Question

A book weighing 5 N rests on top of a table.

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1.A downward force of magnitude 5 N isexerted on the book by the force of

thetable gravity inertia

.

2. An upward force of magnitude _____ isexerted on the _____ by the table.

A. 6N/table

B. 5N/table

C. 5N/book

D. 6N/book

3. Do the downward force in Part A and theupward force in Part B constitute a 3rd law pair?

yes no

4. The reaction to the force in Part A is a force of magnitude_____, exerted on the _____ by the _____. Its direction is _____.

A. 5N/ Earth/ Book/ Upward

B. 5N/ Book/ Table/ Upward

C. 5N/ Book/ Earth/ Upward

D. 5N/ Earth/ Book/Downward

5. The reaction to the force inPart B is a force of magnitude _____, exerted on the _____ by the_____. Its direction is _____.

A. 5N/ Table/ Book/ Upward

B. 5N/ Table/ Earth/ Upward

C. 5N/ Book/ Table/ Upward

D. 5N/ Table/ Book/Downward

E. 5N/ Earth/ Book/ Downard

6. Which of Newton's lawsdictates that the forces in Parts A and B are equal and opposite?

Newton's 1stor 2nd law Newton's 3rdlaw

7. Which of Newton's laws dictates that the forces in Parts B and Eare equal and opposite?

Newton's 1stor 2nd law Newton's 3rdlaw

Answer 1

Concept and reason

The concept required to solve this question is Newton’s second law of motion.

Initially, determine which force exerted on the book by using the Newton’s second law. Finally, calculate the magnitudes of the forces in each part of the question by using the Newton’s second law of motion.

Fundamentals

Newton’s second law of motion states that the acceleration of the object is directly proportional to the net force acting on the object and inversely proportional to the mass of the object.

The net force exerted on the object is,

Here, is the acceleration of the object, is the net force acting on the object and m is the mass of the object.

(1)

An object is attracted towards the center of the earth due to gravitational force of the earth. A force of 5 N acts on the book and the force are directed downward. The table exerts a force equal to the weight of the book. But the direction of the force exerted by the table is upwards.

Inertia is the property by which a body at rest continues to be at rest and a body in uniform motion continues to be in the state of motion.

Gravity is the force by which an object is attracted towards the center of the earth. So the downward force on the book is due to gravity.

(2)

The force exerted on book is 5 N. The force is directed downwards.

Hence, a force of equal magnitude acts in the upward direction.

A force of 5 N acts upward on the book and it is exerted by the table.

(3)

By Newton’s third law of motion, every action has an equal and opposite reaction.

For the downward force and the upward force to constitute the Newton’s third law pair, the downward force should be exerted by the book on the table and the upward force should be exerted by the table on the book.

Thus, the downward force in Part 1 and the upward force in Part 2 do not constitute a 3rd law pair.

(4)

By Newton’s third law of motion, every action has an equal and opposite reaction.

A force of 5 N is acting on the book. The force is exerted by the earth and it is directed downwards.

The reaction force in part 1 is force of magnitude 5 N, exerted on the Earth by the Book. Its direction is upwards.

(5)

A force of 5 N acts upward and it is exerted by the table.

By Newton’s third law of motion, every action has an equal and opposite reaction.

The reaction to the force in Part 2 is a force of magnitude 5 N, exerted on the table by the book. Its direction is downward.

(6)

A downward force is exerted on the book by gravity in part 1. In part 2, an upward force of equal magnitude is exerted on the book by the table. This is not an action reaction pair.

Hence, Newton’s first or second laws dictate that the forces in parts 1 and 2 are equal and opposite.

(7)

By Newton’s third law of motion, every action has an equal and opposite reaction. A force of 5 N acts upward on the book and it is exerted by the table.

The reaction to this force is a force of magnitude 5 N, exerted on the table by the book. Its direction is downwards. Thus, the law that states that the forces in Part 2 and Part are equal and opposite is Newton’s third law of motion.

Ans: Part 1

A downward force of magnitude 5 N is exerted on the book by the force of gravity.