Physics 2.14 - 2.15 - Newton’s first and second laws

Resultant force and Newton's first law

Begin by naming the body whose motion is being explained. Every force included must act
on that one body. Choose a positive direction, then combine forces in that direction. Forces
in the opposite direction subtract. The single force with the same overall effect is the
resultant force.

Newton's first law describes what the body's velocity does:

• If the resultant force is zero, a body at rest remains at rest, while a moving body
  continues at constant velocity. Constant velocity means constant speed in a straight line.
• If the resultant force is not zero, the body's velocity changes. Its speed can change,
  its direction can change, or both can change.

Zero resultant force does not mean that no forces act. A book resting on a table has weight
downward and a support force upward; if those forces are equal, their resultant is zero. A
cyclist at constant velocity can have a forward driving force equal to the total resistive
force. If the cyclist stops pedalling, resistance becomes a backward resultant and the
cyclist slows. The motion has not "used up" a force: a force is changing the velocity.

Newton's second law

Newton's second law makes the cause-and-effect link quantitative. For a body of constant
mass, its acceleration is in the direction of the resultant force. A larger resultant force
causes a larger acceleration; for the same resultant force, a larger mass causes a smaller
acceleration.