Newtonian Mechanics and Forces

Rolling friction occurs when round objects (wheels, balls) roll on surfaces - bicycles, scooters, and cars all use wheels. A rock on a hill experiences static or sliding friction, not rolling friction, as it slides rather than rolls.

When a ship moves through water, it experiences fluid friction (drag) exerted by the water opposing its motion. Gravitational force pulls downward, magnetic force requires magnetic materials, and molecular force acts at atomic scales - none of these are relevant to a ship's water resistance.

Friction exists because no surface is perfectly smooth - even polished surfaces have microscopic irregularities that create resistance when surfaces slide past each other. If surfaces were perfectly smooth, friction would be eliminated.

Fluid friction depends on: speed of body (higher speed = more drag), shape of object (streamlined shapes reduce drag), and nature of fluid (more viscous fluids create more resistance). It does NOT depend on the nature (material/composition) of the object itself.

Air exerts frictional force (drag) on moving objects because air opposes their relative motion through it. Air does not move in the same direction as the body, it doesn't increase speed, and being light doesn't cause friction - the opposition to motion is key.

Work in physics is defined as force multiplied by displacement in the direction of the force (W = F × d × cos θ). When a porter carries a load horizontally, the gravitational force acts downward while the displacement is horizontal, so the angle between them is 90° and cos 90° = 0. Therefore, no work is done against gravity in horizontal carrying.

In uniform circular motion, centripetal force is always perpendicular to instantaneous displacement. Work = force × distance × cos(90°) = force × distance × 0 = zero.

A wound spring stores elastic potential energy due to its deformation. This stored energy can be slowly released to power the watch mechanism.

Hydraulic brakes work on Pascal's law, which states that pressure applied to an enclosed fluid is transmitted undiminished to all parts of the fluid. A small force on the brake pedal creates large force at the wheels through pressure transmission.

Viscosity is the internal friction in fluids that resists flow. It arises due to intermolecular forces between layers of fluid moving at different velocities. Just as solid friction opposes relative motion between surfaces, viscosity opposes relative motion between fluid layers.

 Work is a scalar quantity.

Work is defined as force multiplied by displacement in the direction of the force (W = F × d × cosθ). When pushing against a wall that doesn't move, the displacement is zero, so no work is done regardless of how hard you push. Option A is incorrect because no displacement means no work. Option C is wrong because the wall doesn't move. Option D is incorrect since B is the right answer.

During free fall, the only force acting on a body is gravity (mg). Since there's no support force (normal reaction), the body experiences apparent weightlessness - what we commonly call 'weightless'. This is standard physics terminology despite the body still having mass. Options B, C, and D are incorrect because free fall isn't about having minimum, maximum, or medium weight.

From Newton's second law: F = ma, rearranged gives m = F/a. Mass equals applied force divided by resulting acceleration. Option D (F/Δp) is incorrect because Δp = F·Δt, so F/Δp = 1/Δt, not mass. Options A and B incorrectly use velocity instead of acceleration.