Podcast
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?
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State Newton's Second Law of Motion.
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Is acceleration directly proportional to force, or is it inversely proportional to mass? Give an example.
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Is acceleration directly proportional to mass, or is it inversely proportional to mass? Give an example.
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What is the net force that acts on a 10-N freely falling object?
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Why doesn't a heavy object accelerate more than a light object when both are freely falling?
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What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance?
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What two principal factors affect the force of air resistance on a falling object?
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What is the acceleration of a falling object that has reached its terminal velocity?
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If two objects of the same size fall through air at different speeds, which encounters the greater air resistance?
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Why does a heavy parachutist fall faster than a lighter parachutist who wears the same size parachute?
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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?
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How many forces are required for a single interaction?
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When you push against a wall with your fingers, they bend because they experience a force. Identify this force.
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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?
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State Newton's Third Law.
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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.
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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?
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Do action and reaction forces always act on different bodies? Defend your answer.
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Can you cancel a force on Body A with a force that acts on Body B? Defend your answer.
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How does a helicopter get its lifting force?
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What law of physics is inferred when we say you cannot touch without being touched?
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According to the parallelogram rule, what does the diagonal of a constructed parallelogram represent?
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Consider Nellie in Figure 4.36 If the ropes were vertical, with no angle involved, what would be the tension in each rope?
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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.
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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.
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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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Description
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.