Understanding Thrust, Pressure, Buoyancy, and Density

Posted on Sep 5, 2024 in Physics

Thrust and Pressure

Thrust

Thrust is the force acting on a body perpendicular to its surface. The S.I. unit of thrust is Newton (N). For example, when fixing a poster on a bulletin board, you apply thrust by pressing drawing pins with your thumb.

Pressure

Pressure is the thrust per unit area. It is calculated as: Pressure = Thrust / Area. The S.I. unit of pressure is Pascal (Pa), which is equivalent to Newton per square meter (N/m2). A larger unit, kilopascal (kPa), is often used.

Pressure depends on two factors:

  • Force applied
  • Area over which force acts

The same force can produce different pressures depending on the area it acts upon. For instance, a force applied over a large area produces a small pressure, while the same force applied over a small area produces a large pressure.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the atmosphere at any given location. It varies with place and time. At sea level, atmospheric pressure is approximately 1.01 x 105 Pa, which is also known as 1 atmosphere (1 atmosphere = 760 mm of Hg).

Pressure in Fluids

Fluids encompass both liquids and gases. Like solids, fluids have weight and exert pressure on the base and walls of their container. This pressure is transmitted equally in all directions within the fluid.

In a liquid, pressure is the same at all points at the same horizontal level. Pressure increases with depth.

Buoyancy

When an object is placed in a liquid, the liquid exerts an upward force on it, known as buoyant force or upthrust. This force is why objects appear to lose weight when submerged in a liquid. The weight of an object in water is called its apparent weight, which is less than its true weight.

Buoyancy is the tendency of a liquid to exert this upward force. The buoyant force increases as more of the object’s volume is submerged. However, once the object is fully submerged, further submersion does not increase the buoyant force.

Factors Affecting Buoyant Force

  1. Volume of the submerged object: As the volume of the submerged object increases, the buoyant force also increases, reaching its maximum when the object is fully submerged.
  2. Density of the liquid: Liquids with higher density exert a greater buoyant force. For example, seawater, being denser than freshwater, exerts a greater buoyant force, making it easier to swim in.

Why Objects Float or Sink

Whether an object floats or sinks depends on the relative magnitudes of its weight (W) acting downwards and the buoyant force (B) acting upwards. There are three possibilities:

  1. If B < w, the object sinks.
  2. If B = W, the object floats.
  3. If B > W, the object rises and then floats.

Therefore, an object floats if the buoyant force is equal to or greater than its weight. This condition is met when the object’s density is less than or equal to the liquid’s density.

Archimedes’ Principle

Archimedes’ principle states: “When an object is wholly or partially immersed in a liquid, it experiences a buoyant force (or upthrust) which is equal to the weight of liquid displaced by the object.”

This principle applies to both liquids and gases. Although gases like air exert a buoyant force, it is often negligible. However, the buoyant force of air is what allows balloons to rise.

Applications of Archimedes’ Principle

  • Designing ships and submarines
  • Determining the relative density of a substance
  • Lactometers (measuring milk purity)
  • Hydrometers (measuring liquid density)

Density and Relative Density

Density

Density is the mass of a substance per unit volume. It is calculated as: Density = Mass / Volume. The SI unit of density is kg/m3. Density is a characteristic property of a substance and can be used to assess its purity.

Relative Density

Relative density is the ratio of a substance’s density to the density of water. It is a unitless quantity. Relative density indicates how much heavier or lighter a substance is compared to an equal volume of water. For example, iron has a relative density of 7.8, meaning it is 7.8 times heavier than an equal volume of water.

If a substance’s relative density is greater than 1, it is denser than water and will sink. If it is less than 1, it is less dense than water and will float. For instance, ice has a relative density of 0.9 and floats in water, while iron, with a relative density of 7.8, sinks.