of its fall. Its speed reading (neglecting air resistance) would increase each second by ____.
| a. about 5 m/s | b. about 10 m/s | c. about 15 m/s |
| d. a variable amount | e. depends on its initial speed. |
Answer: B
The acceleration of gravity is approximately 10 m/s/s. Acceleration represents the rate at which the velocity changes - in this case, the velocity changes by 10 m/s every second. So the speed will increase by the amount of 10 m/s every second.
23. Ten seconds after being dropped from rest, a free-falling object will be moving with a speed of ____.
| a. about 10 m/s. | b. about 50 m/s. | c. about 100 m/s. | d. more than 100 m/s. |
Answer: C
Since the speed of a free-falling object increases by 10 m/s every second, the speed after ten of these seconds will be 100 m/s. You could use the kinematic equation
where vi=0 m/s and a = -10 m/s/s and t=10 s
24. A baseball pitcher delivers a fast ball. During the throw, the speed of the ball increases from 0 to 30.0 m/s over a time of 0.100 seconds. The average acceleration of the baseball is ____ m/s2.
| a. 3.00 | b. 30.0 | c. 300. | d. 3000 | e. none of these |
Answer: C
Acceleration is velocity change over time. In this problem, the velocity change is +30.0 m/s and the time is 0.100 s. Thus,
25. On takeoff, a rocket accelerates from rest at a rate of 50.0 m/s2 for exactly 1 minute. The rocket's speed after this minute of steady acceleration will be ____ m/s.
| a. 50.0 | b. 500. | c. 3.00 x 103 | d. 3.60 x 103 | e. none of these |
Answer: C
Use the equation
vf = 0 + (50.0 m/s/s)*(60.0 s) = 3.00 x 103 m/s
(Note that the unit on time must be the same as the time units for which the acceleration is given.)
26. When a rock is dropped, it will accelerate downward at a rate of 9.8 m/s2. If the same rock is thrown downward (instead of being dropped from rest), it acceleration will be ____. (Ignore air resistance effects.)
| a. less than 9.8 m/s2 | b. 9.8 m/s2 | c. more than 9.8 m/s2 |
Answer: B
Whether rising or falling, if the sole force acting upon the object is gravity, then the acceleration is 9.8 m/s/s (often approximated as 10 m/s/s).
27. Consider drops of water that leak from a dripping faucet at a constant rate. As the drops fall they ____.
| a. get closer together | b. get farther apart | |
| c. remain at a relatively fixed distance from one another | ||
Answer: B
Since the drops of water are falling (and probably free-falling), they should be getting farther apart as they fall. This is because the free-falling drops are accelerating and thus gaining speed.
28. Renatta Oyle is again found driving her '86 Yugo down Lake Avenue, leaving the following trail of oil drops on the pavement.
If her car is moving from right to left, then ...
- her velocity has a rightward direction and her acceleration has a rightward direction.
- her velocity has a rightward direction and her acceleration has a leftward direction.
- her velocity has a leftward direction and her acceleration has a rightward direction.
- her velocity has a leftward direction and her acceleration has a leftward direction.
Answer: D
The car is heading leftward and the velocity is always in the same direction as the direction which the object moves. Since the car is speeding up, the acceleration is leftward. Whenever an object speeds up, its acceleration is in the same direction which the object moves. Whenever an object slows down, its acceleration is in the opposite direction which the object moves.
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