Physics
Equilibrium of Forces
Quick fact
An object can be in equilibrium even while moving—as long as its velocity (speed and direction) is constant.
Why this is interesting
You push a book across a table and it stops. But why does it stop? And how can a moving train stay at a constant speed?
Read the full explanation
Understanding Equilibrium of Forces
Imagine a tug-of-war where both teams pull with exactly the same strength—the rope doesn’t move. That’s static equilibrium. Now picture a car cruising on a highway at a steady 100 km/h. All forces (engine thrust, air drag, friction) are balanced, so it doesn’t speed up or slow down—dynamic equilibrium. In both cases, the net force is zero.
A deeper explanation
Equilibrium of forces is a direct consequence of Newton's first law: an object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted upon by an unbalanced force. When forces are balanced (net force = 0), there is no change in motion. This principle allows engineers to design stable structures (like bridges) where all forces are carefully balanced, and it explains why a skydiver reaches a terminal velocity when air resistance equals weight.