the floor is smooth, the wall is rough

the floor is rough, the wall is smooth

the floor and wall both are smooth surfaces

the floor and wall both are rough sur-faces

three forces acting at a point will be in equilibrium

three forces acting at a point can be represented by a triangle, each side being proportional to force

if three forces acting upon a particle are represented in magnitude and direction by the sides of a triangle, taken in order, they will be in equilibrium

if three forces acting at a point are in equilibrium, each force is proportional to the sine of the angle between the other two

balance each other

produce a couple and an unbalanced force

are equivalent

can not balance each other.

2n-3

n-l

′2n-l

n - 2

resultant couple

moment of the forces

resulting couple

moment of the couple