MORE PRECISELY 2-2 Newton's Laws of Motion and Gravitation
  1. Every body continues in a state of rest or in a state of uniform motion in a straight line unless it is compelled to change that state by a force acting on it.

It requires no force to maintain motion in a straight line with constant velocity—that is, motion with constant speed and constant direction in space. The tendency of a body to remain in a state of uniform motion is usually called inertia. When velocity does vary (the speed increases or decreases, or the direction of motion changes), its rate of change is called acceleration. The relation of acceleration to any forces acting on a body is the subject of the second law of motion:

  1. When a force F acts on a body of mass m, it produces in it an acceleration a equal to the force divided by the mass. Thus, a = F/m, or F = ma.

In honor of Newton, the SI unit of force is named after him. By definition, 1 newton (N) is the force required to cause a mass of 1 kilogram to accelerate at a rate of 1 meter per second every second. Newton's third law relates the forces acting between separate bodies:

  1. To every action there is an equal and opposite reaction.
This law means, for example, that you attract Earth with exactly the same force as it attracts you (a force known as your weight). This attraction is governed by one final law:

The Law of Universal Gravitation
("Newton's Law of Gravity")

Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them. In other words, two bodies of masses m1 and m2, separated by a distance r, attract each other with a force F that is proportional to (m1 m2)/r2. The constant of proportionality is known as the gravitational constant, or often simply as Newton's constant, and is always denoted by the letter G. We can then express the law of gravity as

The value of G has been measured in extremely delicate laboratory experiments. In SI units, its value is 6.67 10-11 newton meter2/kilogram2 (N m5%*  úøý=;%ü¬ûÿ=