Download Laws of Motion

Chapter 4
Forces and the Laws of Motion
As we learned, Galileo’s use of the concepts of displacement,
velocity, and acceleration accurately described motion. At the
same time Johannes Kepler successfully described the motion of
planets. But some questions remained:
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What causes motion?
What causes a change of motion?
Why do feathers slow down on the way down
How do birds fly?
Why do planets orbit the Sun in ellipses?
The turning point in the development of physics was the
Principia (1683) by Isaac Newton, arguably the most influential
single publication outside of theology.
Newton realized there where two other variables in nature aside
from those described by Galileo, namely:
Force = A push or a pull, which may change an object’s motion
Mass = Amount of matter in an object. One of its properties is
to resist a change in motion.
We can use an object’s resistance to gravitational motion to
measure it mass, its called weight. Mass is related to weight.
With these two concepts, Newton’s introduced three laws in
nature (although the answer of why we have them and how they
worked remained):
Newton’s Three Laws of Motion
1) An object in motion tends to remain in motion, an object at
rest tends to remain at rest, unless acted by an external
force
2) Fnet = ma
The net force equals mass times acceleration. Note that
force and acceleration are vectors (have direction), mass is
a scalar. The net force is the vector sum of all forces acting
on an object.
The dimensions of force are mass*length/time2
1 Newton  1 kg*m/sec2
3) For every action there is an equal an opposite reaction
Newton’s 1st Law:
 The natural state of an object is to keep its motion
 A change of motion occurs only if there is net force
Inertia = the tendency of an object not to change its motion
Mass is quantitative measure of an object’s inertia
Newton’s 1st Law is also known as the law of inertia. It was
originally suggested by Galileo.
Newton’s first law is not intuitive because of friction. Friction is
a force that opposes motion due to “rubbing” a surface or
“pushing” molecules. In the absence of friction, objects would
keep their velocities forever!
Part of Newton’s genious was to recognize that friction is just
another force among many.
We’ll keep learning more about friction later.
Newton’s 2nd Law:
 Fnet = ma
 This equation tells a story, it is an expression of cause and
effect
 It is not a definition, a model, or a good idea, it is a
fundamental law of nature!
This is the formula that determines most of classical physics, it’s
both simple and deep. The “big deal” is that F=ma predicts and
explains not just all motion, but the internal forces on all objects.
It takes practice to look at a situation and determine all of the
forces at play – practice is the key!
Newton’s 3th Law:
 Forces always exists in pairs
 The pairs of forces act on different objects
This is perhaps the hardest law to understand and make sense of.
The key is that force pairs act on different objects.
Since the forces are equal, do they exert the same action? Do
equal forces have the same effect on all bodies? No! Hence we
can push on things
Summary:
 Force
 Net force
 Mass
 Inertia
 Friction
 Newton’s three laws of motion