Equilibrium

If several forces are in equilibrium, their vector arrows form a closed shape when arranged head to tail: the finishing point returns to the starting point, so the resultant is zero. For equilibrium under just two forces, the forces must be equal in magnitude and opposite in direction.

An object is in equilibrium when all the forces acting on that one object balance, giving a resultant force of zero. It can then remain at rest or move with constant velocity.

An aircraft travelling in a straight line at constant speed can therefore be in equilibrium if thrust balances drag horizontally and lift balances weight vertically.

Equal, opposite arrows show that the forces balance. Their vector sum is zero, so the lamp is in equilibrium. By contrast, if the forward driving force on a car is greater than the combined resistive forces, the free body diagram shows a forward resultant force.