Force is an external effect (push or pull) that: (i) Produces or tries to produce motion in a body at rest. (ii) Stops or tries to stop a moving body. (iii) Changes or tries to change the direction of motion. (iv) Changes the shape of the body at rest.

• Force can act even when no motion results ('tries to produce' covers static situations — a wall being pushed that doesn't move experiences an equal reaction from the wall).
• Changing direction at constant speed (uniform circular motion) requires force — centripetal force. This is effect (iii).
• NEET: 'Which of the following is NOT an effect of force?' — trap option: 'changes mass.' Force never changes mass (in classical mechanics). It can change size/shape but not mass.

Four fundamental forces in order of relative strength (weakest to strongest): Gravitational (1) → Weak Nuclear (10²⁵) → Electromagnetic (10³⁶) → Strong Nuclear (10³⁸). The strong nuclear force is strongest; gravitational is weakest at all scales.

• Nuclear force acts at very short range (< 10⁻¹⁵ m). At larger distances, electromagnetic and gravitational forces dominate.
• Electromagnetic force: acts between electric charges at rest or in motion; includes both electric and magnetic forces. Associated particle: photon.
• NEET: 'The strongest fundamental force at nuclear distances is ___' → Strong Nuclear. 'The weakest fundamental force is ___' → Gravitational. Both are direct recall questions.

Weight is the gravitational force exerted by Earth on a body. W = mg, where m is mass and g is acceleration due to gravity (9.8 m/s² on Earth's surface). Weight is a vector (downward). Mass is a scalar (intrinsic property, same everywhere).

• On the Moon, g_Moon ≈ g_Earth/6, so weight reduces to 1/6 — but mass remains the same. NEET: 'Mass of a body on Moon is ___' → same as on Earth. 'Weight on Moon is ___' → W/6.
• Weight acts through the centre of gravity (≈ centre of mass for uniform fields). For free fall, weight is the only force and acceleration = g.
• NEET trap: 'An astronaut in space has zero weight but non-zero mass.' → TRUE. In free fall (orbit), the astronaut and spacecraft fall together — apparent weight = 0 (weightlessness) — but mass is unchanged.

Normal reaction N (or R) is the contact force exerted by a surface on an object, perpendicular to the surface and directed away from it. It is an electromagnetic force (surface repulsion at atomic scale). Its magnitude varies — it equals only mg in the special case of a body at rest on a horizontal surface with no other vertical forces.

• On a horizontal surface, stationary body: N = mg (balanced with weight). On an inclined plane, N = mg cos θ — less than mg because the surface only cancels the perpendicular component of weight.
• In a lift accelerating upward: N = m(g + a) > mg. Decelerating downward: same. In free fall: N = 0 (weightlessness).
• NEET: 'Normal reaction is always equal to weight' → FALSE. Normal equals weight ONLY on a horizontal surface in static conditions with no other vertical forces. Any inclination, acceleration, or external force changes N.

Tension is the force exerted by the end of a taut string, rope, or chain against a pulling force. It acts along the string, away from the body. For a massless inextensible string, tension is the same at every point. Tension is an electromagnetic force at the microscopic level.

• Direction: always away from the body (the string pulls — it cannot push). A rope can transmit tension but not compression.
• For a massless string: T is the same throughout. For a string with mass m: tension varies along its length because the string's own weight acts on each segment.
• NEET: In an Atwood machine (two masses m₁ > m₂ over a pulley), T = 2m₁m₂g/(m₁+m₂). Both masses experience the same tension (massless string assumption).

Spring force: F = -Kx. K is the spring constant (stiffness, N/m); x is the displacement from the spring's natural (equilibrium) length. Negative sign means the force is restoring — directed opposite to displacement. Compressed spring pushes back; stretched spring pulls back.

• Elastic potential energy stored in spring: PE = ½Kx². This is recovered fully when the spring returns to natural length — spring force is conservative.
• Springs in series: 1/K_eff = 1/K₁ + 1/K₂ (effective spring constant is less — softer). Springs in parallel: K_eff = K₁ + K₂ (stiffer).
• NEET: spring force questions often test: (a) force for given compression, (b) potential energy stored, (c) effective spring constant for series/parallel combinations.