SCI 102: Part I - Energy (Physics) Notes on Inertia and Newton's First Law
Course Structure and Assessment
Attendance & Participation: 15%
Quizzes: 15% total
Quiz #1: Newton’s Laws of Motion
Quiz #2: Waves and Sound
Lab Reports: 40%
Final Exam: 30%
Assessment 2 (as listed in the transcript)
Day 1: Newton’s First Law – Law of Inertia
Activity 1.1: 15 minutes
Debrief Activity 1.2: 15 minutes
Debrief Activity 1.3: 15 minutes
Objectives
Understand Newton’s First Law of Motion and relate it to inertia.
Understand the significance of Newton's law by identifying and refuting classic misconceptions concerning the causes of motion.
Recognize inertia as a property of an object that depends solely upon mass.
Background
Sir Isaac Newton (1643–1727): English scientist and mathematician.
Famous for gravity and the three laws of motion; published them in Philosophiae Naturalis Principia Mathematica (1687).
These laws describe the motion of all objects on scales relevant to everyday life.
Newton’s First Law of Motion (Law of Inertia)
Statement: The law of inertia says that every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero (unbalanced) force.
Origin: Concept originated with Galileo.
Implications: An object will keep doing what it was doing unless acted on by an unbalanced force. If stationary, it stays stationary; if moving at constant velocity, it continues moving at that velocity.
Key idea: It takes a force to change the motion of an object.
What is meant by unbalanced force?
Balanced forces: Forces on an object are equal in magnitude and opposite in direction; no change in motion occurs.
Unbalanced forces: Forces are not equal and opposite; the motion of the object changes.
Historical Quiz Question (From Slide 9)
Question: The first to introduce the concept of inertia was:
a) Galileo
b) Newton
c) both
d) neither
Answer (from the slide): Galileo introduced inertia prior to Newton.
Check Your Neighbor (1 of 2)
Quick-pull demonstration: A sheet of paper withdrawn from under a soft-drink can does not topple the can.
Correct reasoning: The can has inertia; the paper is pulled away, but the can tends to stay in its state of motion (or rest) momentarily due to inertia, causing no toppling when withdrawn rapidly.
Correct option: C. the can has inertia
Check Your Neighbor (2 of 2)
Scenario: Swinging a stone overhead in a horizontal circle; if the string breaks, the stone tends to follow a straight-line path.
Correct option: B. straight-line path
Explanation: When the centripetal force (tension in the string) is removed, the stone’s inertia makes it continue in a straight-line path tangent to the circle.
Thought Experiment: Gravity and Orbits
If gravity between the Sun and Earth suddenly vanished, Earth would move in a straight-line path.
Correct option: b) a straight-line path
Significance: Gravity provides the centripetal force keeping Earth in orbit; without it, inertia would cause a straight-line motion.
Examples of Inertia
Several everyday demonstrations show inertia (e.g., sudden pulls or jolts causing different motion responses).
Specific prompts in the slides include questions like: Why does a sudden downward yank break the bottom string but a slow pull breaks the top string? Why will the coin drop into the glass when a force accelerates the card? Why do downward motion and sudden stop of a hammer handle tighten the hammerhead?
Takeaway: Inertia explains why objects resist changes in their motion and why force and motion are related through Newton’s laws.
Inertia and Balanced vs Unbalanced Forces in Practice
When two teams exert equal force on a rope in opposite directions, the forces are balanced and there is no change in motion.
A ball at rest requires a kick (unbalanced force) to start moving.
Inertia is the resistance to a change in motion, not a force itself.
Why Objects in Motion Do Not Move Forever without a Force
Real-world objects experience unbalanced forces (e.g., friction, air resistance, gravity) that slow them down and eventually stop them.
Examples:
A book sliding on a table slows due to friction.
A ball thrown upward slows due to gravity.
Inertia in Space
In outer space, away from gravity and friction, a rocket launched with a given speed and direction would continue in that same direction and at the same speed indefinitely.
Implication: Inertia alone would keep the motion unaltered in a frictionless, gravity-free environment.
Inertia in Action (1 of 3)
Rapid deceleration is sensed by passengers as a lurch forward due to inertia.
This is tied to Newton’s Second Law because a net external force (e.g., brakes) is needed to stop the object; before that force acts, the body continues moving due to its inertia.
Relevant formula: F = ma where F is the net external force, m is mass, and a is acceleration (including deceleration).
Inertia in Action (2 of 3)
Example: In a cart with a ball resting in the middle, a quick forward jerk can cause the ball to:
a) be hit by the front of the cart,
b) be hit by the back of the cart,
c) remain in the middle as the cart moves forward,
d) all of the above depending on the jerk rate.
