A force is a push or a pull acting on an object. Every movement we see – a ball being kicked, a door being opened, a fruit falling from a tree, or a ship floating on water – happens because of some force. This article explains what force is, its units and effects, the different types of forces and the four fundamental forces of nature, with exam-focused facts and practice questions.
What is Force?
Force is an external agent that changes or tends to change the state of rest or of uniform motion of a body, or changes its shape or size. Force is a vector quantity – it has both magnitude and direction.
According to Newton's second law, Force = mass × acceleration (F = ma).
Units of Force
| System | Unit | Relation |
|---|---|---|
| SI | newton (N) | 1 N = 1 kg m s-2 |
| CGS | dyne | 1 N = 105 dyne |
| Gravitational unit | kilogram-force (kgf) | 1 kgf = 9.8 N |
Force is measured with a spring balance. Its dimensional formula is [M L T-2].
Effects of Force
- It can move a body at rest (pushing a box).
- It can stop a moving body (applying brakes).
- It can change the speed of a moving body (pedalling a bicycle faster).
- It can change the direction of motion (a batsman hitting the ball).
- It can change the shape or size of a body (pressing a sponge or kneading dough).
Balanced and Unbalanced Forces
- Balanced forces: when forces acting on a body cancel each other and the net force is zero. The body's state of motion does not change. Example: a book lying on a table; two teams pulling a rope with equal force in a tug of war.
- Unbalanced forces: when the net force is not zero. The body starts moving, stops, or changes speed or direction. Example: one team winning a tug of war.
Types of Force
Forces are broadly divided into two groups – contact forces, which act only when bodies touch each other, and non-contact forces, which act from a distance.
A. Contact Forces
| Force | Meaning | Example |
|---|---|---|
| Muscular force | Force applied using muscles of the body | Lifting a bucket, animals pulling carts |
| Frictional force | Force that opposes relative motion between two surfaces in contact | A rolling ball slowing down and stopping |
| Normal force | Force applied by a surface perpendicular to it, on a body resting on it | A table supporting a book |
| Tension | Pulling force transmitted through a string, rope or cable | Rope of a well bucket, lift cables |
| Spring (elastic) force | Restoring force in a stretched or compressed spring | Spring balance, shock absorbers |
| Air resistance (drag) | Friction offered by air or liquid on a moving body | Parachute slowing a fall |
| Buoyant force (upthrust) | Upward force exerted by a fluid on an immersed body | Ship floating on water |
B. Non-Contact Forces
| Force | Meaning | Example |
|---|---|---|
| Gravitational force | Force of attraction between any two masses | Fruit falling from a tree; Moon revolving around the Earth |
| Magnetic force | Force exerted by a magnet on magnetic materials or another magnet | Magnet attracting iron nails; compass needle |
| Electrostatic force | Force between charged bodies | A rubbed comb attracting small bits of paper |
Friction in Detail
Friction is caused by the irregularities (roughness) of the two surfaces in contact. It always acts opposite to the direction of motion.
- Static friction: acts when a body is at rest and we try to move it.
- Sliding (kinetic) friction: acts when one body slides over another.
- Rolling friction: acts when one body rolls over another.
Exam point: Static friction > Sliding friction > Rolling friction. That is why it is easier to roll a heavy drum than to slide it, and why wheels and ball bearings reduce effort.
| Friction is useful | Friction is harmful |
|---|---|
| We can walk without slipping | Causes wear and tear of shoes, tyres and machine parts |
| We can write with a pen or pencil | Produces heat and wastes energy in machines |
| Brakes stop vehicles | Reduces the efficiency of engines |
| Matchstick lights by rubbing | Makes moving heavy objects difficult |
Ways to reduce friction: lubricants (oil, grease), ball bearings, polishing surfaces, using wheels, and streamlined shapes for aeroplanes, cars, boats and even birds and fish. Treads on tyres and soles of shoes are made to increase friction for better grip.
Gravitational Force
Newton's Law of Universal Gravitation: Every body in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
F = G m1m2 / r2, where the universal gravitational constant G = 6.67 × 10-11 N m2 kg-2. Its value was first measured by Henry Cavendish (1798).
- Gravity is the gravitational force of the Earth. Acceleration due to gravity, g = 9.8 m/s2 near the Earth's surface.
- g is maximum at the poles and minimum at the equator, because the Earth bulges at the equator and spins on its axis.
- g decreases with height above the surface and with depth below it; it is zero at the centre of the Earth.
- On the Moon, g is about one-sixth of that on the Earth.
- Escape velocity from the Earth is about 11.2 km/s.
- Gravitational force keeps the planets moving around the Sun and causes tides along with the Moon's pull.
Buoyant Force and Archimedes' Principle
Archimedes' Principle: When a body is fully or partly immersed in a fluid, it experiences an upward force (buoyant force) equal to the weight of the fluid displaced by it.
- A body floats if the buoyant force equals its weight, i.e., if its density is less than or equal to that of the fluid.
- An iron nail sinks, but a heavy iron ship floats because its hollow shape displaces a large volume of water.
- About 90% (nine-tenths) of an iceberg stays under water, because the density of ice is about 0.9 times that of water.
