SCIENCE 9 (PHYSICS)

Reviewer: Laws of Motion, Forces, Acceleration, and Reaction and Action + friction + vector and scalar:

Lesson 1: Newton’s First Law of Motion (Law of Inertia)

Definition

Newton’s First Law states that:

An object at rest will remain at rest, and an object in motion will continue moving at a constant velocity unless acted upon by an unbalanced force.

Inertia

  • Inertia is the tendency of an object to resist changes in its motion.

  • The mass of an object is a measure of its inertia.

  • The greater the mass, the greater the inertia.

Key Points

  • Balanced forces doesn’t immediately result in no change in motion.

  • Objects do not need a force to keep moving.

  • A force is only needed to change an object's motion.

Lesson 2: Newton’s Second Law of Motion (Law of Acceleration)

Definition

Newton's Second Law states:

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

Formula

[F=ma][F = ma]

Where:

  • F = Force (Newtons, N)

  • m = Mass (kg)

  • a = Acceleration (m/s²)

Rearranged Formulas

[a=Fm][a = \frac{F}{m}]

[m=Fa][m = \frac{F}{a}]

Relationships

Force and Acceleration

  • More force → Greater acceleration

  • Less force → Smaller acceleration

Mass and Acceleration

  • More mass → Smaller acceleration

  • Less mass → Greater acceleration

Examples

  1. A 10 N force pushes a 2 kg object.

[a=102=5,m/s2][a=\frac{10}{2}=5 , m/s^2]

  1. A 50 N force acts on a 10 kg object.

[a=5010=5,m/s2][a=\frac{50}{10}=5 , m/s^2]

Acceleration

Definition

Acceleration is the rate of change of velocity.

An object accelerates when:

  • Its speed increases.

  • Its speed decreases (deceleration).

  • Its direction changes.

  • Both speed and direction change.

Formula

[a=vf−vit][a=\frac{v_f-v_i}{t}]

Where:

  • (v_f) = final velocity

  • (v_i) = initial velocity

  • (t) = time

Units (taught)

  • meters per second squared (m/s²)

Force

Definition

A force is a push or pull that can change the motion of an object.

Unit

  • Newton (N) / kg⁡m/s2\operatorname{kg}m/s^2

Types of Forces

Balanced Forces

  • Equal in magnitude and opposite in direction.

  • Net force = 0.

  • No acceleration occurs.

Unbalanced Forces

  • Forces are not equal.

  • Net force ≠ 0.

  • Causes acceleration.

Vector and Scalar

Vector - quantity that has magnitude and direction.
Scalar - quantity that has magnitude only.
Net Force - vector sum of all individual forces.


FORMULA:

Fnet​=F1​+F2​+F3​+⋯Fnet​=F1​+F2​+F3​+⋯

​Fnet​=Fapplied​−Ffriction​Fnet​=Fapplied​−Ffriction​

Friction

Definition

Friction is a force that opposes motion between two surfaces in contact.

Important Rule

Friction always acts opposite the direction of motion or the applied force.

Relationship Between Friction and Other Forces

When friction and another force act on an object:

  • Friction opposes the other force.

  • Friction reduces the net force acting on the object.

  • Friction can slow down, stop, or prevent motion.


Finding the Resultant of Perpendicular Forces

When two forces are at right angles (90°), use the Pythagorean Theorem.

Formula

[R=F12+F22][R=\sqrt{F_1^2+F_2^2}]

Where:

  • (R) = resultant force

  • (F_1) and (F_2) = perpendicular forces

Example

[F1=30N][F2=40N][R=302+402][R=900+1600][R=2500][R=50N][F_1 = 30N][F_2 = 40N][R=\sqrt{30^2+40^2}][R=\sqrt{900+1600}][R=\sqrt{2500}][R=50N]

Lesson 3: Newton's Third Law of Motion (Law of Action and Reaction)

Definition

Newton's Third Law states:

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

This means that when one object exerts a force on another object, the second object exerts a force of equal magnitude but in the opposite direction on the first object.

Key Characteristics

  • Action and reaction forces always occur in pairs.

  • The forces are equal in magnitude.

  • The forces are opposite in direction.

  • The forces act on different objects.

  • One force cannot exist without the other.

Formula

Faction=FreactionF_{action}=F_{reaction}

Walking

  • Your foot pushes backward on the ground (action).

  • The ground pushes your foot forward (reaction).

  • This allows you to move forward.

Swimming

  • A swimmer pushes water backward (action).

  • Water pushes the swimmer forward (reaction).

Rocket Launch

  • The rocket pushes gases downward (action).

  • The gases push the rocket upward (reaction).

Gun Recoil

  • The bullet moves forward (action).

  • The gun moves backward (reaction).