Conceptual Physics Chapter 4 Review

Questions and Answers

State the law of inertia (Newton's First Law of Motion)

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

What concept was missing from people's minds in the sixteenth century when they couldn't believe Earth was moving?

The concept of inertia.

When a bird lets go of a branch and drops to the ground below, why doesn't the moving Earth sweep away from the dropping bird?

The bird is moving with the Earth and has inertia.

What kind of path would the planets follow if suddenly their attraction to the Sun no longer existed?

In the absence of a force, the planets would follow straight-line paths. Signup and view all the answers

State Newton's Second Law of Motion.

The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object. Signup and view all the answers

Is acceleration directly proportional to force, or is it inversely proportional to mass? Give an example.

Acceleration is directly proportional to force; push on a brick and it accelerates. Signup and view all the answers

Is acceleration directly proportional to mass, or is it inversely proportional to mass? Give an example.

Acceleration is inversely proportional to mass; if you push an elephant, it won't accelerate as much as if you pushed on a brick. Signup and view all the answers

What is the net force that acts on a 10-N freely falling object?

The net force on a 10-N object is its weight. Signup and view all the answers

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

Free fall is due to the gravitational force due to the Earth; since gravity is constant, gravity does not accelerate heavy objects faster and light objects slower. Signup and view all the answers

What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance?

10 N - 4 N = 6 N Signup and view all the answers

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

The object's frontal area and speed affect the force of air resistance. Signup and view all the answers

What is the acceleration of a falling object that has reached its terminal velocity?

At terminal velocity, a falling object has zero acceleration. Signup and view all the answers

If two objects of the same size fall through air at different speeds, which encounters the greater air resistance?

The faster object encounters greater air resistance. Signup and view all the answers

Why does a heavy parachutist fall faster than a lighter parachutist who wears the same size parachute?

The greater weight is more effective in 'plowing through' air, resulting in more terminal speed for a heavier person. Signup and view all the answers

Previously, we said that a force was a push or pull; now we say it is an interaction. Which is it? A push or pull, or an interaction? And what does it mean to say interaction?

In its simplest sense, force is a push or a pull. In a broader sense, force makes up an interaction between one thing and another. Signup and view all the answers

How many forces are required for a single interaction?

Two forces that are equal in magnitude and opposite in direction make up a single interaction. Signup and view all the answers

When you push against a wall with your fingers, they bend because they experience a force. Identify this force.

The wall is the force that pushes against you. Signup and view all the answers

A boxer can hit a heavy bag with great force. Why can't he hit a sheet of newspaper in midair with the same amount of force?

The first can't exert any force at all unless what is being hit exerts the same amount of force. Signup and view all the answers

State Newton's Third Law.

To every action there is always an opposed equal reaction. Signup and view all the answers

Consider hitting a baseball with a bat. If we call the force on the bat against the ball the action force, identify the reaction force.

The reaction force is the ball exerting a force on the bat. Signup and view all the answers

If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why do the cannonball and cannon have very different accelerations?

The cannon has more mass than the cannonball making the cannon's acceleration less. Signup and view all the answers

Do action and reaction forces always act on different bodies? Defend your answer.

An action-reaction pair of forces are always equal and opposite, but two forces that are equal in magnitude and opposite in direction are NOT always an action-reaction pair. Signup and view all the answers

Can you cancel a force on Body A with a force that acts on Body B? Defend your answer.

An example is a book on a table - not an action-reaction pair. Signup and view all the answers

How does a helicopter get its lifting force?

Helicopters create lift by pushing air down, thereby experiencing an upward reaction force. Signup and view all the answers

What law of physics is inferred when we say you cannot touch without being touched?

Newton's Third Law; every action has a reaction. Signup and view all the answers

According to the parallelogram rule, what does the diagonal of a constructed parallelogram represent?

The sum of two vectors and it is called the resultant. Signup and view all the answers

Consider Nellie in Figure 4.36 If the ropes were vertical, with no angle involved, what would be the tension in each rope?

