Newtonian Mechanics and Forces

Rolling friction occurs when a rounded object (like a ball or wheel) rolls over a surface instead of sliding across it. The contact point continuously changes, creating less resistance than sliding friction.

Rolling objects need friction between wheel and surface to grip (otherwise they'd just spin in place). Writing requires friction between pen/paper to leave marks. Walking requires friction between feet and ground to push forward without slipping. All depend on friction.

Throwing sand on snow-covered tracks increases surface roughness, which increases friction between wheels/feet and the track. This provides better traction on otherwise slippery surfaces. The other options (polishing, greasing, oiling) all decrease friction.

Of the three types of friction listed, rolling friction offers the least resistance to motion. This is why wheels are so efficient - they convert high-resistance sliding friction into low-resistance rolling friction.

The wheel reduces friction by converting sliding friction into rolling friction. This fundamental principle transformed transportation, allowing heavy loads to be moved with much less effort. The wheel doesn't eliminate or generate energy - it reduces wasteful friction.

Drag is the frictional force exerted by fluids (both liquids AND gases) on objects moving through them. Air resistance (gas) and water resistance (liquid) are both forms of drag that oppose motion through the fluid.

Objects lose kinetic energy when moving through fluids due to fluid friction (drag) that opposes their motion. Static, sliding, and rolling friction occur between solid surfaces in contact, not in fluids like air or water.

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.