Clear answers to commonly-asked classical physics questions — discussed further in the community forum.
Gravity's pull grows with mass, but so does inertia, the resistance to acceleration; the two cancel, so everything accelerates equally in a vacuum.
Galileo is said to have tested this at Pisa, and Apollo 15 later dropped a hammer and a feather on the airless Moon — they hit the ground together.
Mass is the amount of matter and is constant everywhere, while weight is the gravitational force on that mass and changes with local gravity.
Ignoring air, range is greatest at 45 degrees; horizontal and vertical motions are independent, with gravity affecting only the vertical part.
Momentum is mass times velocity, a vector conserved in all collisions, while kinetic energy is a scalar that's only conserved in elastic ones.
There's no real outward force; your body's inertia wants to go straight while the ride pulls you inward, and that mismatch feels like a push outward.
Forces come in pairs, so if A pushes B then B pushes A equally and oppositely, but the two act on different objects so they don't cancel.
Conservation of angular momentum: reducing their moment of inertia must increase spin rate to keep total angular momentum constant.
Friction resists sliding surfaces, and the microscopic bonds that repeatedly form and break convert kinetic energy into thermal energy.
It trades distance for force: moving a long arm a large distance produces a large force over a short distance at the load, conserving total work.
It changes form rather than vanishing, from chemical to kinetic to heat; 'using' energy really means degrading it into less useful forms.
Gravity constantly pulls it toward Earth while its sideways speed carries it forward, so it's perpetually falling but always missing the ground.
The steady speed a falling object reaches when air resistance exactly balances gravity, so net force and acceleration drop to zero.
The weight of the water above you grows with depth, pressing harder, so pressure rises about one atmosphere every ten meters.
Its spinning angular momentum resists changes in orientation, so an external torque causes slow precession instead of toppling it over.
An object's resistance to changes in its motion; more mass means more inertia, which is exactly what Newton's first law describes.
Speed is just how fast you're going, while velocity is speed with a direction, so it changes when you turn even at constant speed.
Ignoring air, gravity removes speed on the way up and adds the same amount on the way down, so it comes back at equal speed.
Kinetic energy is the energy of motion (½mv²), while potential energy is stored energy of position, like a raised weight ready to fall.
They're in free fall around the Earth along with their ship, so nothing presses up on them — that's the weightless feeling.
The inward force that keeps something moving in a circle; for a turning car it's friction, and for the Moon it's gravity.
The net work done on an object equals its change in kinetic energy, linking force-over-distance directly to speed.
A thin, low-friction layer at the surface means your foot's sideways push isn't resisted, so it slides out from under you.
They convert the car's kinetic energy into heat through friction between the pads and discs until the car stops.
Pushing a system at its natural frequency makes energy build up dramatically — how a swing goes higher, or a bridge can sway dangerously.
More mass means more kinetic energy and momentum, so the brakes must do more work over a longer distance to bring it to rest.
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