- It is easier to swim in sea water than in river water, because sea water is denser and gives more upthrust.
- Hydrometer (used to measure the density of liquids) and lactometer (used to check the purity of milk) work on this principle.
Centripetal and Centrifugal Force
- Centripetal force is the force needed to keep a body moving in a circular path. It always acts towards the centre. F = mv2/r. Example: tension in a string when a stone is whirled; gravity provides the centripetal force for satellites.
- Centrifugal force is a pseudo (apparent) force felt in a rotating frame, acting away from the centre. Examples: cream separator, washing machine dryer, passengers feeling pushed outward on a turn.
- Roads and railway tracks are banked (outer edge raised) on curves to provide the needed centripetal force.
Pressure: Force on an Area
Pressure = Force / Area. Its SI unit is the pascal (Pa) = N/m2. The same force produces more pressure on a smaller area.
- Knives and needles have sharp edges and tips to produce high pressure.
- School bags have broad straps to reduce pressure on the shoulders.
- Camels have broad feet, and heavy vehicles have wide tyres, so that they do not sink in sand or soft ground.
- Atmospheric pressure is measured with a barometer; a sudden fall in barometer reading indicates a storm.
Moment of Force (Torque)
The turning effect of a force about a fixed point (axis) is called the moment of force or torque. Torque = Force × perpendicular distance from the axis. Its SI unit is N m.
- Door handles are fixed far from the hinges, so less force opens the door.
- A spanner with a long handle makes it easier to loosen a nut.
- Two equal and opposite forces acting at different points form a couple – for example, turning a steering wheel or a tap.
- Levers, see-saws, scissors and pliers work on the principle of moments.
Four Fundamental Forces of Nature
All forces in the universe arise from just four basic forces.
| Force | Acts between | Relative strength | Range | Carrier particle |
|---|---|---|---|---|
| Strong nuclear force | Protons and neutrons (quarks) in the nucleus | 1 (strongest) | Very short (about 10-15 m) | Gluon |
| Electromagnetic force | Charged particles | 10-2 | Infinite | Photon |
| Weak nuclear force | Particles in radioactive (beta) decay | 10-13 | Very short (less than 10-16 m) | W and Z bosons |
| Gravitational force | All bodies with mass | 10-39 (weakest) | Infinite | Graviton (not yet detected) |
- The strong nuclear force holds the nucleus together against the repulsion between protons.
- Friction, tension, normal force and muscular force are all electromagnetic in origin.
- The weak nuclear force is responsible for beta decay and helps power the nuclear reactions in the Sun.
- Although gravity is the weakest, it controls the motion of planets, stars and galaxies because it always attracts and acts over very long distances.
Quick Revision Points
- SI unit of force: newton; 1 N = 105 dyne; 1 kgf = 9.8 N.
- Rolling friction is the least; static friction is the most.
- G = 6.67 × 10-11 N m2 kg-2; g = 9.8 m/s2, maximum at the poles.
- Lactometer and hydrometer work on Archimedes' principle.
- Centrifugal force is a pseudo force; cream separators use it.
- Strongest fundamental force: strong nuclear; weakest: gravitational.
Read also: Units, Dimensions and Newton's Laws of Motion
Practice MCQs
1. 1 newton is equal to:
(a) 103 dyne
(b) 105 dyne
(c) 107 dyne
(d) 9.8 dyne
2. Which type of friction is the least?
(a) Static
(b) Sliding
(c) Rolling
(d) All are equal
3. The value of acceleration due to gravity is maximum at:
(a) Equator
(b) Poles
(c) Centre of the Earth
(d) Top of a mountain
4. A lactometer, used to test the purity of milk, works on:
(a) Pascal's law
(b) Archimedes' principle
(c) Bernoulli's principle
(d) Newton's third law
5. Which is the strongest fundamental force in nature?
(a) Gravitational
(b) Electromagnetic
(c) Weak nuclear
(d) Strong nuclear
6. A cream separator works on the principle of:
(a) Centrifugal force
(b) Gravitational force
(c) Friction
(d) Surface tension
7. Door handles are fixed far from the hinges because it:
(a) Reduces friction
(b) Increases the moment of force
(c) Increases pressure
(d) Looks attractive
8. Who first measured the value of the universal gravitational constant G?
(a) Isaac Newton
(b) Galileo
(c) Henry Cavendish
(d) Kepler
9. The force of friction is ultimately which type of fundamental force?
(a) Gravitational
(b) Electromagnetic
(c) Strong nuclear
(d) Weak nuclear
10. Why does a heavy iron ship float while an iron nail sinks?
(a) The ship is made of lighter iron
(b) The ship displaces water equal to its weight
(c) Sea water has no gravity
(d) The ship has engines
Answers: 1 – (b) | 2 – (c) | 3 – (b) | 4 – (b) | 5 – (d) | 6 – (a) | 7 – (b) | 8 – (c) | 9 – (b) | 10 – (b)
Reference: NCERT Science Class VIII (Force and Pressure; Friction), Class IX (Gravitation) and NCERT Physics Class XI (Physical World; Laws of Motion; Gravitation).