Net Force, State of Motion, and the Law of Inertia

Module focus: Term 1, Week 1, Day 1 of the Science 9 curriculum.
Primary Subject: Laws of Motion, specifically focusing on Net Force, State of Motion, and the Law of Inertia for the duration of the week.
Learning Objectives:
- Identify and describe the five common forces: Gravity, Normal Force, Friction, Applied Force, and Air Resistance, through real-life applications.
- Demonstrate appreciation for how forces influence daily activities, including walking, driving, playing sports, and falling objects.
- Compute and record net force values accurately within the "Force Detective Worksheet."
- Classify physical scenarios as either balanced or unbalanced forces.

Gravity: A ripe apple falling from a tree toward the ground because it is pulled downward toward Earth.
Friction: The force generated when rubbing hands together.
Normal Force: A book resting on top of a surface, such as a table.
Applied Force: A student pushing a shopping cart forward.
Air Resistance: A skydiver opening a parachute to slow down their descent.
Balanced Forces: A person standing perfectly still.
Unbalanced Forces: A soccer ball being kicked, which causes it to start moving.
Net Force: Demonstrated in a tug-of-war where forces are unequal.
Inertia: When a bus stops suddenly, and the passengers move forward.
Constant Velocity: A car moving in a straight line at the same speed for several seconds.

Scenario: A commercial airplane cruising high above clouds at a steady speed in a straight direction.
Gravitational Influence: Gravity acts on the airplane even at high altitudes. The Earth continuously pulls the airplane downward with a force categorized as weight.
The Physics of Flight Stability: The airplane does not fall because of "Lift." Lift is the upward force that balances the downward pull of gravity.
The Four Forces of Flight:
1. Gravity: Acting downward (Weight).
2. Lift: Acting upward.
3. Thrust: Acting forward (providing the motion).
4. Drag: Acting backward (opposing forward motion).
Net Force Calculation ( $F_{net}$ ): When an airplane moves at a constant velocity, the net force is $0\,N$ (Newtons). This is because the forces are balanced: Lift equals Gravity, and Thrust equals Drag.
Motion Characteristics: Because the speed and direction do not change, the object is not accelerating. According to Isaac Newton’s First Law of Motion, an object remains at a constant velocity unless acted upon by an unbalanced force (net force).

Definition: A force is a push or a pull.
Capabilities of Force:
- Start motion.
- Stop motion.
- Change the speed of an object.
- Change the direction of an object.
Examples: Kicking a ball, pushing a shopping cart, pulling a door open, or gravity pulling objects downward.
Unit of Measurement: Forces are measured in Newtons ( $N$ ).

Gravity:
- Definition: A force that pulls all objects toward the center of the Earth.
- Direction: Always acts downward.
- Example: Dropping a book leads it to fall directly toward the floor.
Normal Force:
- Definition: The support force exerted by a surface.
- Direction: Always perpendicular to the surface (upward from a flat surface at a $90^{\circ}$ angle).
- Mechanism: If a book lies on a table, it compresses the atoms in the table; these atoms squeeze upward, creating the support force.
Friction:
- Definition: A force that opposes the motion between two surfaces in contact.
- Direction: Acts in the opposite direction of the motion.
- Example: Sliding a heavy box across the floor.
Applied Force:
- Definition: A force that is applied to an object by a person or another object.
- Example: Pushing a chair across a room.
Air Resistance:
- Definition: A specific type of friction occurring between air and a moving object.
- Direction: Opposes the motion of the object.
- Example: A parachute provides air resistance to slow down a skydiver.

Definition of Net Force: The vector sum (net result) of all forces acting on an object at a given time. It determines if the state of motion will change.
Case 1: Balanced Forces:
- Net Force ( $F_{net}$ ) is $0\,N$ .
- Forces are equal in magnitude/amount but opposite in direction.
- Result: No change in motion (object stays at rest or continues at constant velocity).
- Example: A rock resting on a table where Gravity is $10\,N$ downward and Normal Force is $10\,N$ upward.
Case 2: Unbalanced Forces:
- Net Force ( $F_{net}$ ) is not equal to zero ( $F_{net} \neq 0$ ).
- Forces are not equal.
- Result: Acceleration occurs (change in speed or direction).
- Example: Pushing a $10\,kg$ box with a $20\,N$ force while friction provides $5\,N$ of resistance. The net force is $15\,N$ in the forward direction ( $20\,N - 5\,N = 15\,N$ ).

Scenario 1: Book on a Table
- Gravity: $15\,N$ downward.
- Normal Force: $15\,N$ upward.
- Net Force: $0\,N$ .
- Motion: The object is at rest (not moving).
Scenario 2: Pushing a Box
- Pushing Force: $30\,N$ .
- Friction: $10\,N$ .
- Net Force: $20\,N$ in the direction of the push.
Scenario 3: Car at Constant Speed
- Thrust: $50\,N$ .
- Air Resistance: $50\,N$ .
- Net Force: $0\,N$ .
- Observation: No acceleration is happening, demonstrating Newton’s First Law (rest/constant motion when forces are balanced).
Scenario 4: Kicking a Soccer Ball
- Kick Force: $40\,N$ to the right.
- Air Resistance: $5\,N$ to the left.
- Net Force: $35\,N$ to the right.
- Classification: Unbalanced force; the ball will speed up/accelerate.

Definition of Force: A push or a pull.
Force Pulling Toward Earth: Gravity.
Support Force from Surfaces: Normal Force.
Function of Friction: To oppose motion.
Effect of Zero Net Force: The motion of the object does not change.
Definition of Balanced Forces: Net force is equal to zero ( $F_{net} = 0$ ).
Sample Calculation: If a $20\,N$ force pushes right and a $5\,N$ friction force acts left, the Net Force is $15\,N$ to the right.
Consequence of Unbalanced Forces: Acceleration occurs.
Inertia Principle: An object at rest will stay at rest unless acted upon by an unbalanced force.
Measurement Unit: Forces are measured in Newtons ( $N$ ).