Chapter 3: Force and Pressure

Chapter 3: Force and Pressure

Forces

• Definition: A force is a push or pull resulting from the interaction between objects, which can cause changes in motion (start, stop, slow down, speed up, or change direction).

Contact Forces

• Definition: Forces produced by objects that are in physical contact.

• Examples:

  • Friction: Opposes motion between surfaces in contact.

  • Air-resistance: Frictional force exerted by air on moving objects.

  • Normal Force: Perpendicular force exerted by a surface on an object.

  • Tension: Force exerted by a string, rope, or spring on an object connected to it.

Non-contact Forces

• Definition: Forces that act on objects without physical contact.

• Examples:

  • Gravitational Force: Pull exerted by Earth on an object.

  • Electrostatic Force: Attraction or repulsion between electric charges.

  • Magnetic Force: Attraction or repulsion between magnets.

Mass

• Definition: Measure of the amount of matter in an object, unchanged by the object's shape, size, or volume; determined by the number and composition of atoms.

• Units:

  • SI Unit: Kilogram (kg)

  • Other units: Gram (g) and Tonne.

• Measurement Instruments: Beam Balance, Electronic Balance.

Weight

• Definition: Result of the gravitational pull exerted by the Earth; a type of force with downward direction (toward the Earth's center).

• Units: Newton (N).

• Measured Using: Spring Balance, Weighing Scale.

Relationship Between Mass and Weight

• Weight (W) is calculated using the formula: W = mg where:

  • W: Weight (in N)

  • m: Mass (in kg)

  • g: Gravitational field strength (N/kg)

Differences Between Mass and Weight

• Mass:

  • Amount of matter in a body, scalar quantity.

  • Measured in kilograms (kg); constant across gravitational fields.

  • Measured by beam or electronic balances.

• Weight:

  • Resultant force of gravitational pull, vector quantity (magnitude and direction).

  • Measured in newtons (N); varies with gravitational field strength.

  • Measured by spring or compression balances.

Density

• Definition: Mass per unit volume, a fixed property for pure substances. \rho = \frac{m}{V} where:

  • p : Density (in kg/m³)

  • m: Mass (in kg)

  • V: Volume (in m³)

• SI Unit: kg/m³; also expressed as g/cm³.

Floating and Sinking

• An object floats in a liquid if its density is lower then the liquid.

• An object sinks if its density is greater than the liquid.

Pressure

• Definition: Force per unit area. P = \frac{F}{A} where:

  • P: Pressure (Pa)

  • F: Force (N)

  • A: Area (m²)

• Units: Pascal (Pa) or N/m².

Hydraulic Press

• Uses incompressible liquids to transfer pressure in enclosed systems; pressure is equal across pistons but force differs due to varying areas.

• Example: P = \frac{F}{A} leads to unique force calculations based on area variations.