PHYSICS 2015-16September 16, 2015

Today’s Agenda (Day 22)

1. Housekeeping:

2. Homework Check: Practice Problems 3.2

3. Class Activity:Review PPT – cont’d: a) Section 3.2 - Motion with Constant Acceleration

b) Section 3.3 – Free Fall (if time permits)

HOMEWORK: Complete Practice Problems 3.3 – see p. 4 of document below With a PARTNER, complete the Ch 3 Assessment Questions – see p. 5 of

document below

REMINDERS: Practice Problems 3.3 Wednesday, September 16, 2015 Ch 3 Assessment Problems Thursday, September 17, 2015 Chapter 3 Test Friday, September 18, 2015 – MAY BE POSTPONED

PHYSICS 2015-16

PRACTICE PROBLEMS 3.2Instructions: Read the word problems below. Please show work for deriving the solution to each problem.

1. The graph in Figure 13 below describes the motion of two bicyclists, Akiko and Brian, who start from rest and travel north, increasing their speed with a constant acceleration. What was the total displacement of each bicyclist during the time shown for each?

Hint: Use the area of a triangle: area = (1/2)(base)(height).

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50123456789

Figure 13 (p. 70)

Series2 Series4

Time (s)

Velo

ity (m

/s)

2. The motion of two people, Carlos and Diana, moving south along a straight path is described by the graph in Figure 14 below. What is the total displacement of each person during the 4.0-s interval shown on the graph?

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.51

1.52

2.53

3.54

4.5

Figure 14 (p. 70)

Series2 Series4

Time (s)

Velo

city

(m/s

)

3. A skateboarder is moving at a constant speed of 175 m/s when she starts up an incline that causes her to slow down with a constant acceleration of -0.20 m/s2. How much time

passes from when she begins to slow down until she begins to move back down the incline?

4. A race car travels on a straight racetrack with a forward velocity of 44 m/s and slows at a constant rate to a velocity of 22 m/s over 11 s. How far does it move during this time?

5. A car accelerates at a constant rate from 15 m/s to 25 m/s while it travels a distance of 125 m. How long does it take to achieve the final speed?

6. A bike rider pedals with constant acceleration to reach a velocity of 7.5 m/s north over at time of 4.5 s. During the period of acceleration, the bike’s displacement is 19 m north. What was the initial velocity of the bike?

7. A car with an initial velocity of 24.5 m/s east has an acceleration of 4.2 m/s2 west. What is its displacement at the moment that its velocity is 18.3 m/s east?

8. Sunee is training for 5.0-km race. She starts out her training run by moving at a constant pace of 4.3 m/s for 19 min. Then she accelerates at a constant rate until she crosses the finish line 19.4 s later. What is her acceleration during the last portion of the training run?

CHALLENGE QUESTIONS:1. A car, just pulling onto a straight stretch of highway, has a constant acceleration from 0

m/s to 25 m/s west in 12 s.a) Draw a v-t graph of the car’s motion.b) Use the graph to determine the car’s displacement during the 12.0 –s time interval.c) Another car is traveling along the same stretch of highway. It travels the same distance

in the same time as the first car, but its velocity is constant. Draw a v-t graph for this car’ motion.

d) Explain how you knew this car’s velocity.

2. The car in Figure 16 (see p. 72 of Textbook) travels west with a forward acceleration of 0.22 m/s2. What was the car’s velocity (vi) at point xi if it travels a distance of 350 m in 18.4 s?

PHYSICS 2015-16

PRACTICE PROBLEMS 3.3Instructions: Read the word problems below. Please show work for deriving the solution to each problem.

1. A construction worker accidentally drops a brick from a high scaffold. a) What is the velocity of the brick after 4.0 s?b) How far does the brick fall during this time?

