A push or pull that can change an object's motion. Measured in newtons (N).

Forces that cancel out so the net force is 0 N. The object's motion does not change.

Forces that do not cancel. Net force is not 0 N, so the object's motion changes (it accelerates).

The acceleration of an object depends on the net force and its mass. Conceptually: F = m × a.

Amount of matter in an object and measure of inertia; measured in kilograms (kg).

A push or pull that can change an object's motion; measured in newtons (N).

The force of gravity on an object; depends on mass and gravitational field strength; measured in newtons (N).

Forces and Newton’s Laws: Why Objects Start, Stop, and Move — SHS Science Foundations: Integrated Chemistry, Biology, and Physics

Step 1: What Is a Force?

What Is a Force?

A force is a push or a pull that can change an object's motion: start it, stop it, speed it up, slow it down, or change its direction.

Common Forces

Key everyday forces:

Gravity pulls objects toward Earth.
Normal force is a support force from a surface.
Friction resists motion when surfaces touch.
Applied force is a push or pull you or another object use.

Measuring Force

Force is measured in newtons (N). One newton is the force needed to give a 1 kg mass an acceleration of 1 m/s².

From Motion to Cause

Earlier you learned how to describe motion. Now you will focus on why motion changes: because of forces acting on objects.

Step 2: Balanced vs. Unbalanced Forces

Many Forces at Once

Most objects have several forces acting on them at once. We care about the net force: the overall result after all pushes and pulls are combined.

Balanced Forces

Balanced forces have a net force of 0 N. Motion does not change: rest stays rest; constant motion stays constant motion.

Unbalanced Forces

Unbalanced forces have a net force not equal to 0 N. The object accelerates: it may start, stop, speed up, slow down, or turn.

Box Example

Two people push a box from opposite sides. 50 N vs 50 N → net 0 N (balanced). 50 N vs 30 N → net 20 N toward the stronger push (unbalanced).

Step 3: Newton's First Law – Inertia

First Law: Inertia

Newton's First Law: objects at rest stay at rest, and objects in motion stay in straight-line motion at constant speed unless a net external force acts.

What Is Inertia?

Inertia is resistance to changes in motion. More mass means more inertia, so it is harder to start or stop the object.

Everyday Examples

A soccer ball stays still until kicked. A rolling ball would keep going without friction. In real life, friction and air resistance act to slow it.

Car and Seatbelt

When a car stops suddenly, your body tends to keep moving forward. The seatbelt provides the force that changes your motion and keeps you safe.

Link to Balanced Forces

If forces are balanced (net force = 0), motion does not change. This is exactly what Newton's First Law describes.

Step 4: Newton's Second Law – Force, Mass, and Acceleration

Second Law Overview

Newton's Second Law: an object's acceleration depends on the net force acting on it and its mass.

Force, Mass, Acceleration

More net force → more acceleration. More mass → less acceleration for the same force. Concept formula: `F = m × a`.

Skateboard vs Piano

Push a skateboard and a piano with the same effort. The skateboard (small mass) accelerates a lot; the piano (large mass) hardly accelerates.

Pushing Harder

If you push the same skateboard harder, you apply more force and it speeds up faster. This is Newton's Second Law in action.

Units

Force is in newtons (N), mass in kilograms (kg), and acceleration in meters per second squared (m/s²).

Step 5: Newton's Third Law – Action and Reaction

Third Law Statement

Newton's Third Law: for every action force, there is an equal and opposite reaction force.

Force Pairs

Forces always come in pairs: equal in size, opposite in direction, and acting on different objects.

Pushing a Door

You push on a door; the door pushes back on you with an equal and opposite force. You feel this force in your hand.

Jumping and Walking

Jumping: you push down on the ground; the ground pushes you up. Walking: your foot pushes backward; the ground pushes you forward.

Why They Do Not Cancel

Action and reaction forces do not cancel because they act on different objects, like your foot and the ground.

Step 6: Mass vs. Weight – Not the Same Thing

What Is Mass?

Mass is how much matter is in an object and how much inertia it has. It stays the same on Earth, the Moon, or anywhere else.

What Is Weight?

Weight is the force of gravity on an object. It depends on mass and how strong gravity is, and is measured in newtons (N).

Changing Gravity

Your mass is the same on Earth and the Moon, but your weight is smaller on the Moon because gravity is weaker there.

Key Difference

Mass = amount of stuff and inertia. Weight = how strongly gravity is pulling on that mass right now.

Step 7: Free-Body Ideas in Everyday Situations

Free-Body Ideas

A free-body idea focuses on one object and lists all the forces acting on it. You do not need full diagrams yet, just clear force names.

Book on a Table

For a book at rest: gravity pulls down; the table's normal force pushes up. These are balanced, so the book stays at rest.

Person in an Elevator

Standing still in a stopped elevator: gravity pulls you down; the floor's normal force pushes you up. Balanced forces, no acceleration.

Bike Speeding Up

For a bike on level ground: ground pushes it forward; friction and air resistance push backward; gravity and normal force balance vertically.

Sliding Box Slowing Down

A box sliding after you stop pushing: friction acts opposite motion; gravity and normal force balance. Unbalanced friction slows the box.

Step 8: Thought Exercise – Start, Stop, or Keep Going?

Apply what you know about forces and Newton's laws.

For each situation, decide:

1) Are the forces mostly balanced or unbalanced?

2) Which Newton's law is most helpful to explain it?

Write short answers in your notes or say them out loud.

A hockey puck sliding on nearly frictionless ice keeps moving at almost constant speed.

Balanced or unbalanced?
Which law explains why it keeps moving?

You push harder on a shopping cart, and it speeds up more.

Balanced or unbalanced?
Which law explains the link between your push and the acceleration?

You jump off the ground.

What force actually pushes you up?
Which law describes the action and reaction forces?

A phone rests on your hand.

List the forces on the phone.
Are they balanced or unbalanced?

When you finish, check yourself:

If motion is not changing → think First Law and balanced forces.
If motion is changing → think Second Law and unbalanced forces.
If you see pairs of pushes between two objects → think Third Law.

Step 9: Quick Check – Everyday Forces

Answer this multiple-choice question to check your understanding.

You are riding a bike at a steady speed on a straight, level road. Which statement best describes the horizontal forces on you and the bike?

There are no horizontal forces acting because the speed is constant.
The forward force from the ground on the tires is exactly balanced by friction and air resistance.
The forward force from the ground is greater than friction, but gravity balances it.
Friction is greater than the forward force from the ground, but inertia keeps you moving.

Show Answer

Answer: B) The forward force from the ground on the tires is exactly balanced by friction and air resistance.

At constant speed in a straight line, the net force is zero (First Law). Horizontally, the forward force from the ground on the tires must be balanced by friction and air resistance. Forces can be present and still balanced.

Step 10: Quick Check – Mass, Weight, and Laws

One more question to connect mass, weight, and Newton's laws.

Which statement is correct?

Your mass and your weight both become zero in deep space, far from planets.
Your mass decreases on the Moon, but your weight stays the same.
Your mass stays the same on Earth and the Moon, but your weight changes because gravity is different.
Mass and weight are just two names for the same thing.