02 2-1 (b) The resultant of two like parallel forces
Rule
- Let and be like forces at points and of a rigid body.
- Their resultant is parallel to them, in the same direction, with magnitude
- Its line of action cuts the line at an inner point which divides internally in the inverse ratio of the forces:
- So is nearer to the larger force.
Where the rule comes from
- Add two equal and opposite forces along at and at ; they do not change the effect.
- Combine with the force at , and with the force at , by the parallelogram law. The two resultants meet at a point .
- Resolve them again at : the added horizontal forces cancel and , act together along the line through .
- Similar triangles then give and , so .
✍️ Worked examples
Example 1: Two like forces of 5 N and 7 N act at and with cm. Find their resultant.
Solution:
- N, in the direction of the two forces.
- with , so cm.
- The resultant acts 21 cm from (the end of the 5 N force) and 15 cm from .
Example 2: Where does the resultant of two equal like forces act?
Solution:
- With , , so the resultant acts at the midpoint of with magnitude .