Homework Questions

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

An object in mechanical equilibrium experiences a zero net force.

What relationship between distance traveled and time did Galileo discover for freely falling objects released from rest?

Galileo discovered that distance increased as the time squared.

What is the acceleration of the ball when it is moving up, when it is at the maximum height, and when it is moving down? (Note: The acceleration graph is shown in red.)

It is always negative.

What is the value of the velocity of the ball when it reaches its maximum height?

It is always zero.

How does the acceleration of the ball change when the vertical velocity at which it is thrown upward is increased? (Note: the acceleration graph is shown in red.)

It stays the same.

How does the velocity of the ball at the maximum height change when the initial vertical velocity at which it is thrown upward is increased?

It stays the same.

What two principal factors affect the force of air resistance on a falling object?

The force depends mostly on frontal area and speed.

How does the force of friction for a sliding object vary with speed?

The force of friction is approximately independent of speed.

What general rule can you conclude about the force needed to keep an object in motion at a steady rate?

The force that you need to apply to keep an object moving at a steady rate is equal to the force resisting the motion of the object.

When Dr. Hewitt pulls the tablecloth, what happens to the items on the tablecloth?

They tend to stay where they were

How many forces are required for an interaction?

Two forces, an action and a reaction, are needed for an interaction.

As you read this in your chair, how fast are you moving relative to the chair? Relative to the Sun?

You are not moving relative to the chair, but you are moving over 100,000 km/h relative to the Sun.

When you push against a wall with your fingers, they bend. Identify the force or forces involved.

You exert an action force and the wall exerts an equal and opposite reaction force.

Once the crate is sliding, how hard do you push to keep it moving at constant velocity?

You push with a force equal to and opposite the dynamic friction force.

What is the direction of the net force acting on the object at position B?

to the left The velocity is directed to the right; however, it is decreasing. Therefore, the acceleration is directed to the left—and so is the net force.

Calculate the weight in newtons of a 1800-kg elephant. Express your answer to two significant figures and include the appropriate units.

W = 1.8×10^4 N

To produce a net force on a system, must there be an externally applied net force?

Yes, there must be an external net force on the system.

What is the acceleration of a car that maintains a constant velocity of 100 km/h for 10 s?

0 km/h·s

A car is moving along a straight road at a constant speed. A car is moving along a straight road while slowing down. A car is moving along a straight road while speeding up. A hockey puck slides along a smooth (i.e., frictionless) icy surface. A hockey puck slides along a rough concrete surface. A cockroach is speeding up from rest. A rock is thrown horizontally; air resistance is negligible. A rock is thrown horizontally; air resistance is substantial. A rock is dropped vertically; air resistance is negligible. A rock is dropped vertically; air resistance is substantial. 1. Which of these situations describe the motion shown in the motion diagram at point A? 2. Which of these situations describe the motion shown in the motion diagram at point B? 3. Which of these situations describe the motion shown in the motion diagram at point C?

1. ad 2. be 3. g

What is the net force acting on a 1-kg ball in free fall?

10 N

What is the weight of a 1-kilogram brick resting on a table?

10 N

What is the speed over the ground of an airplane flying at 100 km/h relative to the air caught in a 100-km/h right-angle crosswind?

141 km/h

Consider the video tutorial you just watched. Suppose that we duplicate this experimental setup in an elevator. What will the spring scale read if the elevator is moving upward at constant speed?

18N

How much support force acts on a 200-N girl standing on a weighing scale?

200 N

What is the average speed in kilometers per hour of a horse that gallops a distance of 15 km in a time of 30 min?

30 km/h

What is the net force acting on a falling 1-kg ball if it encounters 2 N of air resistance?

8 N

How does acceleration depend on the net force?

Acceleration is proportional to the net force.

Which ball do you predict will have a greater acceleration as it drops toward the ground, and why?

Ball 1 and ball 2 have equal acceleration.

Which ball will be traveling faster when it hits the ground?

Ball 1 and ball 2 will be traveling equally fast when they hit the ground.

Which ball do you predict will reach the ground first?

Both ball 1 and ball 2 will reach the ground at the same time.

When Dr. Hewitt drops a book and a crumpled piece of paper together from the same height, which object hits the ground first, and why?

Both objects hit the ground at the same time because air resistance is negligible.

Which BEST describes what occurs when a body accelerates?

