Key concept · Interaction

Unit 2: Forces and How They Interact

A force is a push or a pull. Forces never act alone — they always come from an interaction between two objects. This unit explores how forces combine, balance, and change the way objects move.

01 What is a force?

A force is a push or pull that can: change an object's speed, change its direction, or change its shape. Forces are measured in newtons (N).

Forces are drawn as arrows: the length shows the size, and the direction shows which way it acts.

02 Contact and non-contact forces

Contact forces	Non-contact forces
Friction, air resistance, normal force, tension, applied push/pull	Gravity, magnetism, electrostatic force

Contact forces need the objects to be touching. Non-contact forces act across a distance, with no touching needed.

03 Resultant force

When more than one force acts on an object, we combine them into one overall force called the resultant force.

resultant force = sum of forces in one direction − sum of forces in the opposite direction

Balanced forces: resultant force = 0 N. The object stays still or keeps moving at a constant speed in a straight line.
Unbalanced forces: resultant force ≠ 0 N. The object accelerates (speeds up, slows down, or changes direction).

04 Newton's first and second laws (introduction)

Newton's First Law: An object stays at rest, or keeps moving at a constant velocity, unless a resultant force acts on it.

Newton's Second Law: A resultant force causes acceleration. Bigger force → bigger acceleration. Bigger mass → smaller acceleration (for the same force).

F = m × a (force in N = mass in kg × acceleration in m/s²)

V Key vocabulary

Term	Meaning
Force	A push or a pull between two objects, measured in newtons (N).
Resultant force	The single overall force found by combining all forces acting on an object.
Friction	A contact force that opposes motion between two touching surfaces.
Newton (N)	The SI unit of force.

? Quick check

Try to answer before you tap. Answers are hidden underneath.

Two forces act on a box: 15 N right and 6 N left. What is the resultant force?

9 N to the right (15 N − 6 N = 9 N, in the direction of the bigger force).

Give one example each of a contact force and a non-contact force.

Contact: friction, tension, or the normal force from a surface. Non-contact: gravity, magnetism, or electrostatic force.

A car moves at a constant speed in a straight line. What does this tell you about the forces on it?

The forces are balanced — the resultant force is 0 N (Newton's First Law).

← previousUnit 01: The Particle Model of Matter next →Unit 03: Measurement and Scientific Evidence