In the Field the disc never holds steady — it speeds up sweeping toward the sun, slows pulling away, and curves the whole time. Every one of those is acceleration: a change in velocity, in speed or in direction, caused by a net force. Here that force is gravity, and it accelerates the disc at every instant.
Keep an eye on the speed readout: it rises and falls as the disc swings nearer and farther from the sun — that changing speed is acceleration. But notice the path is always curving as well, and a change of direction is acceleration just as much as a change of speed. The disc is never moving in an unchanging straight line, so it is accelerating at every single moment — driven by the net force of gravity.
Acceleration is any change in an object's velocity over time — and velocity means both speed and direction. So speeding up, slowing down, and even turning at a steady speed are all forms of acceleration. It is measured by how much the velocity changes each second, in metres per second per second (m/s²). Acceleration is not the same as going fast: a jet cruising in a straight line at huge speed has zero acceleration, while a slow car rounding a bend is accelerating.
Acceleration is produced by a net force, and the link is Newton's second law: a = F ÷ m. A bigger net force gives a bigger acceleration; a bigger mass gives a smaller one. Calculated as the change in velocity divided by the time, a = Δv ÷ Δt, it can be positive (speeding up), negative (slowing down, or decelerating), or purely a change of direction. When the velocity does not change at all, the acceleration is zero — exactly the no-net-force case of Newton's first law.
In the Field the sun's gravity is a net force on the disc at all times, so the disc is always accelerating. It speeds up as it falls toward the sun and slows as it climbs away — a change in speed — while its path curves continuously, a change in direction. The acceleration is strongest at the closest approach, where gravity is fiercest, and the path bends most sharply there. On the frictionless table, by contrast, the disc between walls has zero acceleration: straight line, steady speed, no net force.
Acceleration is easiest to feel when you can dial it and watch the speed respond. Drive a dot with an acceleration, settle a famous puzzle about the top of a throw, and turn a speed change into a number.
Positive acceleration speeds the dot up; negative slows it down; zero leaves it gliding at a constant speed. The speed keeps changing for as long as the acceleration is not zero.
When the lights turn green, a car's velocity climbs from zero — positive acceleration. The harder the engine pushes and the lighter the car, the bigger that acceleration, exactly as a = F ÷ m predicts.
Drop anything near Earth and it speeds up by about 9.8 m/s every second — the free-fall acceleration g. With no air, a feather and a hammer gain speed together, since gravity accelerates all masses equally.
A car circling a roundabout at a steady speed is still accelerating, because its direction keeps changing. That inward, centripetal acceleration is why you feel pushed sideways in your seat.
Acceleration is how a force shows up as a change in motion. This FAQ travels from the disc speeding round the sun to braking cars, falling apples, roundabouts, fighter-pilot g-forces, and the difference between going fast and accelerating.
Acceleration is any change in an object's velocity over time. Because velocity means both speed and direction, acceleration covers three things: speeding up, slowing down, and changing direction. It is measured by how much the velocity changes each second, in metres per second per second (m/s²). A key point is that acceleration is not the same as speed — an object can move very fast yet not accelerate at all if its velocity stays constant, while a slow object turning a corner is accelerating.
Yes. Velocity is a vector, meaning it has a direction as well as a size, so changing the direction changes the velocity — and any change of velocity is acceleration. A car going round a roundabout at a steady 30 km/h is accelerating the whole way, because its direction is constantly turning. This is why you feel pushed sideways in a fast bend: your body is being accelerated toward the centre of the curve. The acceleration in circular motion is called centripetal acceleration.
Acceleration is the change in velocity divided by the time taken: a = (final velocity − start velocity) ÷ time, often written a = Δv ÷ Δt. For example, a car going from 0 to 28 m/s in 4 seconds has an acceleration of 28 ÷ 4 = 7 m/s², meaning its speed rises by 7 metres per second every second. The units, metres per second squared, come from dividing a speed (m/s) by a time (s), giving a "speed per second".
| Quantity | Tells you | Units |
|---|---|---|
| Speed | How fast (no direction) | m/s |
| Velocity | How fast and which way | m/s (a vector) |
| Acceleration | How fast velocity changes | m/s² |
A net, unbalanced force causes acceleration. Newton's second law makes this exact: a = F ÷ m, so the acceleration equals the net force divided by the mass. If all the forces on an object cancel, the net force is zero and the velocity does not change — no acceleration. As soon as the forces are unbalanced, the object speeds up, slows down, or turns. In the Field, the unbalanced force is the sun's gravity; on a falling apple, it is Earth's gravity.