Change in velocity per unit time

If Dr. Hewitt applies 3 N of force to keep the block moving at a steady rate, what must be the force of friction, and why?

Equal to 3 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.

You are dragging a block on a surface with friction at a steady speed of 2 m/s and exert a force of 5 N to do so. What is the force of friction? Why?

Equal to 5 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.

State Newton's third law of motion.

For every action there is an equal and opposite reaction.

After he gets the block to move, what does Dr. Hewitt do to drag the block across the table at a steady rate?

He continues to apply a steady force.

What occurs if f is less than this sum?

If f is less, then a net force acts on the shoe and it accelerates down the incline.

Consider the system of a single football. If you kick it, is there a net force to accelerate the system? If a friend kicks it at the same time with an equal and opposite force, is there a net force to accelerate the system?

If you kick it, there is a net force; if both of you kick it, there is not.

Distinguish between force and impulse.

Impulse is force times a time interval.

What is inertia?

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

When Dr. Hewitt pulls the tablecloth, why do the items on the tablecloth do what they do?

Inertia makes them stay at rest.

Fill in the blanks: Newton's first law is often called the law of ____; Newton's second law is the law of ____; and Newton's third law is the law of ____.

Inertia, acceleration, action -reaction

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 block of mass 2kg is acted upon by two forces: 3N (directed to the left) and 4N (directed to the right). What can you say about the block's motion?

It could be moving to the left, moving to the right, or be instantaneously at rest.

What is the gain in speed per second for a freely falling object?

It gains approximately 10 m/s.

After the ball is thrown upward, it eventually turns back downward, passing through the zero line (the horizontal axis on the graph). How does the time the ball takes to return to the zero level change when the initial vertical velocity of the ball is increased?

It increases.

Try changing the "Angle of Incline." Then click "Play" to see how the change affects the graph and the dial. As the angle of the inclined plane increases, how does the final speed attained by the ball at the bottom of the inclined plane change?

It increases.

Try changing the "Angle of Incline." Then click "Play" to see how the change affects the graph and the dial. How does the acceleration of the ball change as the angle of the inclined plane increases?

It increases.

Use the "Initial velocity" slider to set the initial vertical velocity of the launch, and then click "Play." How does the maximum height achieved by a ball change when the initial vertical velocity at which it is thrown upward is increased?

It increases.

In the string-pull illustration in Figure 4.8, a sharp jerk on the bottom string results in the bottom string breaking. Does this occur because of the ball's weight or its mass?

Mass

What class of motion did Aristotle attribute to the Moon?

Natural

What state of motion did Aristotle attribute to Earth?

Natural

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.

Which of Newton's three laws focuses on interactions?

Newton's third law

Since an object weighs less on the surface of the Moon than on Earth's surface, does it have less inertia on the Moon's surface?

No.

Which is normally greater: static friction or sliding friction on the same object?

Static friction is normally greater than dynamic friction.

What will be the magnitude of vector S if the rope that supports Mo is vertical?

S=0

Which of the quantities are scalar quantities and which are vector quantities?

Scalar: age, temperature Vector: acceleration, force

Shake something to and fro and you're measuring its what? Lift it against gravity and you're measuring its what?

Shaking measures weight, whereas lifting measures mass.

If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change?

The acceleration is reduced to 1/3 of its original value.

Based on the experiment that Dr. Hewitt demonstrated in the video, what factors determine whether the air resistance experienced by a falling object will affect the motion of the object?

The area of the object in relation to the weight of the object determines the air resistance the object experiences. Objects having greater area but smaller weight experience greater effect of air resistance.

Try changing the "Angle of Incline." Then click "Play" to see how the change affects the graph and the dial. Why does the final speed attained by the ball at the bottom of the inclined plane change in the way that it does when the angle of the incline plane increases?

The ball accelerates at a greater rate for the same duration of time.

Dr. Hewitt has clay "blobs" hanging on either side of his head in this video. When he turns around, what happens to the blobs?

The blobs tend to stay where they were.

In the video, Dr. Hewitt drops a book and a flat piece of paper together from the same height. Which object that hits the ground first?

The book hits the ground first.

What relationship between the Sun and Earth did Copernicus formulate?

The earth revolved in a circle around the sun

In the video, Dr. Hewitt drops a book and a flat piece of paper together from the same height, and one object hits the ground before the other. Why does that object hit the ground first?

