When an object moves with constant velocity in a straight line, it means that the object is not experiencing any acceleration. According to Newton's first law of motion, an object at rest or moving with constant velocity will continue to do so unless acted upon by an external force. Therefore, when an object moves with constant velocity, it experiences two types of forces:
Applied force or external force: An external force must have been applied to the object to set it in motion initially, and this force continues to act on the object to keep it moving at a constant velocity. For example, if you push a box with a constant force across a frictionless surface, the applied force maintains its constant velocity.
Opposing force(s): In the absence of an applied force, the object would eventually come to a stop due to opposing forces acting on it. These opposing forces may include:
a. Frictional forces: If the object is in contact with a surface, there might be friction acting in the opposite direction of motion. Friction can be caused by various factors such as air resistance, surface roughness, or fluid resistance.
b. Drag forces: If the object is moving through a fluid medium like air or water, drag forces will act on it, opposing its motion. Drag forces increase with the speed of the object.
If these opposing forces are balanced and equal in magnitude to the applied force, the object will continue to move with constant velocity. This situation is known as dynamic equilibrium, where the net force acting on the object is zero.
In summary, when an object moves with constant velocity in a straight line, the applied force (external force) maintains its motion, while opposing forces like friction and drag act in the opposite direction to slow it down or bring it to a stop if the applied force is removed.