0.16 Phy1140: force and motion -- introduction  (Page 3/4)

This property of matter not to change its state of rest or of uniform motion at a constant velocity is called inertia .

Force is required to cause a change

Whenever a body changes from a state of rest to a state of motion, or changes its motion from one velocity to a different velocity, a force is required tocause that change.

Newton's contribution

Newton continued Galileo's study of motion and formulated his findings in three laws of motion. The first two laws were formal statements of Galileo'searlier conclusions. The third law formulated a finding that was original to Newton.

Three laws of motion

Unlike those before them, Galileo and Newton saw that in the absence of friction, force was required to change motion, not to maintain it.

The Principia

As mentioned earlier, Newton formulated the earlier findings of Galileo in two laws of motionand added a third law based on his own findings. He published those laws in a document which, when translated from the original Latin, was titled something like: "Mathematical Principles of Natural Philosophy." This document is oftenreferred to simply as the Principia.

The three laws are paraphrased in the following sections.

Law 1

Every body remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by external force actingupon it.

Another interpretation of this law reads as follows:

Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. (This suggests that a state of rest is motion with zero velocity.)

This law is generally regarded as recognition of Galileo's concept of inertia and is often referred to as the "Law of Inertia."

The law explains what force does, but does not suggest how force should be measured.

Law 2

Rate of change of momentum is proportional to the impressed force and takes place in the direction of that force.

Another interpretation of the second law is:

The relationship between an object's mass m, its acceleration a, and the applied force F is F = m*a (where the * indicates multiplication) . Acceleration and force are vectors; in this law the direction of the force vector is the same as the direction of the acceleration vector.

This law, in conjunction with the third law, allows quantitative calculations of dynamics and a definition of mass. In particular, how do velocities change when forces are applied.

The relationships among momentum, force, mass, and accelerations will be explained later.

Law 3

Action and reaction are equal and opposite, and act on different bodies.

Another interpretation of this law is:

For every action there is an equal and opposite reaction.

This law means that forces act in equal and opposite pairs. Whenever two bodies act upon one another, equal and opposite changes of momentum occur in thetwo bodies.

An example of the effect of this law occurs when a person attempts to jump from a small boat to a pier. If the boat is free to move, it will tend to moveaway from the pier as the person attempts to launch himself in the direction of the pier. This may leave the person momentarily hanging in the air between theboat and the pier waiting to experience the acceleration of gravity and fall into the water.