Newton's Second Law
Forces and Motion

Discussion leading to Newton's second law

Teaching Guidance for 14-16 PRACTICAL PHYISCS

Students will have discovered that:

  • a constant force accelerates a given mass with constant acceleration;
  • doubling the force doubles the acceleration, i.e. the acceleration is directly proportional to the force for a given mass. F is proportional to a;
  • the force, F, needed for a given acceleration is inversely proportional to the mass, m
  • for a given force, F, the acceleration, a, is inversely proportional to the mass, m.

(many students find inverse proportion a problem).

Considering these points together leads to F is proportional to ma or F = a constant x ma.

Mass is measured in kg and acceleration in m/s/s but what of force? If the constant is equated to unity, then we are defining a unit of force. In the SI system the force is measured in newtons (symbol N), leading to F = ma.

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law

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