#gravity

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How gravity can act like a lens

A massive object curves spacetime, changing the paths that light follows nearby. When the alignment is suitable, a foreground galaxy or cluster can distort, magnify or multiply the image of something farther away. This gravitational lensing does not require a transparent glass-like object between the source and observer.

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blackholes

What makes a black hole's event horizon a boundary

An event horizon marks a boundary beyond which signals cannot escape to distant observers. It is not a solid surface surrounding a black hole. Outside it, gravity can still allow ordinary orbits, so a black hole does not automatically pull in every nearby object like a cosmic vacuum cleaner.

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physics

Why mass and weight are different quantities

Mass measures an object's inertia, while weight is the gravitational force acting on it. The same object can have different weights in different gravitational fields without changing its mass. In physics, mass is measured in kilograms and weight in newtons, even though everyday language often mixes them.

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tides

Why the Moon raises tides on both sides of Earth

Tides arise from differences in gravitational pull across Earth, rather than the Moon pulling every part equally. The near side is pulled more strongly than Earth's centre, while the far side is pulled less strongly. These differences help produce a two-sided tidal pattern, modified greatly by ocean basins and coastlines.

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gravity

What escape velocity actually describes

Escape velocity is the speed needed to leave a body's gravitational influence without further propulsion, under an idealised model. It depends on the body's mass and the starting distance from its centre. It is not a universal speed limit, and a powered spacecraft can gain energy gradually rather than receiving one instant push.

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