Q: If two forces act on a moving body, could the body possibly move (a) at a constant speed or (b) at a constant velocity? A: If b can be true then a is also true by the same method. If two forces are equal and opposite on an object they will cancel each other out so that there is no net effect on the moving body. Therefore, if an object was traveling with a constant velocity the and two equal and opposite forces were applied to it, the forces would cancel and the object would continue moving with a constant velocity 

Q: A vertical force F is applied to a block of mass m that lies on a floor. What happens to the magnitude of the normal force N on the block from the floor as magnitude F increases from zero if force F is (a) downward and (b) upward? A: If the force F was downward then the magnitude N would increase from its initial value of mg. If the force F was upward then the magnitude N would decrease from mg to zero (when the box would move up off the floor. Example:

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"If no net external force acts on an object, then if the object is a rest it will remain at rest and if it is in motion of any kind it will remain in motion in straight line with constant speed."
Notes:
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass: a = F_{net} / m.
Note:
The most often used formulation of Newton's Second Law is:
F = ma.
When two objects interact, the force exerted on object A by object B is equal
in magnitude to the force exerted on object B by object A and opposite to it in
direction.
F_{AB} =  F_{BA}
Newton's Second Law  F = ma. 
Newton's Second Law (component version)  F_{x}
= ma_{x} F_{y} = ma_{y} F_{z} = ma_{z} 
SI Unit for Force (Newton)  
Weight  W = mg 
Newton's Third Law  F_{AB} =  F_{BA} 
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