Newton's laws, net force and system choice

Newton's laws, net force and system choice

Newton's first law says an object remains at rest or moves at constant velocity when the net external force is zero. Newton's second law connects net force, mass and acceleration: Fnet = ma. Newton's third law describes an interaction pair: when object A exerts a force on object B, B exerts an equal-magnitude, opposite-direction force on A. The two third-law forces act on different objects, so they do not cancel on a single free-body diagram. Begin every problem by naming the system; then include only forces acting on that system and choose a positive direction.

Work it through

A 6.0 kg trolley accelerates at 2.0 m s⁻² on a horizontal surface. For the trolley as the system, Fnet = ma = 6.0 × 2.0 = 12 N in the direction of acceleration. If a push of 20 N acts right, friction must be 8 N left because the forces combine to 12 N right. This separates an applied force from the net force.

Mastery target

Choose a system, draw or interpret forces on it, and calculate or explain net force and acceleration using Newton's laws.