Two forces act on a circular disc as shown. Which diagram shows the line of action of the resultant force?

Question 2

Two forces act on a circular disc as shown.

Which diagram shows the line of action of the resultant force?

Reference: Past Exam Paper – June 2017 Paper 12 Q2

Solution:

Answer: A.

The resultant force of the perpendicular forces 3 N and 4 N can be easily obtained by considering the triangle formed.

Resultant force = √(3²+4²) = 5 N

The resultant force would be located at the point where the line of action of both forces act. The line of action of the 3 N force is vertical while the line of action of the 4 N force. The lines of action cross at the centre of the disc. [B and D are incorrect]

Since the both forces (3 N and 4 N) are not equal, the direction of the resultant force cannot be exactly between them. Here, the resultant force is closer to the 4 N force (since it is larger). [A is correct]

In choice C, the resultant force seems to be closer to the 3 N force, which is incorrect.