Isaac Newton, 1642-1727
Principia Mathematica, 1687
Newton, Philosophiae naturalis principia mathematica (1687) Translated by Andrew Motte (1729)
Lex. II. Mutationem motus proportionalem esse vi motrici impressae, & fieri secundum lineam rectam qua vis illa imprimitur.
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subtracted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both.
Lex. II. Mutationem motus proportionalem esse vi motrici impressae, & fieri secundum lineam rectam qua vis illa imprimitur.
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subtracted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both.
And now, in more contemporary language:
2. Newton's Second Law
An unbalanced force (F) causes an object to accelerate (a).
That means, if you apply a force to an object, and that force is unbalanced (greater than any resisting force), the object will accelerate.
Symbolically:
That's a linear relationship.
Greater F means greater a. However, if the force is constant, but the mass in increased, the resulting acceleration will be less:
a = F / m
That's an inverse relationship.
We have a NEW unit for force. Since force = mass x acceleration, the units are:
kg m / s^2
which we define as a newton (N). It's about 0.22 lb.
There is a special type of force that is important to mention now - the force due purely to gravity. It is called Weight. Since F = m a, and a is the acceleration due to gravity (or g):
W = m g
Note that this implies that: weight can change, depending on the value of the gravitational acceleration. That is, being near the surface of the Earth (where g is approximately 9.8 m/s/s) will give you a particular weight value, the one you are most used to. However, at higher altitudes, your weight will be slightly less. And on the Moon, where g is 1/6 that of the Earth's surface, your weight will be 1/6 that of Earth. For example, if you weight 180 pounds on Earth, you'll weight 30 pounds on the Moon!
2. Newton's Second Law
An unbalanced force (F) causes an object to accelerate (a).
That means, if you apply a force to an object, and that force is unbalanced (greater than any resisting force), the object will accelerate.
Symbolically:
F = m a
That's a linear relationship.
Greater F means greater a. However, if the force is constant, but the mass in increased, the resulting acceleration will be less:
a = F / m
That's an inverse relationship.
We have a NEW unit for force. Since force = mass x acceleration, the units are:
kg m / s^2
which we define as a newton (N). It's about 0.22 lb.
There is a special type of force that is important to mention now - the force due purely to gravity. It is called Weight. Since F = m a, and a is the acceleration due to gravity (or g):
W = m g
Note that this implies that: weight can change, depending on the value of the gravitational acceleration. That is, being near the surface of the Earth (where g is approximately 9.8 m/s/s) will give you a particular weight value, the one you are most used to. However, at higher altitudes, your weight will be slightly less. And on the Moon, where g is 1/6 that of the Earth's surface, your weight will be 1/6 that of Earth. For example, if you weight 180 pounds on Earth, you'll weight 30 pounds on the Moon!
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