Balanced Forces (평형력) — MYP Grade 7 Science

Big idea A force is a push or a pull. When all the forces on an object cancel out, we call them balanced forces — and the object's motion does not change.

01What is a force?

A force is a push or a pull that one object can apply to another. Forces can start motion, stop motion, speed things up, slow things down, or change direction.

We draw forces as arrows: the length shows how strong the force is, and the direction of the arrow shows which way it acts.

02Balanced forces and equilibrium

Forces are balanced when they are equal in size and opposite in direction, so they cancel out. The total (resultant) force is zero.

When forces are balanced, an object stays in equilibrium: if it was still, it stays still; if it was moving, it keeps moving at the same speed in the same direction.

Example. A book resting on a table: gravity pulls it down, and the table pushes up with an equal force. The forces balance, so the book stays still.

03Newton's First Law

Newton's First Law says an object stays at rest, or keeps moving at a constant velocity, unless an unbalanced (resultant) force acts on it.

This tendency to keep doing what it's already doing is called inertia.

04Everyday examples

Many everyday situations involve balanced forces, even when it doesn't look obvious.

A car driving at a steady speed on a flat road (driving force = friction + air resistance)
A hot air balloon floating at a constant height (upward lift = downward weight)
A book on a shelf that is not moving (normal force = weight)

Key vocabulary
Term	한국어	Meaning
force	힘	A push or a pull
balanced forces	평형력	Forces that cancel out; resultant force = zero
equilibrium	평형 상태	A state where balanced forces keep motion unchanged
resultant force	합력	The single overall force after combining all forces
inertia	관성	An object's tendency to resist a change in motion
normal force	수직 항력	The support force a surface pushes back with

Quick check — tap a question to reveal the answer

A ball hangs still from a string. Name the two forces acting on it.

Gravity (weight) pulling down, and tension in the string pulling up. They are equal and balanced.

True or false: balanced forces mean there are no forces at all.

False — there can be several forces, but they cancel out so the resultant is zero.

What does Newton's First Law say happens to a moving object if all forces on it are balanced?

It keeps moving at the same speed in the same direction (constant velocity).

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