2-1 (a) Parallel forces acting on a rigid body

Why a new idea is needed

• For forces acting on one particle the lines of action all pass through a single point, and their resultant passes through that same point.
• For forces acting on a rigid body the lines of action need not meet. Parallel forces never meet, so we must also find where the resultant acts.
• Like forces are parallel and in the same direction. Unlike forces are parallel and in opposite directions.

A balancing experiment

• A uniform ruler balances on a pivot at its centre.
• If two equal stones sit at the ends, the pivot stays at the centre. If one stone is doubled, the pivot must move towards the heavier end.
• The pivot is directly under the point where the resultant of the two weights acts.