Force

Newton’s Laws

1. Newton’s First Law (Law of Inertia)

Definition: An object at rest will stay at rest, and an object in motion will stay in motion with the same speed and direction unless acted upon by an unbalanced (net) force.

Inertia means: the tendency of an object to resist changes in its motion.

An object's inertia is based on its: mass.

Greatest inertia example answer: D) a 15‑kilogram object at rest (largest mass → greatest inertia).

2. Newton’s Second Law

Definition: The acceleration of an object depends on the net force acting on it and the mass of the object.

Formula:
F = m a

Key ideas:

  • Force is a vector (has magnitude and direction)

  • Acceleration is in the same direction as the net force

Example:
F = 4.0 N, m = 2.0 kg → a = 2.0 m/s² east

Other types of forces: gravity (weight), normal force, friction, air resistance, tension, applied force

3. Newton’s Third Law

Definition: For every action force, there is an equal and opposite reaction force.

Bug vs windshield: The bug and windshield exert equal and opposite forces on each other; the bug experiences greater damage due to smaller mass.

Weight

Definition: A force caused by gravity acting on mass.

Formula:
W = m g

Example:
m = 6 kg → W = 58.8 N

Net Force and Motion

Net force: The vector sum of all forces acting on an object.

Three motion states:

  • At rest

  • Constant velocity

  • Accelerating

Constant velocity:
a = 0 → Fnet = 0

Accelerating object: Net force is not zero and points in the direction of acceleration.

Equilibrium

Equilibrium is: when all forces balance and the net force equals zero.

Fnet = 0 → a = 0

Objects at rest or moving at constant velocity are in equilibrium.

Friction

Definition: A force that opposes motion between two surfaces in contact.

Depends on: surface type and normal force (not surface area).

Types of Friction

  • Static friction: prevents motion (greater than kinetic)

  • Kinetic friction: occurs during motion

Special cases:

  • Sliding friction: objects sliding past each other

  • Rolling friction: rolling objects (smaller than sliding)

  • Fluid friction: air or liquid resistance

Formula:
Ff = μFn

Ff = force of friction
μ = coefficient of friction
Fn = normal force

Slippery surfaces: low coefficient of friction

Forces on an Incline

  • Weight acts straight down

  • Normal force is perpendicular to surface

Components of weight:

  • Parallel: mg sinθ (causes sliding)

  • Perpendicular: mg cosθ (balanced by normal force)

Sliding condition:
Object slides when parallel component > friction force

Uniform Circular Motion

Definition: Motion in a circle at constant speed.

Centripetal acceleration: acceleration directed toward the center of the circle.

Formula:
ac = v² / r

  • Velocity is tangent to the circle

  • Acceleration points inward

Centripetal force: the net force causing circular motion

Fc = m v² / r

Supplied by: friction, tension, gravity, or normal force

Universal Gravitation

Definition: Every mass attracts every other mass with a gravitational force.

Formula:
Fg = G m1 m2 / r²

Key ideas:

  • Force increases with mass

  • Force decreases with distance squared

If distance doubles: force becomes 1/4

Weight vs distance: weight decreases as distance from Earth increases

HIGH‑YIELD FLASHCARDS (Quiz Ready)

Flashcard 1

Q: What determines inertia?
A: Mass only

Flashcard 2

Q: Condition for equilibrium?
A: Net force = 0 and acceleration = 0

Flashcard 3

Q: Difference between static and kinetic friction?
A: Static prevents motion; kinetic opposes motion

Flashcard 4

Q: Direction of centripetal force?
A: Always toward the center of the circle

Flashcard 5

Q: What changes in uniform circular motion?
A: Velocity direction (causing acceleration)

Flashcard 6

Q: Formula linking force, mass, and acceleration?
A: F = ma

Flashcard 7

Q: What provides centripetal force in a car turn?
A: Friction between tires and road

Flashcard 8

Q: How does gravity depend on distance?
A: Inversely proportional to distance squared

Flashcard 9

Q: When is friction equal to applied force?
A: At constant velocity

Flashcard 10

Q: Why do lighter objects accelerate more for same force?
A: Smaller mass → larger acceleration

COMPLETE UNIT 4 FLASHCARD SET (ONE SET)

Newton’s Laws & Inertia

Flashcard 1
Q: State Newton’s First Law of Motion.
A: An object at rest stays at rest and an object in motion stays in motion with the same speed and direction unless acted upon by an unbalanced force.

Flashcard 2
Q: What is inertia?
A: The tendency of an object to resist changes in its motion.

Flashcard 3
Q: What property of matter determines inertia?
A: Mass.

Flashcard 4
Q: Which object has the greatest inertia?
A: The object with the greatest mass.

Flashcard 5
Q: State Newton’s Second Law of Motion.
A: The acceleration of an object is directly proportional to the net force and inversely proportional to its mass.

