The Concurrent Forces Viva Questions and Answers
(i) State parallelogram law of forces
Ans: If two forces acting at a point be represented in magnitude and direction by the adjacent sides of a parallelogram drawn from a point, the resultant force is represented in magnitude and direction by the diagonal of the parallelogram drawn from the same point
(ii) State triangle of forces
Ans: If three forces acting at a point can be represented in magnitude and direction by the sides of a triangle taken in order then the forces is in equilibrium,
(iii) What is meant by resultant of a set of force?
Ans: It is that single force which effectively replaces the set of forces
(iv) What are concurrent forces?
Ans: Forces acting at a point are known as concurrent forces.
(v) What is Lami's theorem?
Ans: If three forces acting at a point keep it in equilibrium, then each force is proportional to the since of the angle between the other two.
(vi) 
What is the angle between them
Ans: 45°
vii) Will the forces 15N, 20N and 40N acting at a point keep it in equilibrium?
Ans: No
viii) What are the main sources of error in the experiment?
Ans: (a) Friction at the pulleys. (b) Thread cannot be weight less.
(ix) If there is a lot of friction at the pulleys in what way the result of this experiment be affected?
Ans: The forces represented by the hanging weight will have values lower than what are given by the relation W = mg
(x) Apparatus of Concurrent Forces
Ans: Parallelogram law apparatus consisting of two frictionless pulleys attached to a framework fixed on the top of a vertical drawing board, light inextensible strings, weights and weight hangers and the given body.
(xi) Theory of Concurrent Forces
Ans: If three co-planer concurrent forces P, Q and R keep a point in equilibrium, the resultant of P and Q will be equal and opposite to R, the equilibrant.
(xii). Explain the term, force?
Force is that which changes or tries to change the state of rest or of uniform motion of a body.
(xiii). What is meant by composition of vectors?
The process of adding two or more vector quantities to find their resultant.
(xiv). What is the relation between newton and kg wt?
1 kg wt = 9.8 newton
(xv). Define parallelogram law of forces.
If two vectors (forces) are represented in magnitude and direction along the adjacent sides of a parallelogram, then their resultant can be represented in magnitude and direction by the diagonal drawn from the point of intersection.
(xvi). What is meant by equilibrium?
When a number of forces acting on a body and the body does not change its state of rest or of uniform motion, it is said to be in equilibrium.
(xvii). If there is a lot of friction at the pulleys, in what way the result is affected?
Weight will be lower than the actual value.
(xviii). What is meant by a vector quantity?
Quantities having both magnitude and direction.
(xix). Give some quantities which are vectors.
Displacement, Velocity, Acceleration, Force
(xx). What is meant by a scalar quantity?
Quantities having only magnitude and no direction are called scalars.
(xxi). Give some quantities which are scalar.
Mass, Time, Speed, Work
(xxii). Define concurrent forces
Forces whose lines of action meeting at a single point are called concurrent forces.
(xxii). Define coplanar forces.
Forces acting in a plane are called coplanar forces.