The resultant must be zero
We learned about the resultant force produced by two collinear forces acting at a point in the lesson on resultant force. That knowledge is used directly here.
With one force already applied to a body in some direction, pulling the body the opposite way with a second force makes the magnitude of the resultant smaller. When the two forces are equal the resultant becomes zero.
The condition for equilibrium
For a body to remain in equilibrium the resultant of the forces acting on it must be zero.
F1 − F2 = 0
that is, F1 = F2
But is equality on its own enough?
Does making the two forces equal by itself produce equilibrium? What about the direction they are applied in, and the line they act along? That is what the two activities over the next two pages set out to find.
In this section we look for the conditions needed to maintain equilibrium under two forces acting in opposite directions in the same plane.