if the time needed for the object to cross the same distance decreases by time.
For example:
1st second of motion, the object crosses exactly 1.0 meters of space.
2nd second of motion, the object crosses 3.0 meters of space.
Since the object has crossed more space (3.0m) in the 2nd second, than it did in the
first second(1.0m), we can conclude that the object is moving in an accelerated
motion.
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Another contributor added:
That answer is correct as far as it goes, but it's very important to be aware that
the discussion above only touches on one of several aspects of acceleration.
Somewhere in the past 100 years, somebody decided to name the skinny pedal
on the floor of the car the "accelerator". And ever since that day, everybody
thinks that "acceleration" means "speeding up". It doesn't.
"Acceleration" means: Changing speed (faster OR slower) OR changing direction.
The first answer said: "The time needed to cross the same distance decreases."
That's true. That's acceleration. That's what happens when you stomp the gas ...
you go faster. But that's not the only possible description of acceleration.
When you stomp the brake, you go slower. "The time needed to cross the same
distance increases." According to the strict definition, that's also 'acceleration'.
And what if you're driving along on cruise-control, your speed isn't changing at all,
but you're going around a gentle curve. Hold onto your seat belt, because that's
'acceleration' too. Acceleration doesn't always change your speed. If your direction
is changing, that's 'acceleration' too.
if its velocity is increasing or decreasing
It means that the object is moving.
In common terms, that means that it is moving.
the object is not moving
An object is in motion if it changes position relative to a reference point. Motion is a relative quantity and is expressed in respect to the position of another object. Example: both a car and the Earth are moving (earth is revolving around the sun) but if we express the motion of the car in respect to the Earth, we say the car is moving relative to the Earth. At the same time the earth is moving relative to the sun.
You would say that the object in motion is accelerating. Or you can say that the object's velocity is increasing.
if its velocity is increasing or decreasing
It means that the object is moving.
It means that the object is moving.
In common terms, that means that it is moving.
the object is not moving
That is equivalent to saying that: * The object is moving, or * The object doesn't stay all the time in the same place.
when can you say that the object is in motion
You could say that its motion is uniform, meaning that its speed is constant,and that it's moving in a straight line.
An object is in motion if it changes position relative to a reference point. Motion is a relative quantity and is expressed in respect to the position of another object. Example: both a car and the Earth are moving (earth is revolving around the sun) but if we express the motion of the car in respect to the Earth, we say the car is moving relative to the Earth. At the same time the earth is moving relative to the sun.
The object is moving at a constant speed.
Some other words for still or unmoving:immobilemotionlessstationary