one point

one plane

different planes

perpendicular planes

introduce internal stresses

balance the other forces acting on it

retard its motion

all of the above.

if any number of forces acting at a point can be represented by the sidesof a polygon taken in order, then the forces are in equilibrium

if any number of forces acting at a point can be represented in direction and magnitude by the sides of a polygon, then the forces are in equilibrium

if a polygon representing forces acting at a point is closed then forces are in equilibrium

if any number of forces acting at a point can be represented in direction and magnitude by the sides of a polygon taken in order, then the forces are in equilibrium

magnitude

direction

position or line of action

all of the above

not a replace them by a single force

to replace them by a single force

to replace them by a single force through C.G.

to replace them by a couple