Newton’s Three Laws

Objective: Use ΣF = ma for constant mass in an inertial frame.

For a body of approximately constant mass in an inertial frame, Newton’s second law relates net force and acceleration: ΣF = ma. Acceleration points in the direction of net force, not necessarily in the direction of current velocity. A 6 kg cart under a 12 N net force accelerates at 2 m/s² in the force direction. If mass doubles while net force stays fixed, acceleration halves in this model; if net force doubles at fixed mass, acceleration doubles. Use SI units so kg·m/s² equals N. The equation applies to the sum of external forces on one object, not to an arbitrary single force when others also act.

Objective: Apply newton’s three laws to calculations, evidence, and specified situations.

Newton’s first law describes constant velocity when net external force is zero in an inertial frame. The second law relates net force to acceleration for a constant-mass system: F_net=ma. The third law gives equal and opposite interaction forces on different objects. “Net” matters: an individual force need not equal ma if other forces act. Mass measures inertia rather than weight. These classical laws are models appropriate to ordinary speeds and scales.