Download Forces and Motion

Forces (F)
 Definition:
a push or pull that has
the ability to cause a change in
motion
Standard
unit is the Newton
Where did this unit come
from??
Sir Isaac Newton
 English
physicist and
mathematician
 Laws of Motion and Gravitation
 Invented Calculus
All before the age of 30!
Types of Forces
Forces cause motion
Motion depends on net force
NET means “total” or “sum”
Forces can be
balanced or
unbalanced
Balanced Forces
Equal in size
Opposite in direction
No motion occurs!
(equilibrium)
Like a tie in an
arm wrestling
match!
p. 51
Unbalanced Forces
Unequal in size
Can be in the same direction
OR in opposite directions
Forces must be unbalanced
to cause a change in motion
As a box is sliding down a ramp, a 10 N force
of gravity is pulling the box down the ramp
while a 2 N force of friction acts to stop the
box.
What is the net
(total) force
accelerating the
box
down the ramp?
(Magnitude and
Direction!!)
8 N to the Right
What is the Net Force???
1N
3N
1 N 1N
1 N 1N
Fnet = 1N – 3 N
Fnet
= -2N
= 1N +1 N
Fnet
= +2N
= -1N – 1 N
= -2N
Friction
a force that goes against motion
Four Types of Friction:
1. Air friction (Resistance)
2. Viscous friction
3. Sliding friction
4. Rolling friction
1. Air Friction
Also called Air resistance
(aerodynamics reduce
the air friction)
Ex - falling objects or airplanes
2. Viscous Friction
Moving through
a liquid, or fluid
Ex. - swimming
3. Sliding Friction
Two surfaces rub
against one another
Ex - rubbing hands or clapping
Oil and other lubricants help
reduce friction.
4. Rolling Friction
An object rolls over a surface
Ex – any object with wheels
A_World_Without_Friction.asf
Gravity
Def. - A
downward force
that causes objects
to accelerate to the
center of mass
Gravity causes all falling
objects to accelerate at the
same rate. (9.8 m/s2)
Free
Fall Tower Gizmo
 Apollo
15 Hammer
and Feather Drop
 Mythbusters
Recreates Apollo 15
Drop
Mass (g)
Amount of matter
inside of an object
Is NOT
Weight
(N)
A measure of the force
created as your mass
responds to gravity
Calculating Weight
Formula:
Force of
Weight
(N)
Mass (Kg)
g
m
=
Fw
Gravity =
9.8 m/s2
p. 53
Let’s look at how mass,
weight, and gravity are related.
Mass DOES NOT change, but
your weight will change if
there is a change in gravity
 What
would
happen to your
weight if you
were on the
Moon?
 What would
happen to your
mass if you were
on the Moon?
Weight
and Mass Gizmo
Law of Universal
Gravitation
Newton said that gravitational
force depends on 2 things:
The DISTANCE from the center
of gravity
LESS gravitational force
MORE gravitational force
The MASS of the objects
The yellow object has
more gravitational force than
the red object because the
yellow object has more
MASS.
The pull of gravity
also keeps
satellites orbiting
around the Earth
The mass of
the moon is
much
smaller than
the mass of
the Earth, so
the Moon is
controlled
by the
gravitational
pull of the
Earth.
Does the moon’s gravitational pull effect the Earth?
Yes, the moon’s
gravity is large
enough to actually
pull water out of
the ocean from
space…
Well, not quite
that much...just
enough to create
motion of the
tides.
 As
you move further from the center of the
Earth, the force of gravity DECREASES.
 Where would you weigh less, in Galveston
or in Denver?
 Denver is the “Mile High City”; the force
of gravity is less. Therefore, the force of
gravity would be less because it is further
away from the center of the Earth, and
you would weigh slightly less.
Gravity Song!
Newton’s First Law (Law of Inertia)
 An object at rest stays at rest
and an object in motion keeps
its same motion, UNLESS an
outside force acts on it.
Teenage Terms – Objects will keep
doing what they are doing unless an
outside force changes that!
Inertia
Def.
- The tendency of
an object to resist any
change in motion
 You
must overcome inertia to
start or stop motion
 Depends on the mass of the
object
As mass , inertia will
Which object has the
greatest amount of
INERTIA??
Pencil
Baseball
Bowling Ball
Ping Pong Ball
INERTIA
is what causes a person (or
anything else in the) involved in a head-on
collision to continue moving forward at the
same speed that the car was traveling, until
some outside force causes it to stop (i.e.seatbelt, windshield, etc.)
What’s wrong with this picture??
