A street lamp is fixed to a wall by a metal rod and a cable.
Which vector triangle could represent the forces acting on the end of the rod at point P?
ANSWER: D
The Physics behind the answer:
- When a system is in equilibrium, the sum of all the forces acting on the system must be equal to zero. This is the first condition of equilibrium.
- The sum of these 3 forces must be zero.
- When forces are drawn using vector arrows using tip-to-tail method, the resultant force or the sum of all forces on an object is found through the straight line distance from the tail of the first arrow drawn to the tip of the last vector arrow.
- When an object is in equilibrium, these vector arrows drawn tip-to-tail returns to its origin, that is, a closed figure where the tail of first and tip of last arrows are at the same point. This shows that resultant force is zero.
- There are 3 forces acting at point P of the metal rod [the system].
- the downward weight of the lamp
- the tension in the cable directed along the cable itself
- the compressive force of the rod which is directed to the right
- All the options are closed figures. We need to see which one is consistent with the directions mentioned in bullet 3. Thus, the answer is D.
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