Physics… Force & Motion
FORCE…
 “Any
influence that can cause a
body to be accelerated. It is
measured in NEWTONS.”
A Newton is the force needed to
accelerate one kg one meter per
second per second
 A push or pull exerted by an object
to/on another object
FORCE cont…
 Always occurs in pairs (Sometimes
called Agent & Receiver)
 It can alter the shape or motion of
an object
It is represented by arrows that show
direction and size
Demos
 Rolling Ball
Shows Acceleration
 Push – Pull; Pushing a closed door;
standing on the floor
All show that forces occur in pairs
 Clay ball
Shows how force is required to
change shape
Tennis ball
Change of shape- Elasticity
 Pushing on a table
Forces occur in pairs; Net Force
• Tennis Ball
• http://www.youtube.com/watch?v=sxSm9CRu34
• Rubber Ball
• http://www.youtube.com/watch?v=N8gxND
1kX6k
Paired Verbal Fluency
•Read the biography of Sir
Isaac Newton
•Find a Partner
•Decide who will go first
Isaac Newton
b: 25 Dec 1642 d: 31 March 1727
Principia Mathematica
Philosophiae Naturalis
Newton’s Three Laws of Motion
Law #1:
 “Every body continues in its state of
rest, or of uniform motion in a
straight line, unless it is compelled to
change that state by forces impressed
upon it.”
 Sometimes called the Law of Inertia
What is Inertia?
 “The sluggishness or apparent resistance
a body offers to changes in its state of
motion”
 The larger the mass, the larger its inertia
 A body with less mass is easier to
accelerate
 A body with more mass is harder to
accelerate
Demo
• Tennis Ball and Cup
Tennis ball has inertia; Newton’s First
Law
• http://www.teachersdomain.org/resource/ph
y03.sci.phys.mfw.galileoplane/
• http://www.youtube.com/watch?v=WOvww
O-l4ps
Demos:
Ping pong ball
Small inertia; easy to move & stop
Bowling ball
Larger inertia; hard to move & stop
Granny
Small inertia; easy to move & stop
Mack Truck driven by Granny 
Larger inertia; hard to move & stop
Mass
 Defined as “The quantity of matter in a
body”
 It is the measure of Inertia
m = mass; F = force; a = acceleration
m = F/a
F = m(a)
a = F/m
Momentum
 The product of MASS and
VELOCITY
 It is inertia in motion
 Are you afraid of a piece of grass?
Straw (grass) Through Wood
Small mass, great acceleration
Demos:
One Ounce Projectile
A small mass accelerated at high velocity can
be produce a lot of force
Newton’s Jar
Momentum
Car on a ramp
Momentum
 Eeyore on a car on a ramp
Momentum; inertia in motion
Conservation of Momentum
 All energy stored in a moving object
is not lost, but only changes form
This is just like “Energy cannot be
created nor can it be destroyed, it can
only change form” (Stated by: Albert
Einstein; Phillip Morrison, Mr. Watts…)
Demos:
Newton’s Cradle
Conservation of Momentum
http://www.youtube.com/watch?v=7_AiV12XBbI&saf
ety_mode=true&persist_safety_mode=1&safe=active
Swinging Rock
Potential and Kinetic energies;
Conservation of Momentum
Mass in a Circular Motion
 When object moves in circular path
it has a property called Rotational
Inertia
 AKA: Gyroscopic Inertia
Circular Motion con’t…
 “An object rotating about an axis tends
to remain rotating about that same axis
unless it is interfered with by some
external force.”
 This definition is similar to Newton’s
First Law of Motion
Rotational Inertia Demos:
 Bicycle tire
 Gyroscope
 Top
 Frisbee
 Football
 Moment of Inertia
 X-Zylo
Angular Momentum
A measure of an object’s rotation about a
particular axis
For an object small compared to the
radial distance, it is the product of mass,
velocity and radius
It is the measure of the rotational
property of motion
It is always perpendicular to the
centripetal force
Angular Momentum
Demos:
 Rolling objects
 Washer on a string
 Slingshot (old-fashioned!)
 Bucket with water spun in vertical circles
 Q: What holds the water in the bucket?
 Spin cycle of washing machine
“Whizzer”
 Brendan on the Scrambler
Carnival Ride Know-How
 It’s better to take the OUTSIDE seat of
the car if you’re riding alone
 If you’re riding with someone else, take
the INSIDE seat
 The following formula is similar to
those for Newton’s First Law, but
deals with mass in motion in a
circular path:
r
v
mv = m r
… The mass is not affected by a smaller
radius, while velocity is…
What happens in this situation?
m
vr
= ?vr
Demo
 Spinning on a chair with
weights
Arms Out?
Pulling arms in?
Centripetal Force
 A center-seeking force that causes an
object to follow a circular path.
 “Any force that is directed at right
angles to the path of the moving body
and produces circular motion.”
Centrifugal Force
 A fictitious outward force due to
rotation
 It is a reaction to centripetal force
but there is no agent, therefore it is
experienced relative to position
What we are really experiencing is
ANGULAR MOMENTUM!
Newton’s Second Law
Law #2
The acceleration of a body is
directly proportional to the net
force acting on the body and
inversely proportional to the mass
of the body and is in the direction
of the NET FORCE
2nd Law Formulas
 You’ve seen these before:
 a = F/m
 F = m (a)
 m = F/a
What is the acceleration of these?
Acceleration is Inversely Proportional to mass…
5N
Acceleration = 6 kph
5N
Acceleration = ?
5N
Acceleration = ?
Solutions
Acceleration is Inversely Proportional to mass…
5N
5N
Acceleration = 6 kph
Mass is doubled so
a/ 2 = 3 kph
Mass is tripled so
5N
a/3 = 2 kph
Acceleration is Directly Proportional to force…
5N
Acceleration = 6 kph
10 N =
Acceleration = 12 kph
(2 x f)
15 N =
(3 x f)
Acceleration = 18 kph
What if I want the Acceleration to be the
same for all these bodies?
Acceleration is Directly Proportional to force…
5N
Acceleration = 6 kph
10 N =
Acceleration = 6 kph
(2 x f)
15 N =
(3 x f)
Acceleration = 6 kph
Newton’s Third Law
3rd Law
“Whenever one body exerts a force on
a second body, the second body exerts
an equal and opposite force on the first”
 For every action, there is an equal and
opposite reaction
 You cannot touch without being
touched
Demo
• Syringe with water
• Rocket Balloon
• Beaker of Water on Balance
Common Forces
 Gravitational – non contact
Every object exerts an attractive force
on every other object
 Buoyancy - contact
The tendency to float in a fluid
 Friction - contact
The resistance to motion where two
objects touch
Common Forces
 Magnetic – non contact
The repelling or attraction of a magnet and
ferromagnetic material
 Elastic - contact
Ability to spring back to its original size,
shape, or position after being stretched,
squeezed, flexed, expanded, etc.
 Electrical – non contact
Flow or gathering of a negative charge by
electrons
Other Physics concepts of interest:
 ENERGY
The quality of an object that enables it to
do work
 Power
WORK/ TIME
Potential Energy
The stored energy that a body
possesses because of its
position with respect to other
bodies
Kinetic Energy
The energy of motion
Kinetic Energy = 1/2 mv2
As an object falls,
its energy
transforms
from
all Potential
to
all Kinetic
P
K
P
K
At which point
will the
Potential
and
Kinetic
be equal?
P
K
Right here! ½ way
P
K