Inertia
http://www
g
http://www
contained
Other Examples of 1st Law
 Card
Tricks
 Dishes and table cloth
 Collisions and seatbelts
Crash Test Dummies
 Bike Ramp Crash
 First Law, Inertia, and
Friction Video
Newton’s
nd
2
Law (F = ma)
 The
acceleration of an object is
directly proportional to the force
acting on it and inversely
proportional to its mass.
F,
a (direct relationship)
m,
a (inverse relationship)
Force and Acceleration are
Directly Proportional
F
a
2N
F=ma 2N =(1kg)a
a= 2 m/s2
4N
F=ma
4N =(1kg)a
a= 4 m/s2
Mass and Acceleration are
Inversely Proportional
m,
a (inverse relationship)
F = ma
1 N = (1kg) a
a = 1m/s2
F= ma
1 N = (2kg) a
a = 0.5 m/s2
Solving
nd
2
Law Problems
Use the formula:
Unit for Force is
Newtons (N)
Unit for Mass
is kilograms
(kg)
Unit for
Acceleration is
m/s2
Solving 2nd Law Problems
What force is required to give a 5 kg box
an acceleration of 2 m/s2 ?
Variables Formula Plug-n chug Answer
F= ?
m=5 kg
a= 2m/s2
Solving
nd
2
Law Problems
What force is required to give a 5 kg box
an acceleration of 2 m/s2 ?
Variables Formula Plug-n chug Answer
F= ?
F = ma
m=5 kg
a= 2m/s2
Solving
nd
2
Law Problems
What force is required to give a 5 kg box
an acceleration of 2 m/s2 ?
Variables Formula Plug-n chug Answer
F= ?
F = ma
m=5 kg
a= 2m/s2
F =(5kg)(2m/s2)
Solving
nd
2
Law Problems
What force is required to give a 5 kg box
an acceleration of 2 m/s2 ?
Variables Formula Plug-n chug Answer
F= ?
F = ma
m=5 kg
a= 2m/s2
F =(5kg)(2m/s2)
F= 10N
Solving 2nd Law Problems
 Explains
why you can throw a
baseball further than a bowling ball
 Braking force required to stop a
Volkswagen vs. a dump truck
 Why a Mustang accelerates faster
than a bus
What is the acceleration of a 500
kg ball rolled with a force of 3.6N?
2. A rollercoaster accelerates at a rate
of 30 m/s2. If the rollercoaster
has a mass of 10,345 kg, what size
force must be applied?
3. What is the mass of a tanker truck
that accelerates at 15 m/s2 with a
force of 2500 N?
1.
rd
3
Newton’s
Law
(Action/Reaction)
 Whenever
one object exerts a
force on another (action), the
second object exerts an equal
and opposite force (reaction) on
the first.
 All forces come in pairs.
Newton’s Third Law:
For every action there is an equal
and opposite reaction
Look at the picture below:
What is the action?
The gases push downward
out of the rocket
What is the reaction?
The the rocket is pushed
upward by the gases
Examples of 3rd Law
 Leaning
on wall (action: you push on wall,
reaction: wall pushes back on you)
 Firing a cannon (action: cannon pushes on
cannon ball, reaction: cannon ball pushes
back on cannon)
 Walking (action: you push on the earth,
reaction: the earth pushes back on you)
 Space Shuttle taking off (action: rocket
thrusters push down, reaction: the space
shuttle travels upward)
America’s Funniest Videos
 Dorm
Chair Hit
(Newton’s Third Law & Conservation
of Momentum)
 http://abc.go.com/primetime/afv/ind
ex?pn=videoplayer&id=363529
An object will maintain a constant
state of motion until acted upon by
an “unbalanced” force.
This law shows how force, mass and
acceleration are related.
Force = mass x acceleration
For every action there is an equal
and opposite reaction
Calculating Momentum
•Unit: kg m/s
Formula:
Momentum
(Kg-m/s)
Mass (Kg)
p= mv
Velocity
(m/s)
Will
two moving objects
with the same mass always
have the same momentum??
MASS =
100 kg
V = 1 m/s
(About
220 lbs)
V = 3 m/s
NO!! Their velocities
need to be considered!!
3
Examples of
Momentum
Momentum of a Marble
in a Bowl
Law of Conservation of Momentum
Total amount of momentum
can not change
What is the momentum of
a blackbird with a mass of
0.04 kg flying at 19 m/s?
A meteorite has a momentum of
100 kg-m/s. If the velocity is
1000 m/s, what is its mass?
A frog with a mass of 0.9 kg hops
with a momentum of 10.8 kgm/s. What is its speed?