What is the difference between mass and weight?

Mass is the amount of matter in an object, measured in kilograms, and it never changes. Weight is the gravitational force on that mass, measured in newtons, and it is found by multiplying the mass by g. The same astronaut has the same mass on the Moon but a smaller weight there because gravity is weaker.

Why do action and reaction forces not just cancel out?

Newton's third law forces always act on two different objects, never on the same one. The book pushes down on the table and the table pushes up on the book, but those two forces act on different bodies, so they never appear in the same net force calculation and cannot cancel.

Why does mass cancel out on a frictionless incline?

The force pulling the block down the slope is mg sin theta, but acceleration is force divided by mass, so the m on top and the m on the bottom cancel. That leaves a = g sin theta, which means a heavy block and a light block slide down the same slope with the same acceleration.

Newton's Laws and Forces | EduVerse Learn

Give a shopping trolley a shove and it rolls, push harder and it speeds up faster, load it with bricks and the same push barely moves it. Hidden inside that everyday moment are Newton’s three laws of motion, the rules that connect a force to the way things move. Once you can spot the forces acting on an object and add them up, you can predict exactly how it will move, every single time.

The three laws in plain English

A force is just a push or a pull. Newton noticed that forces explain everything about how objects start moving, stop moving, or change direction, and he boiled it down to three short laws.

First law (inertia). An object stays at rest, or keeps moving at constant velocity, unless a net force acts on it. Things do not change their motion on their own. They are lazy.
Second law. When there is a net force, the object accelerates. The bigger the net force the bigger the acceleration, and the heavier the object the smaller the acceleration. In symbols, $F_{net} = ma$ .
Third law. Every action has an equal and opposite reaction. If you push on a wall, the wall pushes back on you just as hard. The two forces are the same size, point opposite ways, and crucially act on different objects.

The most important word in all three is net. You add up every force on the object as vectors first, and only the leftover net force decides what happens.

The forces you will meet

Almost every VCE question is built from the same small cast of forces. Learn to recognise these and you can draw the picture for any problem.

Weight, the pull of gravity, always points straight down and equals $F_g = mg$ with $g = 9.8$ m/s $^2$ .
Normal force $N$ , the push of a surface, always points at right angles out of the surface.
Tension $T$ , the pull of a rope or string, always points along the rope away from the object.
Friction $f$ , which resists sliding and acts along the surface. Its maximum size is $f = \mu N$ , where $\mu$ is the coefficient of friction.

The clearest way to keep track of all this is a free body diagram: draw the object as a dot or box and every force as an arrow pointing the way it acts.

The blue arrows are the normal force pushing up and the push driving the block to the right. The red arrows are the weight pulling down and the friction resisting the slide. Add up the arrows in each direction and the leftover is the net force.

Newton’s second law is the workhorse

If you remember one equation from this topic, make it $F_{net} = ma$ . It is the bridge between the forces you draw and the motion you measure.

To use it, add up all the forces as vectors to get the net force, then divide by the mass to get the acceleration. The acceleration always points the same way as the net force. If the forces cancel, the net force is zero, the acceleration is zero, and you are back to the first law: rest or constant velocity.

A frictionless incline is the classic example. Gravity points straight down, but only the part of it that runs along the slope can speed the object up. That component works out to $g\sin\theta$ , and because mass cancels, the acceleration down a frictionless slope is simply $a = g\sin\theta$ , no matter how heavy the object is. The part of gravity pressing into the slope is $g\cos\theta$ , and that is balanced by the normal force.

Learn it, then fade the help

1 · I do — fully worked

Acceleration from a net force. A $5.0$ kg box is pushed across a frictionless floor by a horizontal $20$ N force. The only horizontal force is the push, so $F_{net} = 20$ N. Then $a = \dfrac{F_{net}}{m} = \dfrac{20}{5.0} = 4.0 \text{ m/s}^2$ .

2 · We do — your turn, guided

Same steps, your turn. A $4.0$ kg trolley on a frictionless floor is pushed by a horizontal $18$ N force. Find its acceleration. Start with $a = \dfrac{F_{net}}{m} = \dfrac{18}{\;?\;}$ .

Check your step

$a = \dfrac{18}{4.0} = 4.5 \text{ m/s}^2$ , in the direction of the push.

3 · You do — solo

No scaffolding now: a $2.0$ kg block slides down a frictionless incline at $30^\circ$ . Taking $g = 9.8$ m/s $^2$ , find its acceleration.

Reveal solution

The force along the slope is $mg\sin\theta$ , so $a = g\sin\theta = 9.8 \times \sin 30^\circ = 9.8 \times 0.5 = 4.9 \text{ m/s}^2$ . The mass cancels, so it does not matter how heavy the block is.

See it for yourself

Pile on the pushers, change the load, and watch how the net force and the motion respond. Try balancing two equal pushes on opposite sides and notice the object does not move, then add a single extra newton to one side and watch it accelerate.

Interactive simulation, Forces and Motion: Basics Source: PhET Interactive Simulations, University of Colorado Boulder (CC BY 4.0)

How to actually solve one

Nearly every forces question follows the same short recipe.

Draw a free body diagram with every force as an arrow on the object.
Choose a positive direction, then add the forces as vectors to find the net force.
Apply $F_{net} = ma$ to find the unknown, whether that is the acceleration, the mass, or a missing force.
For a surface, remember $N$ balances the perpendicular forces and friction is at most $f = \mu N$ .

Watch the difference between mass and weight. Mass in kilograms goes into $F_{net} = ma$ , while weight in newtons is a force you draw on the diagram using $F_g = mg$ .

Lock it in with active recall

Cover the answer and say each one out loud before you flip. Rate yourself honestly — the cards you find hard come back sooner, the ones you know are spaced further out.

Active recall

Answer from memory first, then flip. Rate yourself and each card returns on a spaced schedule (1 → 3 → 7 → 16 days).

State Newton’s second law and what each symbol means.

What does Newton’s first law say about an object with no net force on it?

Why don’t action and reaction forces cancel out?

How do you find the weight of a mass, and how is it different from mass?

What is the acceleration of a block sliding down a frictionless incline at angle

\theta

Write the formula for the maximum friction force before sliding.

Recall · Momentum and Impulse

How is impulse related to force and time?

Recall · Work, Energy and Power

What is the work done by a force over a distance?

See the recipe in action in the Worked Examples tab, then test yourself in Try It.

Newton's Laws and Forces

The three laws in plain English

The forces you will meet

Newton’s second law is the workhorse

See it for yourself

How to actually solve one

Lock it in with active recall

Try it yourself

Frequently asked questions

Newton's Laws and Forces

Learn

The three laws in plain English

The forces you will meet

Newton’s second law is the workhorse

See it for yourself

How to actually solve one

Lock it in with active recall

Worked examples

Practice questions

Try it yourself

Frequently asked questions