Near Earth's surface, gravity gives every falling object the same acceleration, about 9.8 metres per second squared, written g. That means a falling object gains 9.8 m/s of downward speed each second. Remarkably, g does not depend on the object's mass: in a vacuum a feather and a hammer fall together, because a heavier object feels more gravitational pull but also needs more force to accelerate, and the two effects cancel. On the Moon g is only about 1.6 m/s², so things fall much more slowly there.
Yes — velocity and acceleration are independent. The clearest case is a ball thrown straight up: at the very top of its flight its speed is zero for an instant, but gravity is still pulling it down, so its acceleration is the full 9.8 m/s² downward. That downward acceleration is exactly what stops the rise and starts the fall. So zero velocity does not mean zero acceleration; you can have either one be zero while the other is not.
Newton's second law states that acceleration equals net force divided by mass, a = F ÷ m, often rearranged to F = ma. It captures two intuitions at once: a bigger push gives a bigger acceleration, and a heavier object is harder to accelerate. Push two trolleys equally and the heavier one speeds up more slowly; push one trolley harder and it speeds up faster. This single law lets engineers predict exactly how any object will respond to the forces on it.
Centripetal acceleration is the inward acceleration that any object moving in a circle must have, pointing toward the centre of the circle. Even when the speed is perfectly steady, the direction is changing, and that change is an acceleration directed inward. It is provided by a real inward force — gravity for an orbiting satellite, tension for a ball on a string, friction for a car on a bend. Remove that inward force and the object stops curving and flies off in a straight line.
Acceleration is force divided by mass, and the force here is gravity, which follows an inverse-square law — far stronger up close. So as the disc sweeps in toward the sun, the pull rises steeply and the acceleration peaks at the closest approach, bending the path most sharply and whipping the disc round. As it climbs back out, gravity weakens and the acceleration fades. This is the same reason a comet races fastest at its nearest point to the Sun.
In aviation, "g" is used as a handy unit of acceleration, where 1 g equals 9.8 m/s². Pulling "5 g" in a tight turn means accelerating at five times that, which presses the pilot into the seat with five times their normal weight. High g-forces come from rapid changes of velocity — sharp turns, fast climbs, or hard braking. Around 9 g, the acceleration can drain blood from the brain and cause a blackout, which is why pilots wear special suits and tense their muscles.
No — speed and acceleration are completely different. An object can move extremely fast and have zero acceleration, as long as its velocity is not changing. A spacecraft coasting through deep space at thousands of metres per second, in a straight line at steady speed, has no acceleration at all, because nothing is changing its velocity. Acceleration is about the change in motion, not the amount of it. A slow object that is speeding up, slowing, or turning is accelerating; a fast one moving steadily is not.
Uniform, or constant, acceleration means the velocity changes by the same amount in every equal interval of time — for example, gaining 9.8 m/s each second in free fall. Under uniform acceleration the motion follows simple, predictable rules: the velocity rises in a straight line over time, and the distance travelled grows with the square of the time, so an object falls four times as far in two seconds as in one. These "equations of motion" are the workhorses of introductory mechanics.
In the Field the sun pulls on the disc with a net force at all times, so the disc is always accelerating. You can see it two ways at once: the speed readout climbs as the disc nears the sun and drops as it pulls away, showing a change of speed; and the path is forever curving, showing a change of direction. The acceleration is largest at the closest approach, where gravity is strongest and the path bends most. The frictionless table is the opposite case — straight, steady, zero acceleration.
They describe different things. Momentum is a measure of an object's motion, equal to its mass times its velocity, and it is large for heavy or fast objects. Acceleration is the rate at which velocity changes, and it is large when a force changes the motion quickly. An object can have huge momentum yet zero acceleration — a freight train rolling steadily — or large acceleration yet little momentum — a light ball flicked hard. Force links them: a net force causes acceleration, and over time it changes momentum.
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