Topic 2-03:Newton's First and Third Laws

Newton's First Law is derivable from the Second Law. It says that if there is no force applied on an object, the object either stays stationary or continues to move with a constant velocity. In the Second Law

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if the force f is set to zero, the differentiation of the velocity vector dv/dt becomes zero, so v is a constant. If the object is initially at rest, then its velocity v is zero and thus it is stationary continuously. If the object is initially moving with a constant velocity v, then it will continue to move with the constant velocity v.

The Third Law of Newton says that if a force f is applied to an object, there is always a counter force -f applied by the object to the source of the force f. In Fig.1 a person pushes an object with a force f, and the person is pushed back by the object with a force -f. In Fig. 2 an object of mass m is resting on the floor. The object has a weight mg, and is pushing the floor with a downward force of magnitude mg. The floor is pushing back the object with an upward force of magnitude mg. This upward force of mg from the floor cancels with its weight, a downward force of magnitude mg. Thus, the object remains stationary.