Flashcard 6
Q: Write Newton’s Second Law as a formula.
A: F = ma

Flashcard 7
Q: If force increases and mass stays constant, what happens to acceleration?
A: Acceleration increases.

Flashcard 8
Q: State Newton’s Third Law of Motion.
A: For every action force, there is an equal and opposite reaction force.

Flashcard 9
Q: Why does a bug splatter on a windshield even though forces are equal?
A: The bug has much smaller mass, so it experiences much greater acceleration.

Forces & Weight

Flashcard 10
Q: What is weight?
A: The force of gravity acting on an object.

Flashcard 11
Q: Formula for weight?
A: W = mg

Flashcard 12
Q: What is the value of g on Earth?
A: 9.8 m/s²

Flashcard 13
Q: Difference between mass and weight?
A: Mass is amount of matter (kg); weight is gravitational force (N).

Flashcard 14
Q: List common forces.
A: Gravity, normal, friction, tension, applied force, air resistance.

Net Force & Motion

Flashcard 15
Q: What is net force?
A: The vector sum of all forces acting on an object.

Flashcard 16
Q: What does Fnet = 0 mean?
A: No acceleration.

Flashcard 17
Q: What kind of motion occurs when Fnet = 0?
A: Rest or constant velocity.

Flashcard 18
Q: When does an object accelerate?
A: When the net force is not zero.

Flashcard 19
Q: Why is net force not drawn on a free body diagram?
A: Because it is the result of forces, not an individual force.

Equilibrium

Flashcard 20
Q: What is equilibrium?
A: A state where the net force on an object is zero.

Flashcard 21
Q: Acceleration of an object in equilibrium?
A: Zero.

Flashcard 22
Q: Are moving objects ever in equilibrium?
A: Yes, if moving at constant velocity.

Friction

Flashcard 23
Q: What is friction?
A: A force that opposes motion between two surfaces in contact.

Flashcard 24
Q: What does friction depend on?
A: The nature of the surfaces and the normal force.

Flashcard 25
Q: Does friction depend on surface area?
A: No.

Flashcard 26
Q: Difference between static and kinetic friction?
A: Static prevents motion; kinetic acts during motion.

Flashcard 27
Q: Which is greater: static or kinetic friction?
A: Static friction.

Flashcard 28
Q: Formula for friction force?
A: Ff = μFn

Flashcard 29
Q: What does μ represent?
A: Coefficient of friction.

Flashcard 30
Q: What does a low μ mean?
A: Slippery surface.

Forces on an Incline

Flashcard 31
Q: Direction of weight on an incline?
A: Straight down toward Earth.

Flashcard 32
Q: Direction of normal force on an incline?
A: Perpendicular to the surface.

Flashcard 33
Q: Parallel component of weight?
A: mg sinθ

Flashcard 34
Q: Perpendicular component of weight?
A: mg cosθ

Flashcard 35
Q: When does an object slide down an incline?
A: When the parallel component exceeds friction.

Uniform Circular Motion

Flashcard 36
Q: What is uniform circular motion?
A: Motion in a circle at constant speed.

Flashcard 37
Q: Why is an object accelerating in circular motion?
A: Its velocity direction is constantly changing.

Flashcard 38
Q: Direction of centripetal acceleration?
A: Toward the center of the circle.

Flashcard 39
Q: Formula for centripetal acceleration?
A: ac = v² / r

Flashcard 40
Q: Formula for centripetal force?
A: Fc = mv² / r

Flashcard 41
Q: What supplies centripetal force?
A: Another force such as friction, tension, gravity, or normal force.

Universal Gravitation

Flashcard 42
Q: State Newton’s Law of Universal Gravitation.
A: Every mass attracts every other mass with a force dependent on mass and distance.

Flashcard 43
Q: Formula for gravitational force?
A: Fg = Gm₁m₂ / r²

Flashcard 44
Q: How does force change if distance doubles?
A: It becomes one-fourth.

Flashcard 45
Q: Are gravitational forces equal on both objects?
A: Yes, equal in magnitude and opposite in direction.

Flashcard 46
Q: What causes ocean tides?
A: The Moon’s gravitational pull.

Exam Traps & Key Ideas

Flashcard 47
Q: Can an object move with zero net force?
A: Yes, at constant velocity.

Flashcard 48
Q: Does greater force always mean greater speed?
A: No, it means greater acceleration.

Flashcard 49
Q: Why do heavier objects not fall faster on Earth?
A: Gravity causes the same acceleration for all masses.

Flashcard 50
Q: One-sentence summary of Unit 4?
A: Forces cause acceleration, friction resists motion, and gravity governs motion on Earth and in space.

TOTAL: 50 FLASHCARDS (ONE COMPLETE SET)