Chapter 2

Question 1

Inertia

Answer 1

The property of things to resist changes in motion.

Question 2

Newton’s first law of motion (the law of inertia)

Answer 2

Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force.

Question 3

Force

Answer 3

In the simplest sense, a push or a pull.

Question 4

Net force

Answer 4

The vector sum of forces that act on an object.

Question 5

Vector

Answer 5

An arrow drawn to scale used to represent a vector quantity.

Question 6

Vector quantity

Answer 6

A quantity that has both magnitude and direction, such as force.

Question 7

Scalar quantity

Answer 7

A quantity that has magnitude but not direction, such as mass and volume.

Question 8

Resultant

Answer 8

The net result of a combination of two or more vectors.

Question 9

Mechanical equilibrium

Answer 9

The state of an object or system of objects for which there are no changes in motion. In accord with Newton’s first law, if an object is at rest, the state of rest persists. If an object is moving, its motion continues without change.

Question 10

Equilibrium rule

Answer 10

For any object or system of objects in equilibrium, the sum of the forces acting equals zero. In equation form,
∑F=0.

Question 11

What class of motion, natural or violent, did Aristotle attribute to motion of the Moon?

Answer 11

Natural

Question 12

What state of motion did Aristotle attribute to Earth?

Answer 12

Natural

Question 13

What relationship between the Sun and Earth did Copernicus formulate?

Answer 13

Copernicus reasoned that the simplest way to account for the observed motions of the Sun, Moon, and planets through the sky was to assume that Earth (and other planets) circles around the Sun.

Question 14

What did Galileo discover in his legendary experiment on the Leaning Tower of Pisa?

Answer 14

Galileo found that a stone twice as heavy as another did not fall twice as fast. Except for the small effect of air resistance, he found that objects of various weights, when released at the same time, fell together and hit the ground at the same time.

Question 15

What did Galileo discover about moving bodies and force in his experiments with inclined planes?

Answer 15

If there is no interference with a moving object, it will keep moving in a straight line forever; no push, pull, or force of any kind is necessary.

Question 16

Is inertia the reason for moving objects maintaining motion or the name given to this property?

Answer 16

The name given to this property.

Question 17

How does Newton’s first law of motion relate to Galileo’s concept of inertia?

Answer 17

The tendency of things to resist changes in motion was what Galileo called inertia. Newton refined Galileo’s idea and made it his first law, appropriately called the law of inertia.

Question 18

What type of path does a moving object follow in the absence of a force?

Answer 18

Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force.

Question 19

What is the net force on a cart that is pulled to the right with 100 pounds of force and to the left with 30 pounds of force?

Answer 19

100-30=70

Question 20

Why do we say that force is a vector quantity?

Answer 20

Force requires both magnitude and direction for a complete description.

Question 21

According to the parallelogram rule, what quantity is represented by the diagonal of a constructed parallelogram?

Answer 21

To find the resultant of two vectors that don’t act in exactly the same or opposite direction, we use the parallelogram rule. Construct a parallelogram in which the two vectors are adjacent sides—the diagonal of the parallelogram shows the resultant (The sum of two or more vectors).

Question 22

What is the resultant of a pair of 1-pound forces at right angles to each other?

Answer 22

When a pair of equal-length vectors at right angles to each other are added, they form a square. The diagonal of the square is the resultant, the square root of 2 times the length of either side. 1.41.

Question 23

Consider Nellie hanging at rest in Figure 2.11. If the ropes were vertical, with no angle involved, what would be the tension in each rope?

Answer 23

The tension in each rope would be half of Nellie’s weight.

Question 24

Can force be expressed in units of pounds and also in units of newtons?

Answer 24

Both pounds and newtons are units of weight, which in turn are units of force.

Question 25

What is the net force on an object that is pulled with forces of 80 newtons to the right and 80 newtons to the left?

Answer 25

0N.

Question 26

What is the net force on a bag pulled down by gravity with a force of 18 newtons and pulled upward by a rope with a force of 18 newtons?

Answer 26

0N.

Question 27

What does it mean to say something is in mechanical equilibrium?

Answer 27

When the net force on something is zero, we say that something is in mechanical equilibrium.

Question 28

State the equilibrium rule for forces in symbolic notation.

Answer 28

∑F=0. The symbol ∑ stands for “the vector sum of” and F stands for “forces.”

