NDA Physics · Laws of Motion and Forces

Types of Forces — the Vocabulary of the Chapter

A force is a push or pull that can change a body's state of motion; forces are classified as fundamental vs contact, central vs non-central, and conservative vs non-conservative.

All Laws of Motion and Forces notes NDA Physics strategy

Why this matters

Start here — every later concept assumes this vocabulary. NDA tests it directly as one-line recall: the four fundamental forces, what a contact force is and the laws it obeys, which force is non-conservative (friction), and the three types of mechanical equilibrium. Roughly 7 PYQs across 2018–2024, all EASY or MODERATE; pure memorisation marks.

Concept 1 of 4: What a force is — the foundation

A force is simply a push or a pull. You cannot see a force, only its effects: it can start a body moving, stop it, speed it up, slow it down, or change its direction. Force is a vector — it has both a size and a direction — and its SI unit is the newton (N).

Definition

A force is an interaction that, when unopposed, changes the motion of a body. Its effects are:

Changing speed — speeding up or slowing down (acceleration along the motion).
Changing direction — turning the motion without changing speed (as in circular motion).
Changing shape — deforming a body.

Force is a vector quantity measured in newtons (N), where $1\,\text{N} = 1\,\text{kg m s}^{-2}$ .

Definition of the newton (from Newton's second law)

1\,\text{N} = 1\,\text{kg} \cdot 1\,\text{m s}^{-2}

Nnewton, the SI unit of force
kgkilogram, the SI unit of mass
m s⁻²metre per second squared, the SI unit of acceleration

Worked example

A constant force gives a 2 kg body an acceleration of 3 m/s². What is the size of the force, in newtons?

Practice this concept4 quick reps

Force is a vector — direction matters

Two forces of the same magnitude can produce very different results depending on their directions. When combining forces you must use vector addition (the parallelogram law), never simple arithmetic — adding 3 N and 4 N gives anything from 1 N to 7 N depending on the angle between them.

Concept 2 of 4: Fundamental forces vs contact forces

Every force in the universe is built from just FOUR fundamental forces. Everyday forces like friction, tension, and the normal force are not separate fundamentals — they are large-scale manifestations of the electromagnetic force between atoms. A useful everyday split is contact forces (the bodies touch) vs non-contact / action-at-a-distance forces (gravity, magnetism — they act across a gap).

Definition

There are exactly four fundamental forces in nature: gravitational, electromagnetic, and the strong and weak nuclear forces. A contact force appears only when two bodies are in physical contact (friction, normal force, tension, drag); it obeys Newton's third law and can act between a solid and a fluid. A non-contact (field) force acts across a distance without touching (gravity, electrostatic, magnetic).

Force	Type	Range / note
Gravitational	Fundamental, non-contact	Always attractive; infinite range; weakest of the four
Electromagnetic	Fundamental, non-contact	Source of friction, tension, normal, contact forces at large scale
Strong nuclear	Fundamental	Binds protons and neutrons in the nucleus; very short range
Weak nuclear	Fundamental	Responsible for radioactive (beta) decay; very short range
Friction / Normal / Tension	Contact (derived)	Need physical contact; obey Newton's third law; can act solid-fluid NDA 2024 — contact forces (1) need contact, (2) obey the third law, (3) can act between a solid and a fluid: all three statements are correct.

The four fundamentals are the only true forces; everyday contact forces are electromagnetic in origin. NDA tests "which are fundamental?" (answer: all of gravity, EM, and the two nuclear forces).

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2024 · Q55Easy

Example 2 · Laws of Motion and Forces · Types of Forces

Which of the following forces is/are fundamental in nature ? 1. Gravitational force 2. Electromagnetic forces 3. Strong and weak nuclear forces Select the correct answer using the code given below :

Friction is a contact force; magnetism is non-contact — ALWAYS

NDA 2021 asked whether "friction is a contact force while magnetic force is a non-contact force" is true. It is ALWAYS true: friction requires touching surfaces, while magnetism reaches across a gap. The distractors hedge with "sometimes" or "never" — reject them.

Drill 2 more on fundamental forces vs contact forces

Concept 3 of 4: Conservative vs non-conservative forces; central forces

A conservative force gives back all the work you do against it — lift a book then lower it, and gravity returns the energy. A non-conservative force dissipates energy (usually as heat) so you never get it all back; friction is the classic example. A central force acts along the line joining two bodies (gravity, electrostatic); a non-central force does not (friction acts tangentially along a surface).

Definition

A conservative force does work that depends only on the start and end points, not the path; the work it does around any closed loop is zero (gravitational, electrostatic, spring forces). A non-conservative force dissipates mechanical energy (friction, air drag, viscous force) — its work depends on the path. A central force points along the line joining the two interacting bodies; friction is non-central (tangential) and non-conservative.

