Book edition 7th

Author(s) Douglas C. Giancoli

Pages 978 pages

ISBN 978-0321625922

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Short Answer

You travel from point A to point B in a car moving at a constant speed of $70 km / h$ . Then you travel the same distance from point B to another point C, moving at a constant speed of $90 km / h$ . Is your average speed for the entire trip from A to C, equal to $80 km / h$ ? Explain why or why not.

The average speed for the entire trip from point A to C is less than $80 km / h$ .

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Step by Step Solution

TABLE OF CONTENTS :

TABLE OF CONTENTS

Step 1. Definition of average speed

Speed is a scalar quantity, which means it has only magnitude. Average speed is the ratio of the total distance traveled by an object to the total time taken to cover the distance.

Step 2. Comparison between the times taken for traveling from A to B and B to C

Speed at which your car travels from point A to B, $s_{1} = 70 km / h$

Speed at which your car covers the same distance from point B to C, $s_{2} = 90 km / h$

Thus, you spend more time traveling at $70 km / h$ than at $90 km / h$ , for the same distance.

Step 3. Determination of the average speed of the car

Let the distance from point A to B (or point B to C) be y.

Then the total distance travelled will be $2 y$ .

The time taken (t) by an object to a certain distance is:

$t = \frac{Distance}{Speed}$

Time elapsed in traveling from A to B, $t_{1} = \frac{y}{70 km / h}$

Similarly, time elapsed in traveling from A to B, $t_{2} = \frac{y}{90 km / h}$

The average speed is: $s_{av} = \frac{Total distance}{Total time}$

Therefore, the average speed for the entire trip from A to C is:

$\begin{matrix} = \frac{2 y}{(\frac{y}{70 km / h} + \frac{y}{90 km / h})} \\ = \frac{2 \times 90 \times 70}{90 + 70} km / h \\ = 78.75 km / h \end{matrix}$

Thus, the average speed for the entire journey is less than $80 km / h$ .

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Physics Principles with Applications

Answers without the blur.

Short Answer

You travel from point A to point B in a car moving at a constant speed of $70 km / h$ . Then you travel the same distance from point B to another point C, moving at a constant speed of $90 km / h$ . Is your average speed for the entire trip from A to C, equal to $80 km / h$ ? Explain why or why not.

Step by Step Solution

Step 1. Definition of average speed

Step 2. Comparison between the times taken for traveling from A to B and B to C

Step 3. Determination of the average speed of the car

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Physics Principles with Applications

Answers without the blur.

Short Answer

You travel from point A to point B in a car moving at a constant speed of 70 km/h. Then you travel the same distance from point B to another point C, moving at a constant speed of 90 km/h. Is your average speed for the entire trip from A to C, equal to 80 km/h? Explain why or why not.

Step by Step Solution

Step 1. Definition of average speed

Step 2. Comparison between the times taken for traveling from A to B and B to C

Step 3. Determination of the average speed of the car

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