Question 29

Consider a book that weighs 15 N at rest on a flat table. How many newtons of support force does the table provide? What is the net force on the book in this case?

Answer 29

The table provides 15N of upward force, making the net force equal to 0N.

Question 30

When you stand at rest on a bathroom scale, how does your weight compare with the support force by the scale?

Answer 30

Your weight is equal in magnitude and opposite in direction to the support force from the scale.

Question 31

A bowling ball at rest is in equilibrium. Is the ball in equilibrium when it moves at constant speed in a straight-line path?

Answer 31

Yes.

Question 32

What is the net force on an object in either static or dynamic equilibrium?

Answer 32

0N .

Question 33

If you push on a crate with a force of 100 N and it slides at constant velocity, how great is the friction acting on the crate?

Answer 33

100N, opposite the direction that you push it.

Question 34

What concept was not understood in the 16th century when people couldn’t conceive of a moving Earth?

Answer 34

Inertia.

Question 35

A bird sitting in a tree is traveling at 30 km/s relative to the faraway Sun. When the bird drops to the ground below, does it still move at 30 km/s, or does this speed become zero?

Answer 35

It continues to move at 30km/s relative to the Sun, but it has a speed of 0 km/s relative to the ground.

Question 36

Stand next to a wall that travels at 30 km/s relative to the Sun. With your feet on the ground, you also travel at the same 30 km/s. Do you maintain this speed when your feet leave the ground? What concept supports your answer?

Answer 36

When you jump, you continue to move at 30 km/s due to your inertia.

Question 37

What two classes of motion did Aristotle advocate?

Answer 37

Natural motion and violent motion

Question 38

What did Galileo discover in his legendary experiment on the Leaning Tower of Pisa?

Answer 38

Galileo found that a heavier stone does not fall significantly faster than a lighter one.

Question 39

What did Galileo discover about moving bodies and force in his experiments with inclined planes?

Answer 39

In the absence of a retarding force, a body will keep moving at a constant speed in a straight line forever.

Question 40

If while riding in a smooth-riding train, you toss a coin upward, the coin will normally land ______.

Answer 40

in your hand.

Question 41

What is the net force on a cart that is pulled to the right with 100 pounds of force and to the left with 30 pounds of force?

Answer 41

The net force is 70 pounds to the right.

Question 42

What is the resultant of a pair of one pound forces at right angles to each other?

Answer 42

The resultant is a force of 1.41 pounds in a direction bisecting the 90-degree angle between the two vectors.

Question 43

Consider a book that weighs 15 N at rest on a flat table. How many newtons of support force does the table exert on the book?

Answer 43

15 newtons up

Question 44

Stand next to a wall that travels at 30 km/s relative to the Sun. With your feet on the ground, you also travel at the same 30 km/s. Do you maintain this speed when your feet leave the ground? What concept supports your answer?

Answer 44

When you jump, you continue to move at 30 km/s due to your inertia.

Question 45

What type of path does a moving object follow in the absence of a force?

Answer 45

It continues to move in a straight line at a constant speed.

Question 46

When Dr. Hewitt pulls the tablecloth, why do the items on the tablecloth do what they do? (Tablecloth trick)

Answer 46

Their inertia is sufficient to keep them there.

Question 47

What class of motion did Aristotle attribute to the Moon?

Answer 47

Natural.

Question 48

What is the net force on a bag pulled down by gravity with a force of 18 newtons and pulled upward by a rope with a force of 18 newtons?

Answer 48

The net force is zero newtons.

Question 49

What does it mean to say something is in mechanical equilibrium?

Answer 49

An object in mechanical equilibrium experiences a zero net force.

Question 50

When you stand at rest on a bathroom scale, how does your weight compare with the support force from the scale?

Answer 50

Your weight is equal in magnitude and opposite in direction to the support force from the scale.

Question 51

A bowling ball at rest is in equilibrium. Is the ball in equilibrium when it moves at constant speed in a straight-line path?

Answer 51

Yes, the bowling ball is in equilibrium because ΣF = 0.

Question 52

If you push on a crate with a horizontal force of 100 N and it slides at constant velocity, what is the magnitude and direction of the frictional force acting on the crate?

Answer 52

The frictional force on the crate is 100 N opposite the direction of motion.

Question 53

What is the net force on an object in either static or dynamic equilibrium?

Answer 53

The net force is zero.