Gravitation and Fluids
Learn gravitational interaction, gravity near Earth, and the behaviour of fluids under pressure. Explore free fall, mass and weight, thrust and pressure, pressure in fluids, buoyancy, and Archimedes' principle. Use the interactive simulator below to explore gravity: adjust masses and distance to see how force and motion change. Use Launch to start and Play / Pause to control it.
- Gravitation β mutual attraction between masses (N)
- Gravity (g) β acceleration due to Earth's gravity (m/sΒ²)
- Mass & Weight β mass is intrinsic; weight depends on g
- Pressure β force per unit area (Pa)
- Buoyancy β upward force exerted by fluids
Key formulas
Real-world applications
Satellites and planetary motion
Orbits are governed by gravity; F = Gmβmβ/rΒ² provides the centripetal force for circular orbits.
Key insight: Orbital speed and period depend on mass of central body and orbital radius.
Falling objects and skydiving
Free fall gives a = g; terminal velocity when air resistance balances weight.
Key insight: Before terminal velocity, acceleration is g; after, net force is zero.
Hydraulic systems
Pressure is transmitted through fluids; P = F/A means a small force on small area can lift large load on large area.
Key insight: Pascal's principle: pressure in an enclosed fluid is transmitted undiminished.
Dams and submarines
Pressure increases with depth (P = hΟg); dams and hulls must withstand hydrostatic pressure.
Key insight: Deeper structures experience greater pressure; design must account for it.
Floating ships and balloons
Upthrust equals weight of displaced fluid; ships displace enough water to balance their weight.
Key insight: Floating when weight of object equals weight of displaced fluid.
Common misconceptions & tips
In the absence of air resistance, all objects fall with the same acceleration g. Weight is greater for heavier objects, but so is mass (F = ma β a = F/m = mg/m = g).
π Greater force is offset by greater mass; acceleration is the same.
π’ a = g for all (in vacuum)
π§ͺ In the gravity simulator, acceleration does not depend on the mass of the falling body.
Mass is the amount of matter (kg), a scalar. Weight is the force due to gravity (N), W = mg. On the Moon, your mass is unchanged but your weight is less because g is smaller.
π Mass is intrinsic; weight depends on the gravitational field.
π’ W = mg
π§ͺ Weigh yourself on Earth vs (conceptually) on the Moon.
Pressure in a fluid at rest depends on depth and density (P = hΟg), not on the total volume of the container. A tall thin column and a wide shallow pool can have different pressures at the bottom for the same volume.
π Pressure at a point depends on depth h and Ο, not total volume.
π’ P = hΟg
π§ͺ Pressure at the bottom of a dam is due to depth, not how much water is behind it.
Objects float when the buoyant force (upthrust) equals their weight. A ship floats because it displaces a large volume of water; the weight of that displaced water equals the ship's weight. Density (mass per volume) matters: if average density of object is less than fluid, it floats.
π Floating depends on weight vs upthrust (Archimedes), not just being "light".
π’ Upthrust = weight of displaced fluid
π§ͺ A heavy ship floats; a small heavy stone sinks.
Tip: Use the gravity simulator to see how force and motion depend on mass and distance; compare with the formulas above.
Chapter Guide
How to Study This Chapter
- Start with Universal Law of Gravitation
- Build concepts: g β Free fall β Mass and weight
- Connect ideas: Pressure β Fluids β Buoyancy
- Apply Archimedes' principle in the simulator
What You'll Learn
- Relate gravitation, g, and weight
- Use F = Gmβmβ/rΒ² and P = hΟg
- Understand pressure in fluids
- Interpret buoyancy and floating
Subtopics β Gravitation and Fluids
Each subtopic has a dedicated page with clear explanations and an interactive simulator where relevant.
Universal Law of Gravitation
Every mass attracts every other mass with a force proportional to the product of masses and inversely proportional to the square of the distance between them.
Read more βAcceleration due to Gravity
The acceleration g of a body near Earth's surface due to Earth's gravity; g β 9.8 m/sΒ² downward.
Read more βFree Fall
Motion under the influence of gravity only; acceleration a = g (assuming no air resistance).
Read more βMass and Weight
Mass is the amount of matter (kg); weight is the force due to gravity, W = mg (N).
Read more βThrust and Pressure
Thrust is total force; pressure is force per unit area. P = F/A; unit is the pascal (Pa).
Read more βPressure in Fluids
In a fluid at rest, pressure increases with depth: P = hΟg (hydrostatic pressure).
Read more βBuoyancy
The upward force exerted by a fluid on an immersed or floating body; it opposes the weight.
Read more βArchimedes' Principle
The upthrust on a body immersed in a fluid equals the weight of the fluid displaced by the body.
Read more β