Force and Pressure

📌 Force

A force is a push or pull exerted on an object that may change its state of rest or motion, its speed, its direction, or its shape. Force is a vector quantity. SI unit: Newton (N).

Effects of Force

• Sets a stationary object in motion.
• Stops or slows down a moving object.
• Changes the direction of motion of an object.
• Changes the shape or size of an object.

Types of Forces

⚖️ Balanced and Unbalanced Forces

• Balanced forces: Equal forces acting in opposite directions on an object. The object remains stationary or continues with uniform motion. Net force = 0.
• Unbalanced forces: The resultant force is not zero. The object accelerates, decelerates, or changes direction.

🔄 Turning Effect of a Force (Moment)

A force applied at a distance from a pivot causes rotation. This rotating effect is called the moment of a force or torque.

• SI unit of moment: Newton-metre (N·m)
• Principle of Moments (Law of Levers): For a body in rotational equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments.
• Examples: opening a door, using a spanner, a see-saw, a pair of scissors.

📐 Pressure

Pressure is defined as the force acting perpendicularly per unit area of a surface.

• SI unit of pressure: Pascal (Pa) = N/m²
• Pressure increases when the same force is applied over a smaller area.
• Pressure decreases when the same force is applied over a larger area.

💧 Pressure in Fluids

• Fluids (liquids and gases) exert pressure in all directions.
• Pressure in a liquid increases with depth: P = h × ρ × g (where h = depth, ρ = density, g = acceleration due to gravity).
• Pressure at the same horizontal level in a connected liquid is the same — this is the basis for the hydraulic press.
• Pascal's Law: Pressure applied to an enclosed fluid is transmitted equally in all directions.
• Applications of Pascal's Law: hydraulic brakes, hydraulic lift, hydraulic jack.

🌍 Atmospheric Pressure

• The atmosphere exerts pressure on all surfaces due to its weight. This is called atmospheric pressure.
• Standard atmospheric pressure at sea level ≈ 101,325 Pa (≈ 1 atm).
• Atmospheric pressure decreases with altitude (less air above).
• Measured using a barometer (invented by Torricelli).
• Aneroid barometer is used in altimeters and weather prediction.

🚿 Archimedes' Principle (Buoyancy)

• When an object is partially or fully submerged in a fluid, it experiences an upward force called the buoyant force (upthrust).
• Archimedes' Principle: The buoyant force on a submerged object equals the weight of fluid displaced by it.
• An object floats if buoyant force ≥ its weight; it sinks if weight > buoyant force.
• Applications: ships, submarines, hot-air balloons, hydrometers.

🏷️ Key Terms