2. A student drops a ball from a window 3.5 m above the sidewalk. How fast is it moving when it hits the sidewalk?

3. A tennis ball is thrown straight up with an initial speed of 22.5 m/s. it is caught at the same distance above the ground. a) How high does the ball rise?b) How long does the ball remain in the air? (Hint: The time it takes the ball to rise equals the

time it takes to fall.)

4. You decide to flip a coin to determine whether to do your physics or English homework first. The coin is flipped straight up.a) What are the velocity and acceleration of the coin at the top of its trajectory?b) If the coin reaches a high point of 0.25 m above where your released it, what was its

initial speed?c) If you catch it at the same height as you released it, how much time did it spend in the

air?

PHYSICS 2015-16

CHAPTER 3 ASSESSMENT PROBLEMS (p. 82 – 85 of TB)

Directions: Read and respond to the questions listed below. Show all work, as appropriate.53. How are velocity and acceleration related?54. Give an example of each of the following:

a) An object that is slowing down, but has a positive accelerationb) An object that is speeding up, but has a negative accelerationc) An object that is moving at a constant speed but has an acceleration

56. If an object’s velocity-time graph is a straight line parallel to the t-axis, what can you conclude about the object’s acceleration?57. Rank the following objects according to the magnitude of the acceleration, from least to greatest. Specifically indicate any ties.

a) a falling acorn accelerates from 0.50 m/s to 10.3 m/s in 1.0 s.b) a car accelerates from 2 m/s to rest in 1.0 s. c) a centipede accelerates from 0.40 cm/s to 2.0 cm/s in 0.50 s.d) while being hit, a golf ball accelerates from rest to 4.3 m/s in 0.40 s.

59. The graph illustrated below describes the motion of an object moving east along a straight path. Find the acceleration of the object at each of these times:

a) during the first 5.0 min of travelb)) between 5.0 min and 10.0 minc) between 10.0 min and 15.0 mind) between 20.0 min and 25.0 min

0 5 10 15 20 25 3005

101520253035

Velocity v. Time

Time (min)

Velo

city

(m/m

in)

63. What quantity does the area under a velocity-time graph represent?67. A race car is slowed with a constant acceleration of 11 m/s2 opposite the direction of motion.

a) if the car is going 55 m/s, how many meters will it travel before it stops?

b) how many meters will it take to stop a car going twice as fast?69. Explain why an aluminum ball and a steel ball of similar size and shape, dropped from the same height, reach the ground at the same time.71. Suppose an astronaut drops a feather from a height of 1.2 m above the surface of the Moon. If the free-fall acceleration on the Moon is 1.62 m/s2 downward, how long does it take the feather to hit the Moon’s surface?74. You throw a ball downward from a window at a speed of 2.0 m/s. How fast will it be moving when it hits the sidewalk 2.5 m below?82. Draw a velocity-time graph for each of the graphs shown below.

Displacement-Time Graph

Time

Disp

lace

men

t

Displacement-Time Graph

Time

Disp

lace

men

t

Displacement-Time Graph

Time

Disp

lace

men

t

83. The value of free-fall acceleration on the Moon is about one-sixth of its value on Earth. a) Would a ball dropped by an astronaut hit the surface of the Moon with a greater, equal or lesser speed than that of a ball dropped from the same height on Earth?b) Would it take the ball more, less, or equal time to fall?

85. Rock A is dropped from a cliff, and rock B is thrown upward from the same position. a) When they reach the ground at the bottom of the cliff, which rock has a greater

velocity?b) Which has a greater acceleration?c) Which arrives first?

91. Engineers are developing new types of guns that might someday be used to launch satellites as if they were bullets. One such gun can give a small object forward velocity of 3.5 km/s while moving it through a distance of only 2.0 cm.

a) What acceleration does the gun give this object?b) Over what time interval does the acceleration take place?

92. Highway safety engineers build soft barriers, so that cars hitting them will slow down at a safe rate. Suppose a car traveling at 110 km/s hits the barrier, and the barrier decreases the car’s velocity at a rate of 32 m/s2. What distance would the car travel along the barrier before coming to a stop?