It moves faster or slower
An object is accelerating if its velocity is changing.
If an object is not accelerating, it means that the sum of all forces acting on the object is equal to zero. This is in accordance with Newton's first law of motion, which states that an object will remain at rest or in constant motion unless acted upon by an external force.
When a falling object stops accelerating but is falling at a constant velocity, it is called terminal velocity.
curve
You would say that the object in motion is accelerating. Or you can say that the object's velocity is increasing.
No, an accelerating object cannot have zero speed. Acceleration is the rate at which the speed of an object changes over time. If an object is accelerating, its speed is increasing or decreasing, but it cannot be zero.
On a spacetime diagram, the worldline of an object accelerating away from you would appear as a curve that slopes upwards and to the right, indicating the object's increasing distance from you over time.
An object is not accelerating when it is moving in a straight line at an unchanging speed (which may be zero).
If an object is not accelerating, then one knows for sure that it is moving at a constant speed.
The force-time graph of an accelerating object would typically show a non-zero, positive force applied over time. As the object accelerates, the force applied to it would increase proportionally until it reaches a constant force when the acceleration stops.
Accelerating...or was accelerating.
You can tell an object is accelerating if its velocity is changing, either in speed or direction. This can be observed by noticing a change in the object's position over time.