Force	Conservative?	Central?
Gravitational	Conservative	Central
Electrostatic	Conservative	Central
Spring (elastic restoring)	Conservative	Central (along the spring)
Friction	Non-conservative	Non-central NDA 2019 — the force that is BOTH non-central AND non-conservative is friction (electric and gravitational are central and conservative).
Air resistance / viscous drag	Non-conservative	Non-central

Friction is the only common force that is simultaneously non-central and non-conservative — a frequent NDA distractor target.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2019 · Q100Moderate

Example 3 · Laws of Motion and Forces · Types of Forces

Which one of the following forces is non-central and non-conservative?

Concept 4 of 4: Equilibrium and restoring forces

A body is in equilibrium when the net force on it is zero. But not all equilibria are alike — nudge the body and watch what happens. If it returns, the equilibrium is stable; if it keeps moving away, it is unstable; if it stays put in the new spot, it is neutral. The force that pulls a displaced body back is a restoring force.

Definition

A body is in equilibrium when the net (resultant) force on it is zero. Three types based on the response to a small displacement:

Stable — a restoring force returns it to the original position (a ball in a valley; a pendulum bob).
Unstable — a small push drives it further away (a ball balanced on top of a dome or rod).
Neutral — it stays wherever you leave it (a ball on a flat table).

A restoring force always points back toward the equilibrium position; gravity provides the restoring force for a swinging pendulum.

Type	Response to small push	Example
Stable	Returns to original position	Ball at the bottom of a bowl; pendulum bob
Unstable	Moves further away	Ball balanced on top of a vertical rod NDA 2018 — a ball balanced on a vertical rod is in UNSTABLE equilibrium.
Neutral	Stays in the new position	Ball resting on a flat horizontal table

Restoring force is the signature of stable equilibrium; for a pendulum, gravity supplies it (NDA 2018).

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2018 · Q81Easy

Example 4 · Laws of Motion and Forces · Types of Forces

A ball balanced on a vertical rod is an example of

Gravity acts as the RESTORING force for a pendulum

NDA 2018 asked what kind of force gravity provides for a vibrating pendulum bob. The answer is restoring force — not "applied" (no external push) and not "frictional". The component of weight along the swing always points back to the mean position.

Drill 1 more on equilibrium and restoring forces

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

What a force is — the foundation
Definition of the newton (from Newton's second law)
$1\,\text{N} = 1\,\text{kg} \cdot 1\,\text{m s}^{-2}$

Reference tables (3)

Fundamental forces vs contact forces5 rows

Force	Type	Range / note
Gravitational	Fundamental, non-contact	Always attractive; infinite range; weakest of the four
Electromagnetic	Fundamental, non-contact	Source of friction, tension, normal, contact forces at large scale
Strong nuclear	Fundamental	Binds protons and neutrons in the nucleus; very short range
Weak nuclear	Fundamental	Responsible for radioactive (beta) decay; very short range
Friction / Normal / Tension	Contact (derived)	Need physical contact; obey Newton's third law; can act solid-fluid NDA 2024 — contact forces (1) need contact, (2) obey the third law, (3) can act between a solid and a fluid: all three statements are correct.

The four fundamentals are the only true forces; everyday contact forces are electromagnetic in origin. NDA tests "which are fundamental?" (answer: all of gravity, EM, and the two nuclear forces).

Conservative vs non-conservative forces; central forces5 rows

Force	Conservative?	Central?
Gravitational	Conservative	Central
Electrostatic	Conservative	Central
Spring (elastic restoring)	Conservative	Central (along the spring)
Friction	Non-conservative	Non-central NDA 2019 — the force that is BOTH non-central AND non-conservative is friction (electric and gravitational are central and conservative).
Air resistance / viscous drag	Non-conservative	Non-central

Friction is the only common force that is simultaneously non-central and non-conservative — a frequent NDA distractor target.

Equilibrium and restoring forces3 rows

Type	Response to small push	Example
Stable	Returns to original position	Ball at the bottom of a bowl; pendulum bob
Unstable	Moves further away	Ball balanced on top of a vertical rod NDA 2018 — a ball balanced on a vertical rod is in UNSTABLE equilibrium.
Neutral	Stays in the new position	Ball resting on a flat horizontal table

Restoring force is the signature of stable equilibrium; for a pendulum, gravity supplies it (NDA 2018).

Watch out for (3)

Force is a vector — direction matters
→ What a force is — the foundation
Friction is a contact force; magnetism is non-contact — ALWAYS
→ Fundamental forces vs contact forces
Gravity acts as the RESTORING force for a pendulum
→ Equilibrium and restoring forces

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