The tension in each rope would be equal to the other and half of the total tension. Signup and view all the answers

Can it be said that, when a pair of vectors is at right angles to each other, the resultant is greater than either of the vectors separately? Defend your answer.

Yes, the resultant is equivalent to the hypotenuse of a right triangle which will always be greater than either of its other sides. Signup and view all the answers

When a vector at an angle is resolved into horizontal and vertical components, can it be said that each component has less magnitude than the original vector? Defend your answer.

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Briefly summarize Newton's three laws of motion.

1st law of motion: An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. 2nd law of motion: Acceleration = net force/mass. 3rd law of motion: To every action there is always an opposed equal reaction. Signup and view all the answers

Study Notes

Newton's Laws of Motion

Law of Inertia (First Law): Objects remain at rest or in uniform motion unless a nonzero force acts upon them.
Historical Context: The concept of inertia was absent in the sixteenth century, leading to skepticism about Earth's motion.
Inertia Example: A bird dropping from a branch stays in motion with the Earth due to its inertia, so the Earth doesn’t move away from the bird.
Planetary Paths: If the Sun's gravitational attraction ceased, planets would move in straight-line paths without influence.

Acceleration and Forces

Newton's Second Law: Acceleration is directly proportional to net force and inversely proportional to mass; represented as ( a = \frac{F_{net}}{m} ).
Force and Acceleration: If a brick is pushed, it accelerates; demonstrating direct proportionality between force and acceleration.
Mass and Acceleration: Pushing an elephant yields less acceleration than pushing a brick, showing acceleration is inversely proportional to mass.

Free Fall Dynamics

Net Force in Free Fall: The net force acting on a freely falling 10-N object is its weight (10 N).
Acceleration in Free Fall: Heavy and light objects fall at the same rate due to constant gravitational force; gravity does not differentiate between weights.
Air Resistance: The net force on a falling object counters forces such as air resistance, resulting in different outcomes depending on the resistance encountered.
Factors of Air Resistance: Frontal area and speed of the object significantly impact air resistance during free fall.

Terminal Velocity

Terminal Velocity: When reaching terminal velocity, the acceleration of a falling object becomes zero as forces balance out.
Impact of Weight: A heavier parachutist falls faster than a lighter one with the same parachute size due to greater downward force overcoming air resistance.

Nature of Forces

Definition of Force: Fundamentally, a force is a push or pull; broadly, it involves interaction between entities.
Interacting Forces: A single interaction consists of two equal and opposite forces acting on different bodies.
Example of Force Interaction: When pushing against a wall, the wall's force counteracts the push, causing fingers to bend.

Newton's Third Law

Third Law Summary: For every action, there is an equal and opposite reaction.
Baseball Bat Example: The reaction force to the bat hitting a baseball is the ball's force exerted back on the bat.
Different Accelerations: The cannonball and cannon experience different accelerations despite equal force due to differing masses.

Vector Dynamics

Action-Reaction Forces: Action and reaction forces act on different bodies, making them an interactive pair, but not necessarily equal pairs in force magnitude.
Can Forces Cancel?: Forces acting on separate bodies (like a book on a table) do not cancel each other out; they do not form action-reaction pairs.
Helicopter Lift: Lift is created by helicopters pushing air downward, resulting in an upward reaction force.

Parallelogram Rule and Vectors

Parallelogram Rule: The diagonal of a parallelogram formed by two vectors represents the resultant vector.
Right-Angled Vectors: In a right triangle formed by vectors, the resultant is always greater than either vector.
Resolving Vectors: When a vector is resolved into components, each component has a lesser magnitude than the original vector.

Summary of Newton's Laws

First Law: An object remains at rest or in uniform motion unless acted upon by an unbalanced force.
Second Law: Acceleration is given by the equation ( a = \frac{F_{net}}{m} ).
Third Law: Every action has an equal and opposite reaction, solidifying the interplay of forces in nature.

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Test your understanding of important principles from Chapter 4 of Conceptual Physics with these flashcards. Revise key concepts like Newton's First Law of Motion and the historical context of inertia. Perfect for students looking to reinforce their knowledge.