These notes are to supplement the reading of Chapter four pages 123-157 in the Holt Text Physics Text Book by Serway
and Faughn.
Additionally the following are helpful.
An explanation of Newton’s 1st law by JSM http://www.youtube.com/watch?v=BwkUNrSCNMg
- An explanation of Newton’s 1st law by JSM http://www.youtube.com/watch?v=Hyw9uNF4nmE
- An explanation of Newton’s 2st law by JSM http://www.youtube.com/watch?v=amNWC0Af8OQ
- An explanation of Newton’s 2st law by JSM http://www.youtube.com/watch?v=amNWC0Af8OQ
- An explanation of Newton’s 3st law by JSM http://www.youtube.com/watch?v=PZVhhB12QXI
- An explanation of Newton’s 3st law by JSM http://www.youtube.com/watch?v=X95Yk9y5Pqc
- An overview of Newton’s 3 laws http://www.youtube.com/watch?v=iH48Lc7wq0U
- A website with great info http://www.physicsclassroom.com/Class/newtlaws/
- 1st law info with graphic http://www.physicsclassroom.com/mmedia/newtlaws/cci.cfm
- allows you to practice free body diagrams http://www.physicsclassroom.com/shwave/fbd.cfm
-
VA SOL’s covered:
PH.3
PH.4
PH.5
The student will investigate and understand how to demonstrate scientific reasoning and logic. Key concepts
include
a) analysis of scientific sources to develop and refine research hypotheses;
b) analysis of how science explains and predicts relationships;
c) evaluation of evidence for scientific theories;
d) examination of how new discoveries result in modification of existing theories or establishment of new
paradigms; and
e) construction and defense of a scientific viewpoint (the nature of science).
The student will investigate and understand how applications of physics affect the world. Key concepts include
a) examples from the real world; and
The student will investigate and understand the interrelationships among mass, distance, force, and time through
mathematical and experimental processes. Key concepts include
d) Newton’s laws of motion;
Newton’s Laws and Forces
So Far
-
Aristotle (nat. state @ rest need force for constant v)
Galileo (once in motion will stay in motion)
Sir Isaac Newton
Born the year after Galileo died (1643 AD)
- Father died before he was born. Socially backward, couldn’t handle family farm. – tutor saw potential
- Goes to school – bubonic plague – goes home
- Wrote a article on light, wasn't well received
- Invented calc (Fluctuations) and laws of motion in “Principia”
- Did other work w/ gravity, religion, invented reflecting telescope.
- Became famous said “If I have seen further it is b/c I stand on the shoulder of giants.”
- Took over mint – improved efficiency- knighted
- Friend left – nervous breakdown
- 2yrs later he recovered, wrote “Optics”
3 Laws of Motion
- Newton’s 1st law : Objects at rest will remain at rest and objects in motion will remain in straight line motion
unless acted on by a net force.
 Inertia is the term for used to describe the first part of the 1st law “objects at rest will
remain at rest” and is directly related to mass. More mass = more inertia.
 An example of this is your head moving backwards when you accelerate in a car.
 Space probes are example of object in motion staying in motion, as they do not need to
fire their engines after they are far enough away from earth’s orbit.
nd
- Newton’s 2 law : F=ma
 After you add up all of the forces acting on an object it will equal the mass times the
acceleration of that object.
 If the forces are balanced (F =0 =ma) there will be no acceleration.
- Newton’s 3rd law: -F = F For every force there is an equal (in magnitude) and opposite (in direction) force.
 Because of this you can never punch a wall (or anything) harder than your fist gets hit. If
you hit a wall with 400N of force, the wall hits you with -400N of force.
 An example of this is the kick of a gun. The force on the bullet is the same force the gun
will kick back with. The higher caliber bullet will have a greater kick.
 Because –F=F then –ma=ma so what we find is the larger mass has less acceleration.
 This also means that when you jump, the force of weight the Earth exerts on you is
equal to the force you exert on the earth. Since the Earth has such a larger mass its
acceleration is very small.
What is a Force anyway?
A force is a push or a pull. The unit we use to measure this is the Newton “N” which is just an abbreviation
for a kgm/s2
Types of Forces
- Force of Friction (Ff)
o Is a force that resist motion as and creates heat.
o It is caused by small bumps on the surface of solids and static electricity.
o Two types of friction
 Static –is friction of a non moving object, and always larger than kinetic
 Kinetic – is friction of a moving object, and always smaller than static
o Formula is Ff=μFn where μ is the coefficient of friction (relates how much resistance there is
between two surfaces.)
Force of weight (Fw) also known as Force of gravity
o Force of Weight is the measure of the force of gravity pulling on the mass. It will change if gravity
changes. The SI Unit is the Newton (N) By the way 4.448N=1 lbs.
o Mass is a measure of the amount of matter in an object. It is not affected by gravity. The SI Unit: is
the Kg.
o Formula is Fw=mg
- Force Normal (FN)
o Is a force that acts perpendicular outward from a surface that something is being pushed into.
- Force applied (Fa)
o Is a generic term for any outside push or pull on an item.
Free Body Force Diagrams
With free body diagrams, force vectors are drawn out of objects in the direction of the force acting
on them.
-
Some Examples:
Example Problems:
A car (mass 500kg) skids to a stop over a distance of 20.0m. The car was initially moving at 25.0m/s, what force did the
tires produce to sop the car.
Ff=?
Vf2=vi2+2ad
∑F=ma
d=20m
a=( Vf2-vi2)/2d
Ff=(500kg)(-15.6m/s)
Vi=25m/s
a=-15.6m/s
Ff= -7810N
Vf=0m/s