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Laws of Motion

Thus far we have studied the laws of motion, without asking, “What is it that causes a body to move?” We know that a force is needed in order to change the motion of a body, that is, in order to accelerate it or decelerate it, or to change its direction. In the absence of a force, a body will either remain at rest, or continue to move with constant speed in a straight line. These conclusions are summed up in three statements known as laws of motion.

The First Law. — The statement of the first law of motion is as follows: A body at rest remains at rest and a body in motion remains in motion at a constant speed in a straight line, unless acted upon by an external force.

Thus the first law involves the idea of motion and the idea of force. It explains what is to be understood by force: it is that which tends to change the state of rest of a body, or of uniform motion in a straight line. The first law, however, tells us more than this. It tells us that if a body is kept free from the action of forces, it will remain in its state of rest or of uniform motion in a straight line. Thus the normal state for a body to be in is one of rest or of uniform motion in a straight line, i. e. motion with uniform velocity; it is only the presence of force which can alter this normal state. The property by virtue of which a body tends to remain in either of the natural states, and to resist being accelerated, is called iner­tia. In this sense, inertia is an absolute quality possessed in equal degree by all bodies, because all bodies are completely inert.

The Second Law. — The second law deals with the change in motion of a body when force is applied to it.

This law is stated as follows:

Rate of change of motion of a body is proportional to the applied force and is in the direction in which the force acts.

The expression change of motion requires explanation. By motion is here meant quantity of motion or momentum. It is defined as the product of the velocity and a quantity called the mass of the body.

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Using the word momentum, the second law may also be stated as follows: Rate of change in momentum of a body is proportional to the applied force and is in the direction in which the force acts.

The Third Law may be stated as follows.

To every action there is always an equal and opposite reaction. It is a matter of common observation that a body A cannot exert force on a second body В without В at the same time exerting force on A. Thus all forces occur in pairs, which may conveniently be spo­ken of as action and reaction. The third law of motion tells us that the two forces which constitute such a pair are equal in magnitude and opposite in direction.

For example, when we stretch a rubber band, holding one end in each hand, you must pull as hard with your left hand as you do with your right [2, С. 57 - 58].



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