Newton's first law of motion is also known as law of inertia. The law states that every body remains in its state of rest or uniform motion in the same direction except when it is made to change its state with an external force.
The first law establishes frames of reference that are called inertial frames. The external force acting on the body is the vector sum of all the different forces that are acting on the object. Thus, one has to find the vector sum of the forces to get the net force. If this sum is zero, the state of the object does not change according to Newton's first law of motion.
Hence, two conclusions can be drawn from Newton's first law of motion. First is that if the net force on a static object is zero, the will remain in its state of rest until an external net force acts on it to change the state. Hence a ball that is resting on the ground will remain in its place forever till an external force acts on it to change its position.
The second conclusion is that if a body is in motion, the state of its motion will change if a net force acts upon it. The result of the action of the force on the motion of the body will be either to increase (accelerating) or decrease (decelerating) the velocity of the body. You might be thinking how an object that is in motion keeps on moving forever. This is because we all have seen objects in motion stopping after some time if left to itself.
Let’s take an example. If a ball is thrown on the ground, we find the ball stopping after some time. The ball was in motion due to the action of the force on the ball. But when an object moves on any surface, an equal and opposite force acts on it. This opposite force is called Frictional Force. Thus, as the ball moves, an opposing frictional force acts on the ball against its motion, resulting in decreasing the motion of the ball.
It is difficult to demonstrate the first law as the result of frictional force can only be seen in the slowing of the ball. Similarly, in outer space, the gravitational force acts on objects whose results are seen. In spite of this, the law provides the elementary cause for the change of state of an object, be it in motion or at rest.
law of motion
