Question

How does newton's first law of motion differ from the second law of motion?

Answer 1

First law of motion or law of Inertia states that an object at rest continues to be at rest or in motion unless compelled by any external force
But
Second law of motion states the rate of change of momentum is directly proportional to the applied unbalanced force in the direction of motion.
The first tells us the nature of objects in motion or at rest.
while second law tells us rate of change of momentum (p)
is equal to applied unbalanced force (MV)

Answer 2

When no net force act on a body , the body either remains at rest or moves with constant velocity. once a body has been set in motion, no net force is needed to keep it moving
In other words, "If no force act on a body , then the body's velocity does not change, i.e. the body does not accelerate.

Second law states that if a net external force acts on a body, the body accelerates. The direction of acceleration is same as the direction of the net force. The net force acting on a body is equal to the product of the body's mass and acceleration of the body.
F=m×a
S.I unit of force is Newton(N) or kgms^-2