Answers

2016-03-05T16:09:53+05:30
Let us consider an isolated system( no ext force is acting) 
acc. to Newton's law
F = ma 
for an isolated system, it will
ma= 0
m can't be zero and as no force is acting the acce (a) = 0 which means the body will remain in its state of rest of uniform motion as what our newton's first law states. hence, Newton first law of motion can obtain from the second law of motion.
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2016-03-07T11:59:54+05:30
Newton's First Law of Motion states that a body at rest will remain at rest unless an outside force acts on it, and a body in motion at a constant velocity will remain in motion in a straight line unless acted upon by an outside force.

If a body experiences an acceleration ( or deceleration) or a change in direction of motion, it must have an outside force acting on it. Outside forces are sometimes called net forces or unbalanced forces.

The property that a body has that resists motion if at rest, or resists speeding or slowing up, if in motion, is called inertia. Inertia is proportional to a body's mass, or the amount of matter that a body has. The more mass a body has, the more inertia it has.

The Second Law of Motion states that if an unbalanced force acts on a body, that body will experience acceleration ( or deceleration), that is, a change of speed. One can say that a body at rest is considered to have zero speed, ( a constant speed). So any force that causes a body to move is an unbalanced force. Also, any force, such as friction, or gravity, that causes a body to slow down or speed up, is an unbalanced force. This law can be shown by the following formula

F= ma
F is the unbalanced force
m is the object's mass
a is the acceleration that the force causes
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