In physics, when is an object accelerating?
The Moon circles Earth at a nearly constant speed, and it has done so for billions of years. Nothing about its pace ever seems to change.
Is the Moon accelerating?
You have already seen the rule that decides this: an object accelerates whenever its velocity changes, and a change of direction counts.
In uniform circular motion the speed is constant, but the velocity is not. The direction changes at every instant.
The Moon's direction of travel changes all the way around its orbit. The Moon is accelerating, right now.
'Constant speed, so no acceleration' is the trap. The rule reads on velocity, not on speed.
A helicopter flies a circular search pattern at a steady 40 m/s. Is it accelerating?
A subway train cruises a long straight tunnel at a steady 20 m/s. Is it accelerating?
Run the rule on one car. The car gains speed down a straight on-ramp. Is it accelerating? Yes: its velocity is changing.
The same car holds a steady speed on the straight highway. Is it accelerating? No: its speed and direction both hold, so its velocity holds.
The same car holds that same steady speed around a circular interchange loop. Is it accelerating? Yes: its direction is changing, so its velocity is changing.
An airliner flies a circular holding pattern at a steady 250 mph. Is it accelerating?
Is its velocity changing? Check both halves.
Its speed holds at 250 mph.
Its direction of travel changes at every instant on the circle.
A change in direction is a change in velocity, and a changing velocity is an acceleration.
Yes. The airliner is accelerating the whole way around the pattern.
A freight train rolls along a long straight track at a steady 25 m/s. Is it accelerating?
Is its velocity changing? Check both halves.
Its speed holds at 25 m/s.
Its direction holds too: the track is straight.
Neither half of its velocity changes.
No. The train is not accelerating.
Golda says: 'A race car holding a steady 150 mph around a circular track is accelerating.' Is Golda correct?
Which of these movers is accelerating?
A horse on a steadily turning carousel is accelerating. What makes that true?
A weather satellite circles Earth at a constant speed on a circular orbit. Which change makes it count as accelerating?