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

Disable node explorer

Off
IOP DOMAINS Physics CPD programme

Energy CPD videos

Our new set of videos gives teachers and coaches of physics a preview of the training we offer ahead of this term's live support sessions.

Find out more