SPI Physics · Laws of Motion and Forces

Fundamental Forces and Levers

Every force in nature comes from four basic forces, of which gravity is the weakest, and a lever is a rigid bar turning about a fixed point, sorted into three classes by what sits in the middle.

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Why this matters

Two questions in the bank, one EASY from the 2022 paper (the weakest basic force) and one MODERATE (which class of lever a hinged door is). Both are named facts, so the tables below are the whole job.

Concept 1 of 2: The four fundamental forces

Gravity holds the planets in orbit, so it feels mighty. But between two small particles it is unbelievably weak: a small magnet lifts a pin against the pull of the whole Earth. It seems strong only because planets and stars are so massive.

Definition

All forces in nature come from four fundamental forces: gravitational, weak nuclear, electromagnetic and strong nuclear.
From weakest to strongest: gravitational < weak nuclear < electromagnetic < strong nuclear.
Gravitation is the weakest, about $10^{-39}$ times the strong nuclear force. It is always attractive and acts between all masses.
The strong nuclear force is the strongest. It holds protons and neutrons together in the nucleus and acts only over very short distances.
The electromagnetic force (the electrostatic force between charges is part of it) lies in between.

Force	Strength (strong nuclear = 1)	Range	Acts between
Strong nuclear	1 (strongest)	About 10⁻¹⁵ m, inside a nucleus	Protons and neutrons
Electromagnetic	About 10⁻²	Unlimited	Charged particles
Weak nuclear	About 10⁻¹³	Less than 10⁻¹⁶ m	Particles in radioactive (beta) decay
Gravitational	About 10⁻³⁹ (weakest)	Unlimited	All bodies with mass

Weakest to strongest: gravitational, weak nuclear, electromagnetic, strong nuclear.

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

SPI · 2022 · SPI Entrance Examination 2022 · Q86Easy

Example 1 · Laws of Motion and Forces · Fundamental Forces and Levers

The weakest force of interaction is ____.

Thinking gravity is strong because planets feel it

Gravity rules the motion of planets only because they are so massive. Between two particles it is the weakest of all the fundamental forces.

Concept 2 of 2: The three classes of lever

A lever has three points: the fulcrum it turns on, the effort you apply and the load it moves. Whichever of the three sits in the middle decides the class.

Definition

First class: the fulcrum is in the middle, between effort and load.
Second class: the load is in the middle, between fulcrum and effort. The effort arm is always longer, so it always multiplies force.
Third class: the effort is in the middle, between fulcrum and load. It never multiplies force, but it moves the load faster and farther.
Principle of the lever: effort $\times$ effort arm $=$ load $\times$ load arm, where each arm is measured from the fulcrum.
A door on hinges is second class: the hinge is the fulcrum at one end, you push at the handle at the far end, and the door's weight acts in between.

Class	In the middle	Examples
First	Fulcrum	See-saw, scissors, pliers, crowbar, beam balance
Second	Load	Nutcracker, wheelbarrow, bottle opener, door on hinges
Third	Effort	Fire tongs, forceps, fishing rod, the human forearm

Memory key, in class order 1, 2, 3, the middle point is F, L, E.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionModerate

Example 2 · Laws of Motion and Forces · Fundamental Forces and Levers

A hinged door is an example of

Classing by where the fulcrum is

The class depends on which of the three points is in the MIDDLE. A fulcrum at the end can belong to a second or a third class lever; look at whether the load or the effort is in between.

Calling scissors a second class lever

In scissors the pivot (the fulcrum) sits between the handles (effort) and the blades (load), so scissors are first class.

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.

Reference tables (2)

The four fundamental forces4 rows

Force	Strength (strong nuclear = 1)	Range	Acts between
Strong nuclear	1 (strongest)	About 10⁻¹⁵ m, inside a nucleus	Protons and neutrons
Electromagnetic	About 10⁻²	Unlimited	Charged particles
Weak nuclear	About 10⁻¹³	Less than 10⁻¹⁶ m	Particles in radioactive (beta) decay
Gravitational	About 10⁻³⁹ (weakest)	Unlimited	All bodies with mass

Weakest to strongest: gravitational, weak nuclear, electromagnetic, strong nuclear.

The three classes of lever3 rows

Class	In the middle	Examples
First	Fulcrum	See-saw, scissors, pliers, crowbar, beam balance
Second	Load	Nutcracker, wheelbarrow, bottle opener, door on hinges
Third	Effort	Fire tongs, forceps, fishing rod, the human forearm

Memory key, in class order 1, 2, 3, the middle point is F, L, E.

Watch out for (3)

Thinking gravity is strong because planets feel it
→ The four fundamental forces
Classing by where the fulcrum is
→ The three classes of lever
Calling scissors a second class lever
→ The three classes of lever

Test yourself on a real paper

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