Question

Which of the following is not a condition of equilibrium?
A. The resultant of all the forces acting on the body should be equal to zero.
B. The resultant of all the forces acting on the body about the point of rotation should be equal to zero.
C. Both A and B
D. None

Answer 1

Hint:If all the forces acting on a body are balanced then the object is considered to be in a state of equilibrium.
By this definition we shall observe the conditions of equilibrium in this solution.

Complete step by step solution:
Velocity changes with change in speed. So, this change in velocity is known as acceleration.
For a body to remain at rest the acceleration of the body should be zero i.e. there should not be any change in motion.
For acceleration to be zero there should not be any applied force. This is the first and foremost condition for equilibrium to exist.
When the net force is zero the body is in equilibrium.
For the net force to be zero each axis of motion i.e. both x-axis and y-axis should be zero.
Again if a body is in rest in dynamic motion, it may not be at rest in static motion.
For example- If a girl sits in a seesaw she is at rest but since the seesaw is moving the body is in dynamic motion.
So, the dynamic motion should also be zero for the body to be in equilibrium.
So, from the above observation we can say that the resultant of all the forces acting on the body should be equal to zero and also the resultant of all the forces acting on the body about the point of rotation should be equal to zero for equilibrium to exist.
Hence, option D is correct. Both the conditions given in the question are conditions of equilibrium.

Note:The velocity should be zero or constant for equilibrium to happen. Here we should not get confused with the word resultant of all forces which means sum of all forces. The sum of all forces acting on the body should be zero and not only one force.