Linear Motion (Lessons 1- Chapters 1- 3)

An object travels 8 meters in the first second of travel, 8 meters again during the third second. Its acceleration in meters per second is: a. 0 b. 5 c. 8 d. 10 e. More than 10.

a. 0

5. What is the acceleration of a car that maintains a constant velocity of 100 km/hr for 10 seconds? a. 0 b. 10 km/hr/s c. Both of these d. None of these

a. 0 ******************** Reasoning: Acceleration is related to change in velocity. Since the velocity remains constant, it means the acceleration is zero.

*If a projectile is fired straight up at a speed of 10 m/s, the time it takes to reach the top of its path is about: a. 1 second b. 2 seconds c. 10 seconds d. Not enough information to estimate.

a. 1 second Reasoning: At the top, its speed must go to zero. But acceleration (i.e.change of speed in every second) is 10 m/s/s. Thus its speed decreases by 10 m/s every second. Since it started with 10 m/s, it will take 1 second for its speed to go to zero and reach the top.

Galileo's use of inclined planes allowed him to effectively a. Slow down the acceleration of free fall b. Increase the acceleration beyond that of free fall. c. Eliminate the acceleration of free fall d. All of these e. None of these

A. ***** Rolling objects down the inclined plane allow him to study their motion at a lower acceleration than the free fall

The average speed of a horse that gallops a distance of 10 kilometers in a time of 30 minutes is a. 10 km/h b. 20 km/h c. 30 km/h d. More than 30 km/h

B. 20 km/h ******************* Reasoning: V(average) = (10 km) / (.5 hr) = 20 km/h

*If an object falling freely downward were somehow equipped with an odometer to measure the distance it travels, then the amount of distance it travels each succeeding second would be: a. Constant b. Less and less c. Grater than the second before.

C. Greater than the second before. ************************* Reasoning: the distance traveled each second increases due to the fact that the velocity is increasing. See page 46.

An airplane that flies at 100 km/h with a 10 km/h tailwind travels at 110 km/h relative to the ground. If it instead flies into a 10 km/h headwind, its ground speed is a. 90 km/h b. 100 km/h c. 110 km/h d. 120 km/h

a. 90 km/h

6. As an object freely falls downward, its a. Velocity increases b. Acceleration increases c. Both of these d. None of these

a. Velocity increases ******************** Reasoning: In a free fall velocity keeps increasing. However, the acceleration remains constant and is = 9.8 m/s/s.

What is the average speed of a car that travels 45 km in 3 h? a) 135 km/h b) 15 km/h c) 10 km/h d) 9.8 km/h

b) 15 km/h; v = 45 km / 3 h = 15 km/h

**Consider a ball that is thrown straight upward at the edge of a canyon with an initial velocity of 20 m/s. Three seconds later, where is it located? Take its initial position, at the edge of the canyon, to be the origin; that is, yi = 0. a) 30 m b) 15 m c) - 10 m d) - 30 m

b) 15 m y = yi + vi t + (1/2) a t2 = 0 + (20 m/s) (3 s) + (1/2) (- 10 m/s2) (3 s)2 = (0 + 60 - 45) m = 15 m **review this problem**

*Consider a car that starts at rest and accelerates at 2 m/s2 for 3 seconds. At that time, t = 3 s, how far has it gone? a) 12 m b) 9 m c) 6 m d) 3 m

b) 9 m x = xi + vi t + (1/2) a t2 = 0 + 0 + (1/2) (2 m/s2) (3 s)2 = 9 m

*An apple falls from a tree and hits the ground 5 meters below. It hits the ground with a speed of about: a. 5 m/s b. 10 m/s c. 15 m/s d. 20 m/s e. Not enough information given to estimate.

b. 10 m/s Reasoning: For the apple to fall 5 m in its fall, it must be falling for about 1 second, (read pages 46-47, and chack Table 3.3). Thus in one second, its speed increases by about 10 m/s. It hits the ground with a speed of 10 m/s.

If an object falling freely downward were somehow equipped with a speedometer on a planet where the acceleration due to gravity is 20 meters per second per second, then its speed reading would increase each second by a. 10 m/s b. 20 m/s c. 30 m/s d. 40 m/s e. Depends on its initial speed.

b. 20 m/s

If a car increases its velocity from zero to 60 km/h in 10 seconds, its acceleration is: a. 3 km/h s b. 6 km/h s c. 10 km/h s d. 60 km/h s e. 600 km/h s.

b. 6 km/h s Reasoning: acceleration = (change in V) / (time) = (60 - 0 km/h) / (10 s) = 6 km/h/s

A heavy object and a light object are dropped at the same time from rest in a vacuum. The heavier object reaches the ground: a. Sooner than the lighter object b. At the same time as the lighter object c. Later than the lighter object.

b. At the same time as the lighter object Reasoning: In a vacuum, there is no air resistance. The acceleration of the light and heavy object is the same.

