Newton’s Laws and How They Apply to Astronomy
Written by: VinceSummers • Edited by: RC Davison
Updated Oct 2, 2011 • Related Guides: Sun | Comet [pic]
Even though deduced in the 17th century, Newton's first, second, and third laws of motion, plus his law of gravitation are extremely important to astronomers today in the research they conduct to understand the Universe.
Sir Isaac Newton, historically one of the most inspired of scientists, is famous for his three laws relating to force, mass, momentum, and acceleration. Although we’re living in times not entirely defined by classical mechanics, those laws still apply and are fundamental in the field of astronomy.
Newton's First, Second, and Third Laws of Motion
Newton’s First Law: This is called the “law of inertia.” A body remains in a constant state of motion or at rest unless acted upon by an external force. In other words, if all forces exerted on an object add up to zero, its state of motion will remain constant; its acceleration will be zero.
Newton’s Second Law: A force applied to an object produces a time rate of change of its linear momentum.
In mathematical jargon, the force is equal to the derivative of the linear momentum with respect to time.
F = d(mv)/dt
Since by classical mechanics mass is constant, for most purposes the expression may be simplified,
Newton’s Third Law: For every action, there is an equal and opposite reaction. This tells us that if a force, F, is applied to an object, then that object responds with an equal force of opposite direction, -F. Combined with the second law, its equivalent is,
m1a1 = -m2a2
As an example, a hunter pulls the trigger that fires a bullet from a gun. A reactive force is very real; the gun recoils. In the case of a rocket, the fuel ignites and pushes the rocket forward. The rocket pushes back and the exhausted products exit from the rear. In this instance, both the second and third laws obviously apply.
If an applied...