Chap 2 - homework questions

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?

15 newtons up

Why do we say that force is a vector quantity?

A force has a magnitude and a direction.

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

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

What is inertia?

Inertia is the tendency of a body to maintain its state of motion in the absence of applied forces.

What state of motion did Aristotle attribute to Earth?

The Earth has a Natural class of motion

What relationship between the Sun and Earth did Copernicus formulate?

The Earth revolved in a circle about the Sun.

What class of motion did Aristotle attribute to the Moon?

The Moon has a Natural class of motion

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?

The net force is 70 pounds to the right

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?

The net force is zero newtons.

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?

The net force is zero newtons.

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

The net force is zero.

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

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

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

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

State the equilibrium rule for forces in symbolic notation

ΣF = 0

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

An object in mechanical equilibrium experiences a zero net force.

What are the units for force?

Force can be expressed in newtons or pounds.

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

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

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?

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

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

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

A ball rolling along a floor doesn't continue rolling indefinitely. Is it because it is seeking a place of rest or because some force is acting upon it? If the latter, identify the force.

It is because some force is acting upon it. The force is friction

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

Newton refined Galileo's concept of inertia and made it his first law of motion.

Your friend says that inertia is a force that keeps things in their place, either at rest or in motion. Do you agree? Why or why not?

No, I dont agree because Inertia is a PROPERTY of objects. Inertia is property that makes objects want to keep doing what theyre doing.

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

The diagonal is the resultant, or sum, of two vectors.

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?

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

Consider the normal force on a book at rest on a tabletop. If the table is tilted so that the surface forms an inclined plane, will the magnitude of the normal force change? If so, how?

The normal force is always perpendicular to the object. So if the table is tilted the Normal Force will also tilt with the book so it is pointed at an angle relative to the horizontal.

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

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

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?

The tension in each rope would be half of the gravity force.

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

They did not understand inertia.

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?

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