10. If an object falls with constant acceleration, the velocity of the object must: a. Be constant also b. Continuously change by the same amount each second c. Continuously change by varying amounts depending on its speed d. Continuously decrease e. None of these.

b. Continuously change by the same amount each second

A ball is thrown upwards and caught when it comes back down. In the presence of air resistance, the speed with which it is caught is: a. More than the speed it had when thrown upwards. b. Less than the speed it had when thrown upwards. c. The same as the speed it had when thrown upwards.

b. Less than the speed it had when thrown upwards. Reasoning: In the presence of air resistance, the ball is going to reach its highest point which is shorter than the case when there is no air resistance. As the ball falls, its acceleration is going to be less than 9.8 m/s2. Thus at the end its final velocity will be less than the no air restsnce case.

*Drop a rock from a 5-m height and it accelerates at 10 m/s2 and strikes the ground 1 s later. Drop the same rock from a height of 2.5 m and its acceleration of fall is: a. About half b. The same c. More.

b. The same

**Consider a ball that is thrown upward at the edge of a canyon with an initial velocity of 20 m/s. Three seconds later, what is its velocity? a) 30 m/s b) 15 m/s c) - 10 m/s d) - 30 m/s

c) - 10 m/s v = vi + a t = 20 m/s + ( - 10 m/s2) (3 s) = (20 - 30) m/s = - 10 m/s

What is the average speed of a motorcycle that travels 20 m in 2 s? a) 40 m/s b) 20 m/s c) 10 m/s d) 9.8 m/s

c) 10 m/s v = 20 m / 2 s = 10 m/s

*Consider a car that starts at rest and accelerates at 2 m/s2 for 3 seconds. At that time, t = 3 s, how fast is it going? a) 12 m/s b) 9 m/s c) 6 m/s d) 3 m/s

c) 6 m/s v = vi + a t = 0 + (2 m/s2) (3 s) = 6 m/s

To measure the time needed to investigate motion, a) Aristotle used the pendulum clock which had just been invented b) Ptolemy used a sundial c) Galileo invented his own water clocks d) Newton invented the pendulum clock

c) Galileo invented his own water clocks

Kinematics is a description of motion. Motion was first well understood a) by Aristotle and the ancient Greeks b) by Ptolemy in Egypt c) by Galileo in Italy d) not until the beginning of the twentieth century

c) by Galileo in Italy

*Ten seconds after starting from rest, an object falling freely downward will have a speed of about: a. 10 m/s b. 50 m/s c. 100 m/s d. More than 100 m/s

c. 100 m/s Reasoning: Speed increases at a rate of 10 m/s (actually 9.8 m/s) every second. Thus after 10 seconds, the speed is 10 x 10 = 100 m/s.

If a rocket initially at rest accelerates at a rate of 50 m/s2 for one minute, its speed will be a. 50 m/s b. 500 m/s c. 3000 m/s d. 3600 mm/s e. None of these.

c. 3000 m/s Reasoning: Speed is increasing at the rate of 50 m/s every second. Thus in one minute (i.e. in 60 seconds) speed = (5 m/s/s) x (60 s) = 3000 m/s.

It takes 6 seconds for a stone to fall to the bottom of a mine shaft. How deep is the shaft? a. About 60 m b. About 120 m c. About 180 m d. More than 200 m

c. About 180 m Reasoning: Using d = (1/2) g t2 = (1/2)(9.8)(36) = about 180 m.

Disregarding air resistance, objects fall with constant: a. Velocity b. Speed c. Acceleration d. Distances each successive second.

c. Acceleration

The two measurements necessary for calculating average speed are a. Acceleration and time b. Velocity and time c. Distance and time d. Distance and acceleration e. Velocity and distance

c. Distance and time ******************** Reasoning: The definition of the average speed is (covered distance)/(elapsed time).

A ball is thrown upwards and caught when it comes back down. Neglecting air resistance, its speed when caught is: a. More than the speed it had when thrown upwards. b. Less than the speed it had when thrown upwards. c. The same as the speed it had when thrown upwards.

c. The same as the speed it had when thrown upwards.

*Consider a train that has an acceleration of 3 m/s2. Initially, at time t = 0, it has a velocity of vi = 10 m/s. What is its speed at t = 3 s? a) 40 m/s b) 30 m/s c) 23 m/s d) 19 m/s

d) 19 m/s v = vi + a t = 10 m/s + (3 m/s2) (3 s) = (10 + 9) m/s = 19 m/s

*A car accelerates from rest at 2 meters per second per second. What is its speed 3 seconds after the car starts moving? a. 2 m/s b. 3 m/s c. 4 m/s d. 6 m/s e. None of these.

d. 6 m/s

*In each second of fall, the distance a free falling object will fall is: a. About 5 m b. About 10 m c. The same, but no 5 m or 10 m d. Increasing e. None of these.

d. Increasing Reasoning: In the first second (after the object is dropped) the distance is = 5 m. But in each second afterwards, the distance keeps increasing. See Table 3.3 on page 47.

*While a car travels around a circular track at constant speed its: a. Acceleration is zero b. Velocity is zero c. Both of these d. None of these.

d. None of these. Reasoning: Around a circular track velocity is changing because its direction is changing. The accelartion is changing as well. Thus (d) is the answer.

*At one instant an object in free fall is moving upward at 50 meters per second. One second later its speed is about: a. 100 m/s b. 60 m/s c. 55 m/s d. 50 m/s e. 40 m/s.

e. 40 m/s. Reasoning: In a free fall, the acceleration is about 10 m/s/s. Thus the objects's speed will decrease by 10 m/s every second.