6 - Forces in equilibrium

vector

magnitude and direction e.g displacement

scalar

magnitude e.g direction

displacement

vector quantity

when is an object at equilibrium?

when two forces (equal and opposite) act on a point object

moment

force multiplied by perpendicular distance of the line of action of a force to a pivot

body

an object that is not a point object

principle of moments

sum of clockwise moments = sum of anticlockwise moments

centre of mass

point through which a single force on the body has no turning effect

couple

pair of equal and opposite forces acting on a body but not along the same line

when is a body in stable equilibrium?

if the body is displaced and released, it will return to its original position

why does an object in stable equilibrium return to equilibrium when it is displaced and released?

centre of mass of object is directly below the point of support when the object is at rest, weight acts as centre of mass, support force=weight
when displaced, line of action of weight no longer passes through point of support so weight returns object to equilibrium

tilting

where an object at rest on a surface is acted on by a force that raises it up on one side

free body force diagram

shows only the forces acting on an object