Translational equilibrium
Also known as: first condition of equilibrium
Translational equilibrium is the state in which the net force on an object is zero, so it has no linear acceleration. The object is either at rest or moving in a straight line at constant velocity.
An object is in translational equilibrium when the vector sum of all forces acting on it equals zero: ΣF = 0. By Newton's second law (F = ma), zero net force means zero acceleration — the object's velocity is not changing. This is sometimes called the first condition of equilibrium.
Crucially, equilibrium does not mean the object is motionless. A book resting on a table is in static equilibrium (at rest), but a car cruising down a straight highway at a constant 60 mph is in dynamic equilibrium — it is moving, yet its net force is still zero because the engine's drive force exactly balances friction and air resistance. In both cases, forces along each axis cancel: ΣFx = 0 and ΣFy = 0.
Solving equilibrium problems means drawing a free-body diagram, resolving forces into components, and setting the sums to zero. A hanging sign supported by two angled cables, a block on an inclined plane held by friction, or a skydiver at terminal velocity are all standard setups. Translational equilibrium pairs with rotational equilibrium (net torque = 0); an object in complete equilibrium satisfies both conditions.
The MCAT tests translational equilibrium in its physics foundations, often through force-balance problems involving tension, inclines, or terminal velocity. Recognize that "constant velocity" in a problem statement is code for zero net force, and set up component equations accordingly.
Key takeaways
- Translational equilibrium means the net force on an object is zero (ΣF = 0), so acceleration is zero.
- Objects in equilibrium are either at rest (static) or moving at constant velocity in a straight line (dynamic).
- Force components must cancel independently along each axis: ΣFx = 0 and ΣFy = 0.
- Complete equilibrium also requires rotational equilibrium — net torque of zero.
- The MCAT signals equilibrium problems with phrases like "constant velocity" or "at rest."