When two forces act simultaneously on an object, whether at rest or moving with constant velocity, the object will either remain at rest or continue moving with constant velocity if the forces balance each other. This concept is known as equilibrium.
There are two types of equilibrium:
Static Equilibrium: An object is in static equilibrium when it is at rest and the net force acting on it is zero. In this case, the forces acting on the object are balanced, and there is no resultant force to cause any acceleration. For example, if you push a stationary box with a force of 10 Newtons to the right, and an equal but opposite force of 10 Newtons is applied to the left, the box will remain at rest.
Dynamic Equilibrium: An object is in dynamic equilibrium when it is moving with constant velocity in a straight line, and the net force acting on it is zero. In this case, the forces acting on the object are balanced, resulting in no acceleration. For instance, if a car is traveling at a constant speed of 60 km/h to the east, and the forces pushing it to the east (e.g., the engine) are balanced by forces opposing its motion (e.g., air resistance and friction), the car will continue moving at a constant velocity.
In both cases, the forces acting on the object cancel each other out, resulting in no change in motion. This can be mathematically expressed as the vector sum of the forces being zero:
ΣF = 0
Where ΣF represents the net force acting on the object. If the net force is not zero, the object will experience acceleration, causing a change in its state of motion.
It's important to note that the concept of equilibrium applies to forces acting in a straight line. If forces act at an angle or in different directions, they need to be resolved into their components along the appropriate axes to determine whether equilibrium is achieved.