# Physics

The power ball has now reached its highest point above the ground and starts to descend again. The motion

diagram representing the velocity vectors is the same as that after the initial release, as shown in the figure of Part

B. Indicate whether the velocity and acceleration of the ball at its highest point are positive (upward), negative, or

zero.

Use P, N, and Z for positive (upward), negative, and zero, respectively. Separate the letters for velocity and

acceleration with a comma.

**Hint 1. **Velocity as a continuous function of time

In Part D you found that the velocity of the ball is positive during the upward motion. Once the ball starts its

descent, its velocity is negative, as you found in Part B. Since velocity changes continuously in time, it has to

be zero at some point along the path of the ball.

**Hint 2. **Acceleration as a continuous function of time

In Part D, you found that the acceleration of the ball is negative and constant during the upward motion, as

well as once the ball has started its descent, which you found in Part B. Since acceleration is a continuous

function of time, it has to be negative at the highest point along the path as well.

2.- 2 . 0 Ã2 . 0

0 – 0 Ã0

P,N

11/10/2014 HW_Week2

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ANSWER:

Correct

These examples should show you that the velocity and acceleration can have opposite or similar signs or that

one of them can be zero while the other has either sign. Try hard to think carefully about them as *distinct*

physical quantities when working with kinematics.

Analyzing Position versus Time Graphs: Conceptual Question

Two cars travel on the parallel lanes of a twolane

road. The

cars’ motions are represented by the position versus time

graph shown in the figure. Answer the questions using the

times from the graph indicated by letters.

Part A

At which of the times do the two cars pass each other?

**Hint 1. **Two cars passing

Two objects can pass each other only if they have the same position at the same time.

ANSWER:

Z,N

11/10/2014 HW_Week2

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Correct

Part B

Are the two cars traveling in the same direction when they pass each other?

ANSWER:

Correct

Part C

At which of the lettered times, if any, does car #1 momentarily stop?

**Hint 1. **Determining velocity from a position versus time graph

The slope on a position versus time graph is the “rise” (change in position) over the “run” (change in time). In

physics, the ratio of change in position over change in time is defined as the velocity. Thus, the slope on a

position versus time graph is the velocity of the object being graphed.

ANSWER:

A

B

C

D

E

None

Cannot be determined

yes

no

A

B

C

D

E

none

cannot be determined

11/10/2014 HW_Week2

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Correct

Part D

At which of the lettered times, if any, does car #2 momentarily stop?

**Hint 1. **Determining velocity from a position versus time graph

The slope on a position versus time graph is the “rise” (change in position) over the “run” (change in time). In

physics, the ratio of change in position over change in time is defined as the velocity. Thus, the slope on a

position versus time graph is the velocity of the object being graphed.

ANSWER:

Correct

Part E

At which of the lettered times are the cars moving with nearly identical velocity?

**Hint 1. **Determining Velocity from a Position versus Time Graph

The slope on a position versus time graph is the “rise” (change in position) over the “run” (change in time). In

physics, the ratio of change in position over change in time is defined as the velocity. Thus, the slope on a

position versus time graph is the velocity of the object being graphed.

ANSWER:

A

B

C

D

E

none

cannot be determined

11/10/2014 HW_Week2

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Correct

Conceptual Question 2.05

Part A

Suppose that an object is moving with constant nonzero acceleration. Which of the following is an accurate

statement concerning its motion?

ANSWER:

Correct

Conceptual Question 2.09

Part A

The graph in the figure shows the position of an object as a function of time. The letters HL

represent particular

moments of time. At which moments shown (H, I, etc.) is the speed of the object

A

B

C

D

E

None

Cannot be determined

In equal times its speed changes by equal amounts.

In equal times its velocity changes by equal amounts.

A graph of its position as a function of time has a constant slope.

In equal times it moves equal distances.

A graph of its velocity as a function of time is a horizontal line.

11/10/2014 HW_Week2

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(a) the greatest?

ANSWER:

Correct

Part B

(b) the smallest?

ANSWER:

Correct

Conceptual Question 2.04

Part A

H

I

J

K

L

H

I

J

K

L

11/10/2014 HW_Week2

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When can we be certain that the average velocity of an object is always equal to its instantaneous velocity?

ANSWER:

Correct

Conceptual Question 2.18

Part A

Two objects are thrown from the top of a tall building and experience no appreciable air resistance. One is thrown

up, and the other is thrown down, both with the same initial speed. What are their speeds when they hit the street?

ANSWER:

Correct

Conceptual Question 2.16

Part A

The figure represents the velocity of a particle as it travels along the *x*axis.

At what value (or values) of *t *is the

instantaneous acceleration equal to zero?

always

only when the acceleration is constant

never

only when the acceleration is changing at a constant rate

only when the velocity is constant

The one thrown down is traveling faster.

The one thrown up is traveling faster.

They are traveling at the same speed.

11/10/2014 HW_Week2

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ANSWER:

Correct

Problem 2.27

Part A

A package is dropped from a helicopter moving upward at 15 m/s. If it takes 8 s before the package strikes the

ground, how high above the ground was the package when it was released if air resistance is negligible?

ANSWER:

Correct

Problem 2.10

Part A

*t *= 1 s

*t *= 0.5 s and *t *= 2 s

*t *= 0

190 m

114 m

152 m

228 m

11/10/2014 HW_Week2

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The position of an object as a function of time is given by *x = bt2 ct*,

where *b *= 2.0 m/s2 and *c *= 6.7 m/s, and *x *and

*t *are in SI units. What is the instantaneous velocity of the object when *t *= 2.3?

ANSWER:

Correct

Conceptual Question 2.02

Part A

If the graph of the position as a function of time for an object is a horizontal line, that object cannot be accelerating.

ANSWER: