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Science · Physics

Forces and Newton's Laws

Free-body diagrams and Newton's three laws — the foundation for every mechanics problem after this one. Nearly every force question comes down to correctly adding up every force acting on an object before applying F = ma. Every question comes with a written walkthrough of the forces involved.

Practise forces and newton's laws nowSee sample questions
3
Difficulty tiers
35s
Target pace
50%
Average first-attempt score

What's covered

  • Newton's First Law — apply the principle that an object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force.
  • Newton's Second Law — apply F = ma to relate the net force on an object to its mass and resulting acceleration.
  • Newton's Third Law — identify the equal and opposite reaction force that every force produces, acting on a different object than the original force.
  • Free-body diagrams — draw and sum every force acting on an object — gravity, normal force, friction, applied force, tension — to find the net force.
  • Friction — distinguish static friction (preventing motion from starting) from kinetic friction (opposing motion already in progress), and apply f = μN to calculate its magnitude.

Where students lose marks

Using one force instead of the net force in F = ma

F = ma requires the net force — the vector sum of every force acting on the object — not just one of the forces present, like an applied push ignoring friction and gravity.

Pairing a Newton's Third Law force with the wrong object

An action-reaction pair always acts on two different objects — a table pushing up on a book (normal force) pairs with the book pushing down on the table, not with gravity pulling down on the book, which is a completely separate force.

Confusing mass and weight

Mass is a fixed amount of matter, measured in kilograms; weight is the force of gravity on that mass, measured in newtons, and changes depending on the local gravitational field — the two aren't interchangeable, even though they're proportional on Earth.

Three sample questions

Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.

Sample 01Foundation

A 10 kg object experiences a net force of 25 N. What is its acceleration?

Sample 02Core

A 60 kg skater pushes off a wall, and the wall exerts a force of 120 N back on the skater. According to Newton's Third Law, what force does the skater exert on the wall?

Sample 03Advanced

A 5 kg block sits on a horizontal surface with a coefficient of kinetic friction of 0.3. A horizontal force of 20 N is applied to the block, causing it to slide. Using g = 10 m/s², find the block's acceleration.

How to practise this

Force problems reward drawing the free-body diagram first — nearly every error comes from missing a force or forgetting that F=ma needs the net force, not just one contributor.

Draw every force before writing F = ma

Sketch a free-body diagram listing every force acting on the object — gravity, normal, friction, applied — before summing them. Untimed practice is where that habit prevents missed forces.

Then 35 seconds a question

Move to timed sessions once free-body diagrams and net force calculations are fast.

Fold into a mock

Pair forces and Newton's laws with kinematics in a mock — most multi-step mechanics problems combine both.

Stop missing a force.
Start drawing every one.

  • A force-by-force walkthrough on every item
  • Timed or untimed sessions, any length you like
  • Tracked separately — see your accuracy and pace on forces and Newton's laws over time
Start practising free

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