Newtons first law of motion states that a body subject to no net external forces continues in a state of uniform motion. His second law of motion in a sense makes his first law complete by telling us what will happen in the event of a body being subject to a net external force: it will accelerate.
The acceleration will be in the direction of the net external force, and proportional to it. Newton's second law may be conveniently stated in the equation
Since
we may also state Newton's second law as: The net external force applied to a body is proportional to the rate of change of momentum of that body.
Both equations above are stated in terms of SI units, so the force is in newtons, the mass in kg, the velocity in m/s, the time in s, momentum in kg m/s and the acceleration in m/s/s or
Newtons second law is verified by many experiments and applies throughout the universe.
Though the second law is true for any number of forces acting in any direction on a body, we are especially interested when the forces act along the same line, often parallel to a surface.
Example: The mass of the block below is 5 kg. Find the acceleration.
The resultant force og the block is 7-4 =3N to the right.
The acceleration is
to the right.
If the base is tilted and the 4N force removed, asbelow, then the only force acting down the slope is a component of gravity. Resolving down the slope, (note
),
