Question: In my text book they said that:

> 
> 1. If no resultant force acts on an object, it will not accelerate; **it will remain at rest or it will** **continue to move at constant speed**
> 2. If an object is at rest or is moving at a constant speed in a straight line ,we can say that there is no resultant force on it.
> 
> 
> 

Now my question as it concentrates more on the first point.

How if the resultant forces or force are zero still the object moves at a constant speed and some times it does not move (or it is at rest as they say)?

Answer: 
Recall Newton's Second Law,
*F=ma*

This means that resultant forces acting on an object will produce acceleration, which is a change in velocity. Remember that velocity is a vector, which includes both *speed* **and** *direction*. So if an object is accelerating, it has either its speed and/or direction of movement undergoing a change.

If an object is traveling at constant speed and still moving in the same direction, it is not experiencing acceleration, and thus the resultant forces acting on it are zero. This is exactly the same for an object at rest, except that its speed happens to be zero.
Basically, if no \*\*net \*\* force is acting on a body, it's acceleration is zero...

Now, acceleration is the change in velocity in unit time... 

So, if velocity is constant or zero, it's acceleration is zero...

Hope this helps!!!