Gravitation
Class 11 · Physics
4 ideas in Class 11 Physics, explained in plain words, with games and quizzes to practise.
Kepler's Laws and Universal Gravitation
Part of: Laws of Gravity Near and Far
Planets loop the Sun in ellipses, sweep equal areas in equal times, and time their years by orbit size cubed. Every pair of masses pulls each other with force growing with mass and shrinking with distance squared. The constant G is the same everywhere.
Why it matters. One rule steers apples, moons and planets alike. It turns sky patterns into predictions.
Formulas
- F = G m1 m2 / r^2
- Kepler third law: T^2 / r^3 = constant
Key words
- Ellipse
- : The oval orbit path with the Sun at one focus.
- Kepler's second law
- : A planet sweeps equal areas in equal times.
- Kepler's third law
- : The square of the period grows with the cube of orbit size.
- Universal gravitation
- : Every pair of masses pulls each other with force shrinking with distance squared.
- Gravitational constant
- : The fixed number G linking masses and distance to force.
Try it in two minutes
- Draw an ellipse with two pins and a loop of string.
- Mark equal-time arcs near and far from one focus.
- Compare year lengths of near and far planets.
Try at home. Time how the Moon shifts nightly against fixed stars.
Acceleration due to Gravity
Part of: Laws of Gravity Near and Far
Weight is mass times the local pull g, which weakens with height and depth. Climbing one Earth radius quarters it; sinking halfway to the centre halves it. The Moon's pull is a sixth of Earth's, so weights shrink there while mass stays put.
Why it matters. Weight shifts with place but mass never does. Mixing them up breaks every scale reading off Earth.
Formulas
- W = m g
- g = G M / R^2 at the surface
- At height h: g falls with 1 / (R + h)^2
- At depth d: g = surface value x (1 - d / R)
Key words
- Weight
- : Mass times local pull, measured in newton.
- Acceleration due to gravity
- : The pull g linking mass to weight.
- Variation with height
- : Gravity weakens with the square of distance from the centre.
- Variation with depth
- : Gravity falls steadily toward the centre.
Try it in two minutes
- Weigh a bag and compute its mass from the reading.
- Halve the pull and recompute the same bag's weight.
- List what changes and what stays on the Moon.
Try at home. Weigh luggage, then find what the scale would read on the Moon.
Potential Energy and Escape Speed
Part of: Energy, Escape and Life in Orbit
Bound systems carry negative energy measured from zero at infinity: freeing them needs outside work. Escape speed from Earth is 11.2 kilometres per second and ignores the escaping body's mass. Weak worlds like the Moon cannot hold fast gas molecules.
Why it matters. Escape decides atmospheres, launches and returns. Negative energy is the signature of anything gravitationally bound.
Formulas
- U = - G M m / r
- Escape speed ve = root(2 G M / R)
- Orbital speed v = root(G M / r)
Key words
- Gravitational potential energy
- : Stored energy from separation, measured zero at infinity.
- Bound system
- : A system with negative total energy, needing outside work to free.
- Escape speed
- : The launch speed that just frees a body, 11.2 km/s from Earth.
- Orbital speed
- : The steady sideways speed of a satellite at its height.
Try it in two minutes
- Roll a ball in a bowl and link depth with escape effort.
- Double a throw speed and feel energy quadruple.
- Compare holding breath in a bowl versus on a plate.
Try at home. Roll marbles out of bowls of different depths and compare effort.
Satellites and Weightlessness
Part of: Energy, Escape and Life in Orbit
A satellite is forever falling around Earth, with sideways speed balancing the inward pull. Low skimmers circle in about 85 minutes; a geostationary one matches Earth's spin to hover over one spot. Riders float because craft and crew fall together, not because gravity vanished.
Why it matters. TV, weather and maps ride on these paths. Weightlessness is shared falling, a fact every space story gets wrong.
Formulas
- Orbital period T = 2 pi root(r^3 / (G M))
- Total orbital energy E = - G M m / (2 r)
- Geostationary period = 24 hours.
Key words
- Satellite
- : A body forever falling around a planet, not floating free of gravity.
- Geostationary orbit
- : A 24-hour equatorial orbit hovering over one spot.
- Low orbit period
- : About 85 minutes for a surface skimmer.
- Weightlessness
- : Falling together with the craft, so no support push is felt.
Try it in two minutes
- Swing a ball on a string to feel orbit balance.
- Spin a globe and track one fixed point below.
- Drop two linked objects and watch them fall as one.
Try at home. Track a bright satellite pass and note its steady glide.
What your child will practise
- Acceleration due to Gravity: Ocean Quick Fire (Quick Fire)
- Kepler's Laws and Universal Gravitation: Jungle Puzzle (Clue Puzzle)
- Kepler's Laws and Universal Gravitation: Jungle Quick Fire (Quick Fire)
- Potential Energy and Escape Speed: Candy Quick Fire (Quick Fire)
- Satellites and Weightlessness: Desert Puzzle (Clue Puzzle)
- Quiz: Acceleration due to Gravity (8 questions)
- Quiz: Kepler's Laws and Universal Gravitation (8 questions)
- Quiz: Potential Energy and Escape Speed (8 questions)
- Quiz: Satellites and Weightlessness (8 questions)