General Science

Water has maximum density at 4°C. When a lake freezes from the top, the ice layer insulates the water beneath. The water at the bottom remains at approximately 4°C because this is the temperature at which water is densest and sinks. The water below the ice doesn't reach 0°C or below because the ice acts as an insulator.

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.

Kinetic energy is directly proportional to the square of velocity (KE = 1/2 mv²). When speed doubles (v → 2v), KE increases by factor of 2² = 4.

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

According to Archimedes' principle, buoyant force equals weight of fluid displaced. Sea water has higher density than river water due to dissolved salts, so less volume needs to be displaced to balance the ship's weight.

Volume = mass / density = 7.8 g / 1.3 g/L = 6 L. This is a straightforward density calculation rearranged to find volume.

Glass has lower density than brass, so in air it experiences greater buoyant force. In vacuum, buoyancy disappears, reducing the upward force on the glass side more than the brass side, making the glass pan go down (decrease).

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.

As altitude increases, atmospheric pressure decreases significantly. The helium/hydrogen inside the balloon remains at initial pressure, creating large pressure difference between inside and outside that exceeds the balloon's elastic limit.

At high altitudes, cabin pressure is lower than ground level. The ink inside the pen was at equilibrium with ground pressure, so at reduced ambient pressure, the internal pressure exceeds external pressure, forcing ink out.

Rapid decrease in atmospheric pressure is a classic indicator of approaching storm systems. Low-pressure centers are associated with stormy weather, cyclones, and turbulent atmospheric conditions.

Mercury's very high density (13.6 g/cm³) allows for a reasonably short barometer column (~76 cm) to measure atmospheric pressure. Less dense fluids would require impractically tall columns (over 10 meters for water).

When a bird flies inside an air-tight box, it pushes air downward with its wings. By Newton's third law, this creates an equal upward force on the bird, and the downward force on the air is transmitted to the box bottom. The total downward force on the box equals the bird's weight, so the weight reading remains unchanged.

A hydraulic lift works on Pascal's law, which states that pressure applied to a confined fluid is transmitted undiminished in all directions. This allows a small force on a small piston to create a large force on a large piston, enabling heavy loads to be lifted with minimal effort.

Surface tension is the property of liquid surfaces to shrink to the minimum possible area. For a given volume, the shape with minimum surface area is a sphere. This is why liquid drops and bubbles naturally acquire spherical shapes - surface tension minimizes the surface area.

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.

Pure water reaches its maximum density at approximately 4°C. At this temperature, 1 gram of water occupies exactly 1 cubic centimeter of volume. This is why water is densest at 4°C - below this temperature, the molecules start forming an open crystal structure (approaching ice).

Heat required to melt 10g ice at 0°C = 10 × 80 = 800 cal. Heat available from 10g water cooling from 10°C to 0°C = 10 × 1 × 10 = 100 cal. Since only 100 cal is available, only 1.25g of ice can melt (100/80). The remaining 8.75g ice stays at 0°C, so final temperature is 0°C with some ice still present.

A bi-metal rod bends when heated because the two metals have different coefficients of linear expansion. One metal expands more than the other for the same temperature increase, causing the composite rod to curve toward the side with less expansion. This principle is used in thermostats and temperature-control devices.