Newton's First Law of Motion, or the Principle of Inertia, states that any body will maintain the state of rest or Uniform Rectilinear Motion (URM) in which it finds itself, unless a force is applied to it.
This statement means that a marble moving linearly on the ground will continue to roll in a straight line ad infinitum unless a force acts on it. Also, a marble at rest will not move until a force acts on it.
Of course, in the real world, the moving marble will eventually stop due to the force of friction between the marble and the ground, a force that opposes the marble's movement. Without this force, the marble would never stop its Uniform Rectilinear Motion. The same marble launched into space, without air resistance nor friction, would maintain its motion ad infinitum.
To maintain its immobility or its constant velocity, an object may be subjected to several forces. However, to maintain its state of motion, the sum of all the forces acting on it must be equal to zero. This idea is described by the principle of inertia.
However, if the sum of the forces is not zero, the net force will cause the object to accelerate. The object will therefore not move at constant velocity.
Inertia is the natural tendency of a body to maintain its state of rest or Uniform Rectilinear Motion.
Inertia depends on mass. Since a heavier object will have greater inertia than a lighter one, greater force must be exerted to overcome the object's inertia.
A truck driver attaches a pendant to the rear-view mirror of his stationary truck. How does the pendant react when the truck accelerates forward?
From the trucker's point of view, the pendant moves backward as the truck accelerates forward, but this is only true from a person inside the truck. In reality, the pendant tends to retain its initial resting state as the truck moves forward. As a result, the truck driver will notice that the pendant is moving backward, whereas it simply tends to remain stationary as the truck moves forward.
A car is rolling at high velocity in a straight line on the freeway until the driver spots a small animal on the road. He then swerves the steering wheel to the left. How will the driver react to this change of direction?
Although the body is offset to the car's right, this representation is not quite exact. In fact, the driver's body will tend to keep its constant velocity in a straight line as the car abruptly changes direction by turning left. Since the body keeps its initial motion as the car moves to the left, the body will seem to crash into the right-hand side of the car, when in fact it is simply maintaining its Uniform Rectilinear Motion.