Newton's Laws of Motion: Why Things Move the Way They Do

Isaac Newton's three laws of motion are among the most successful ideas in all of science: simple sentences that predict how objects move, from a rolling ball to a launching rocket. The first law explains why things keep doing what they're doing; the second connects force, mass, and acceleration in one tidy equation; the third explains why forces always come in pairs. Together they're the backbone of mechanics.

This guide explains each law with everyday examples, plus the common forces involved, with worked and practice questions matched to the level seen in ACT Science and physics at Northside Tutoring.

Why Newton's Laws Matter

Newton's laws explain the motion of everyday objects and are central to physics courses and many ACT Science passages. The second law, F = ma, is one of the most-used equations in all of science. (The SAT has no science section.)

First Law: Inertia

An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net force. This tendency to resist changes in motion is called inertia, and it's why you lurch forward when a car brakes — your body "wants" to keep moving.

Second Law: F = ma

The net force on an object equals its mass times its acceleration:

This says a bigger force produces more acceleration, and a heavier object accelerates less for the same force. Rearranged, a = F/m. It's the workhorse equation of mechanics — solve it for whichever quantity a problem asks for.

Third Law: Action-Reaction

For every action, there is an equal and opposite reaction. When you push on a wall, the wall pushes back on you with equal force. A rocket pushes exhaust gases down, and the gases push the rocket up. Forces always come in pairs acting on different objects.

Common Forces

Several forces show up again and again: gravity (pulls mass toward Earth, giving weight), the normal force (a surface pushing up on an object), friction (opposes motion between surfaces), and tension (a pull through a rope or string). The net force is the combined total of all forces acting on an object.

Putting Them Together

To solve a motion problem: identify all the forces, find the net force, then apply F = ma to get acceleration (or work backward). If forces are balanced (net force zero), the first law applies — no acceleration. If unbalanced, the second law tells you how the object accelerates.

Where You'll See This — Test by Test

Newton's laws support ACT Science physics passages; the SAT has no science section and the SSAT doesn't test it. They're core high-school and AP Physics.

Watch the Lesson

Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.

Worked Example Problems

These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.

Common Mistakes to Avoid

Practice Problems — You Try

Three problems below. Work each before checking the solution.

The Northside Method — How We Teach This 1-on-1

Reading a blog is a great starting point. But there's a meaningful gap between understanding a concept and reflexively applying it under timed conditions. That gap is exactly what our tutors close.

Every Northside student works through a four-step framework:

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