Engineering Mechanics | The three laws of motion
The Three Laws of Motion
The basis of all solutions to mechanics problems are the Newton’s laws of motion in one form or the other. The laws are:
First law: A body does not change its state of motion unless acted upon by a force. This law is based on observations but in addition it also defines an inertial frame . By definition an inertial frame is that in which a body does not change its state of motion unless acted upon by a force. For example to a very good approximation a frame fixed in a room is an inertial frame for motion of balls/ objects in that room. On the other hand if you are sitting in a train that is accelerating, you will see that objects outside are changing their speed without any apparent force. Then the motion of objects outside is changing without any force. The train is a non-inertial frame.