04 2-1 (d) Finding one force from the resultant
Method
- Take the unit vector along the resultant and use with signs.
- Take moments about the point where the resultant acts; the sum of moments of the forces about that point is zero.
✍️ Worked examples
Example 4:
- One of two parallel forces is 20 N.
- The resultant is 35 N, and its line of action is 15 cm from the 20 N force.
- Find the other force and its position in each case: (a) the 20 N force and the resultant have the same direction, (b) opposite directions.
Solution:
- (a) , so N, in the same direction.
- Moments about (point of the resultant): , so cm. The 15 N force is on the other side of , so cm.
- (b) The resultant (35 N) is opposite to the 20 N force, so and N in the direction of the resultant.
- Moments about : , so cm.
- Both and the 55 N force are on the same side of the 20 N force, so cm.