One sandbox for all three of Newton's laws: glide with no net force, accelerate at a = F/m on a live velocity-time graph, and push off to see equal and opposite forces.
With no net force (friction off) the box keeps gliding at constant velocity forever, that is Newton's first law. Force is not needed to keep it moving; friction is what makes real objects stop.
The three laws
Inertia: no net force → velocity does not change.
F = ma: a net force gives an acceleration a = F/m.
Action-reaction: forces come in equal, opposite pairs on different bodies.
Pick a law, then use the controls to change what happens.
Inertia demo. Friction is off, so the box glides at constant velocity.
The rules that govern all motion
Almost every everyday motion, a rolling ball, a launched rocket, a walk to school, obeys
three rules written down by Isaac Newton. The sandbox above lets you play with all three:
switch between Inertia, F = ma, and Action-Reaction and change what happens.
First law: inertia (no net force, no change)
An object keeps doing what it is doing, at rest or moving at constant velocity, unless a
net force acts on it. The everyday reason things slow down is friction, not that
their motion “runs out”.
Second law: F = ma (net force makes acceleration)
A net force gives an object an acceleration: a = F / m. The bigger the force the
bigger the acceleration; the heavier the object the smaller the acceleration.
Third law: action-reaction (equal and opposite)
When A pushes B, B pushes back on A with an equal and opposite force. The two forces
are the same size, point opposite ways, and act on different objects, which is exactly
why they never cancel out.
The three laws together
Law
In one line
Key idea
First (inertia)
No net force, no change in velocity
Force changes motion, it does not maintain it
Second (F = ma)
Net force = mass times acceleration
a = F/m
Third (action-reaction)
Forces come in equal, opposite pairs
The pair acts on two different bodies
Two traps worth knowing
Heavier objects do not fall faster. In a vacuum a hammer and a feather land
together, because a = mg/m = g for any mass.
The reaction force is not weaker, and the pair does not cancel. Action and reaction
are always equal, and because they act on different objects they cannot cancel.
First law (inertia): an object stays at rest, or keeps moving at constant velocity in a straight line, unless a net force acts on it. Second law: a net force gives an object an acceleration equal to force divided by mass (a = F/m, or F = ma). Third law: for every action force there is an equal and opposite reaction force, acting on a different object.
What is Newton's first law of motion?
Newton's first law, the law of inertia, says an object's velocity only changes if a net force acts on it. With no net force, something at rest stays at rest and something moving keeps moving at the same speed and direction forever. Everyday objects slow down only because of friction and air resistance, not because motion 'runs out'. In deep space a nudged object glides on indefinitely.
What is Newton's second law of motion?
Newton's second law relates force, mass, and acceleration: net force = mass times acceleration (F = ma), so acceleration = force / mass (a = F/m). Double the net force and the acceleration doubles; double the mass and the acceleration halves. The acceleration points in the same direction as the net force.
What is Newton's third law of motion?
For every action there is an equal and opposite reaction: if object A pushes on object B, then B pushes back on A with a force of the same size in the opposite direction. The two forces are always equal in strength and they act on DIFFERENT objects, which is why they do not cancel each other out. A swimmer pushes water back and the water pushes the swimmer forward.
Why don't heavier objects fall faster?
Ignoring air resistance, all objects fall with the same acceleration (about 9.8 m/s squared on Earth). A heavier object feels a bigger gravitational force, but it also has more mass to move, and by a = F/m the two effects cancel exactly: a = mg/m = g for any mass. A hammer and a feather dropped in a vacuum land at the same instant.
What is the difference between mass and weight?
Mass is the amount of matter in an object, measured in kilograms, and it is the same everywhere. Weight is the gravitational force on that mass, measured in newtons, and it changes with location (weight = mass times g). You would have the same mass on the Moon but weigh about one sixth as much, because the Moon's g is smaller.