Understanding: Inertia depends on how quickly the external force is applied and how mass resists acceleration.
Inertia in Action (3 of 3)
In a high-speed airplane, flipping a coin shows that the coin keeps up with the observer: it behaves as if the airplane were at rest from the coin’s point of view.
Takeaway: In a non-accelerating (inertial) frame of reference, objects in motion tend to stay in motion with the same velocity relative to that frame.
Everyday Demonstrations: Bus, Train, and Motion
Bus: While the bus rides smoothly, a coin dropped from above your head tends to land at your feet (or very close) because you share the same horizontal velocity as the bus and you and the coin maintain that velocity when gravity acts vertically.
Question posed: In a smoothly riding bus, dropping a coin from above your head will land at your feet (roughly).
This illustrates inertia and the independence of horizontal and vertical motions in the absence of a horizontal external impulse.
Practice MCQs: Quick Review Questions (Based on the Slides)
Q23: A quick pull on a sheet of paper beneath a box of crackers doesn’t topple the box. Best illustrates that:
A) there is an action-reaction pair of forces
B) the box has inertia
C) gravity tends to hold the box secure
D) the box has no acceleration
Answer: B
Q24: When you whip a tablecloth beneath a set of dishes on a table, you’re demonstrating:
A) constant motion
B) inertia
C) friction
D) ΣF = 0
Answer: B
Q25: A bird sitting in a tree moves about 30 km/s relative to the Sun. If the bird takes 1 second to drop to a worm below, logic may tell us that the worm would be 30 km downrange from the bird as it reaches the ground. This doesn’t happen, thanks to Newton’s:
A) law of gravity
B) three laws of motion
C) law of inertia
D) none of the above
Answer: C
Q26: Earth travels about 30 km/s relative to the Sun. When you leap upward in front of a wall, the wall doesn’t slam into you at 30 km/s because the wall:
A) and you move at the same horizontal speed, before, during, and after your jump
B) has too little gravity to influence you
C) moves in the opposite direction to you
D) has negligible inertia compared with the Sun
Answer: A
Q27: When you leap straight upward inside a high-speed train traveling at constant velocity, you land:
A) slightly ahead of your original position
B) at your original position
C) slightly behind your original position
Answer: B
Q28: If you leap straight up inside a high-speed train while it gains speed, you land:
A) slightly ahead of your original position
B) at your original position
C) slightly behind your original position
D) none of the above
Answer: A
Q29: If you leap straight up inside a high-speed train while it slows down, you land:
A) slightly ahead of your original position
B) at your original position
C) slightly behind your original position
D) none of the above
Answer: C
Key Formulas and Concepts (Summary)
Newton’s First Law (Law of Inertia):
Statement: An object at rest stays at rest, and an object in motion stays in motion in a straight line at constant speed, unless acted on by a nonzero (unbalanced) external force.
Interpretation: Motion only changes when an external unbalanced force acts on the object.
Balanced vs Unbalanced Forces:
Balanced: \sum \mathbf{F} = 0 → no acceleration
Unbalanced: \sum \mathbf{F} \neq 0 → acceleration
Newton’s Second Law (for reference):
\mathbf{F} = m \mathbf{a}
Net external force causes acceleration proportional to mass.
Inertia depends on mass: greater mass → greater resistance to changes in motion.
Real-world implications include safety considerations (seat belts), vehicles, and everyday demonstrations (tablecloth, dropping coins, etc.).
Connections and Real-World Relevance
Inertia underpins how we design safety systems (seat belts, airbags) to provide the external force needed to change motion safely.
Understanding frames of reference and how motion appears in different contexts (e.g., inside a moving vehicle vs. outside) helps explain everyday observations like coins dropping straight down on a moving bus.
The ideas connect to orbital mechanics (gravity as the centripetal force) and why planets remain in orbits rather than flying off on a straight-line path.
Historical and Foundational Context
Galileo’s contribution to inertia predates Newton and laid groundwork for Newton’s Laws.
Newton synthesized observations into three laws and published them in Principia Mathematica (1687), which remain foundational to classical mechanics.
Practical Implications and Ethical/Philosophical Notes
The concept of inertia highlights the limitation of assuming that forces are the only active agents in motion; it emphasizes the role of the environment (friction, gravity, resistance) in changing motion.
Philosophically, inertia challenges naive intuitions about motion and causality, illustrating that continued motion does not require continuous force.
Practically, inertia informs engineering, safety design, transportation, sports, and everyday problem solving by predicting how objects will respond to forces.