The heavier object--the book--hits the ground first because the other, lighter, object is more influenced by air resistance.

What happens to the magnitude of the normal vector on a block resting on an incline when the angle of the incline increases?

The magnitude of the normal vector decreases.

Which has a greater momentum: a heavy truck at rest or a moving skateboard?

The moving skateboard.

An object cannot remain at rest unless which of the following holds?

The net force acting on it is zero.

If a block is moving to the left at a constant velocity, what can one conclude?

The net force applied to the block is zero.

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 in either static or dynamic equilibrium?

The net force is zero.

chapter 5 What is the direction of the net force acting on the object at position A?

The net force is zero. The velocity vectors connecting position A to the adjacent positions appear to have the same magnitude and direction. Therefore, the acceleration is zero—and so is the net force.

Why doesn't a heavy object accelerate more than a light object when both are freely falling?

The ratio of the weight to mass is the same for all objects in the same locality.

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.

A person is hanging from two ropes, as shown in the figure. Which rope is supplying the higher tension force?

The rope to the left has the higher tension. The resultant of the two tension forces must point directly upward (to support the force of gravity). Since the left rope is directed more vertically than the right rope, the left rope must have a higher tension such that the horizontal component of the two tensions cancels out.

When Dr. Hewitt drops a book and a crumpled piece of paper together from the same height, which one hits the ground first?

They both hit the ground at the same time.

Why do the alphabet letters tend to do what they do when you rotate the bowl?

They have inertia−the tendency of an object at rest to stay at rest.

Why do the clay blobs do what they do when Dr. Hewitt turns around?

They have inertia−the tendency of an object at rest to stay at rest.

Some alphabet letters are floating in a bowl of soup. You want to move a letter that is far away from you closer to you. How could you rotate the bowl to do that?

Unfortunately, neither rotating the bowl clockwise nor rotating it counterclockwise will work.

How does the magnitude of f relate to the vector sum of mg and N when the shoe is in equilibrium?

Vector f will have the same magnitude as the vector sum of mg and N.

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.

If a car moves with a constant velocity, does it also move with a constant speed?

Yes, because constant velocity requires constant speed in the same direction.

If the system of (Figure 1) is only the orange, is there a net force on the system when the apple pulls?

Yes, there is the action of the apple on the orange.

You tell your friend about the neat tablecloth trick that Dr. Hewitt demonstrated. Your friend tries to repeat this trick at home and later complains to you that it failed. Which of the following is most likely the reason that it failed?

Your friend pulled the tablecloth too slowly.

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.

Find the acceleration for a force of 2 N acting on 2 kg

a = 1 m/s2

Calculate the acceleration of a 250000-kg jumbo jet just before takeoff when the thrust on the aircraft is 110000 N .

a = 0.44 m/s2

Select the correct equations that show that the acceleration of 4.4 m/s2 results from a net force of 11 N exerted on a 2.5-kg cart.

a=Fnet/m=11N/2.5kg=4.4m/s2

When Monkey Mo dangles by a rope and pulls on his cage, each at different angle from the vertical, the rope tension will be greater than _______.

both his weight and the pull on his cage

Click "Play" and see what happens on the graph and on the dial as the ball rolls down the plane. How can you use the graph of velocity versus time to estimate the acceleration of the ball?

by estimating the value of the slope of the graph

Two forces, of magnitude 4N and 10N, are applied to an object. The relative direction of the forces is unknown. The net force acting on the object __________.

cannot have a magnitude equal to 5N

For which situation is the tension of the ropes higher?

case A Since the vertical components of the two tension vectors must add up to the person's weight, the tension must be higher when the ropes are more horizontal.

What is the direction of the net force acting on the object at position C?

downward The horizontal component of the velocity does not change. The vertical component of the velocity increases. Therefore, the acceleration—and hence the net force—is directed straight downward.

A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________.

moving with a constant nonzero acceleration

An impulse can be increased by

increasing the force or increasing the time interval.

The length of a force vector indicates the ________.

magnitude of the force

Are there any quantities of the following, which are neither vector quantities nor scalar quantities: force, age, acceleration, temperature?

no

Two vectors are shown in the figure. Which of the four vectors also shown best represents the resultant vector?

vector C The horizontal component of the resultant vector is equal to the sum of the individual horizontal components of the two vectors, and the vertical component of the resultant vector is equal to the sum of the individual vertical components of the two vectors.