12.3 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE
Forces in equilibrium ESBFB
At the beginning of thischapter it was mentioned that resultant forces cause objects to acceleratein a
straight line. If an object is stationary or movingat constant velocity theneither,
- no forces are acting onthe object, or
- the forces acting on that object are exactly balanced.
In other words, for stationary objects or objects moving with constant velocity, the resultantforce
acting on the object is zero. Additionally, if there is a perpendicular moment of force, then theobject
will rotate. You will learn more about momentsof force later in this chapter.
Therefore, in order for an object not to move orto be in equilibrium, the sum of the forces (resultant
force) must be zero andthe sum of the momentsof force must be zero.
DEFINITION: Equilibrium
An object in equilibriumhas both the sum of theforces acting on it andthe sum of
the moments of the forces equal to zero.If a resultant force acts on an object then that object can be brought intoequilibrium by applyingan
additional force that exactly balances this resultant. Such a force is calledthe equilibrant and is equal
in magnitude but opposite in direction to the original resultant force acting on the object.
DEFINITION: Equilibrant
The equilibrant of any number of forces is the single force required to produce equi-
librium, and is equal inmagnitude but oppositein direction to the resultant force.F 1
F 2
Resultant of
F 1 and F 2F 3
Equilibrant of
F 1 and F 2In the figure the resultant of F 1 and F 2 is shown. The equilibrant of F 1 and F 2 is then the vector
opposite in direction tothis resultant with the same magnitude (i.e. F 3 ).
- F 1 , F 2 and F 3 are in equilibrium
- F 3 is the equilibrant of F 1 and F 2
- F 1 and F 2 are kept in equilibrium by F 3