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  Newton’s Second Law
  Newton’s second law of motion describes how
acceleration, mass, and net force are related.
Acceleration is the rate at which the velocity
of an object changes over time. The net forces
acting on an object can change an object’s
velocity by causing it to speed up, slow down,
or change direction. All of these changes can
be called accelerations.
  The formula that describes the relationship
between force, mass, and acceleration is:
 
  Force = Mass × Acceleration
  The formula is often written as F = m × a.
It indicates that the stronger the force acting
on an object, the more that object will accelerate.
  The formula also indicates that a force will cause
an object with small mass to accelerate more
than an object with large mass. The acceleration
due to a small force on a small object can be
the same as the acceleration of a big force on
a big object.
   
  The train engines must exert
enough force to pull the
freight cars behind them.
The more freight cars there
are, the more force the
engines must exert.