Question

Assertion: The forces acting on a body can be replaced by the resultant force only as regards the motion of the body as a whole.
Reason: The resultant force cannot replace the several forces acting on a body in other respects.
A) If both the assertion and reason are true and the reason is a true explanation of the assertion.
B) If both the assertion and reason are true but the reason is not true the correct explanation of the assertion.
C) If the assertion is true but reason false
D) If both the assertion and reason are false.

Answer 1

Hint: Whenever multiple forces act on a body; we can take the resultant of all the forces which dictate the dynamics of the body. We will use this fact to determine whether the assertion and reason are correct or not.

Complete step by step solution
In physics, we use the concept of resultant force to represent all the forces that act on the object by combining all the forces in a vector addition of all the forces. This is often done with the help of a free body diagram.
If all the forces acting on the body are in the same direction, they will add up directly but if they are in different directions, we will have to take the vector addition of all the forces that act on the body. After taking the vector addition, we obtain the resultant force and we can now instead replace all the individual forces that act on the body with a single resultant force.
This simplifies our calculations since now we only have to take into account the effect of one net force on the body and not treat every force individually. The motion of the body will be determined only due to the resultant force then.
So, the assertion statement is correct as we can replace all the individual forces with a resultant force however the reason statement is false since it states that we cannot do so.
Hence the correct choice is option (C).

Note
The resultant force can only be calculated if all the forces in the system are acting at the same point. This means our body must be a rigid body so that it is not deformed under force and all the forces that act on the body will act on the centre of mass of the body.