Download What is a Force?

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Isaac Newton
(1642 – 1727)
Newton’s Laws
The Father of Force
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What is a Force?
Force can be defined as a push or a pull…
or anything else that has the ability to change motion.
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Formula
Force = mass X acceleration
F = ma
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Force:
kilogram x m/sec2
kg x
2
m/s
Easier than saying kilogram  m/sec2
N
Is even easier!
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To Understand Force…
You must understand the difference between
AND
2 Kg Mass
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What is Mass?
The amount of “stuff” in an object
1 Kg
Mass
1.5 Kg
Mass
Mass is measured in kilograms.
2 Kg
Mass
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How is Mass Different Than Weight?
Weight is measurement of FORCE.
2.2 pounds
3.3 pounds
9.8 N
14.8 N
(1 kg x 9.86 m/s2)
(1.5 kg x 9.8
Acceleration
due to gravity
(9.8 m/s2)
1.0 Kg
Mass
4.4 pounds
19.7 N
(2 kg x 9.86 m/s2)
6m/s2)
Acceleration
due to gravity
(9.8 m/s2)
Acceleration due
to gravity
(9.8 m/s2)
1.5 Kg
Mass
2.0 Kg
Mass
DO NOT USE kilograms (kg) as a measurement for weight (force).
USE pounds (lb) or N for force.
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WEIGHT
WEIGHT = mass X acceleration
due to gravity
W = mg
W = ma
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Weight = Force
W = mg
kilogram x m/sec2
kg x m/s2 = N
Easier than saying kilogram  m/sec2
N
Is even easier!
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1 Kg
Mass
Different Gravity?
Mass is the Same:1 kg
But the WEIGHT…
2.2 Pounds
On the moon 1.0 kg would weigh 0.3 lbs.
Acceleration due to gravity 1.6 m/s2
On the sun 1.0 kg would weigh 59.5 lbs.
On Mars 1.0 kg would weigh 0.8 lbs.
What’s “Your Weight On Other Worlds?”
http://www.exploratorium.edu/ronh/weight/
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The Story of Newton’s Apple
Newton sat under an apple tree, and an apple fell on his head.
That falling apple gave him a revelation
and led him to describe a force called
GRAVITY.
But DON’T think of gravity as falling…
It is the pull (force) that
an object has on another object.
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Newton’s apple was falling because the
massive gravitational force of the Earth was
pulling the apple towards Earth...
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Gravity is the force that ALL objects in the
UNIVERSE exert on each other...
Newton said that gravitational force
depends on 2 things:
The DISTANCE of the objects
The MASS of the objects
LESS gravitational
force compared to...
MORE gravitational
force here.
The blue object has more
gravitational force than the
green object because the
blue object has more
MASS.
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Newton’s apple fell because the Earth’s pull of
gravity....
The itty-bitty pull of apple is too
small to pull the Earth, so it is
pulled to the Earth...
was more massive
than the apple’s pull
of gravity on the
Earth
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The moon is a
satellite too..
The pull of gravity keeps
satellites orbiting around the
Earth
Satellites are sent away and placed in the
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exact spot where the Earth’s gravity pulls
them into a “continuous free fall” or orbit...
The Earth is round and
the satellite continues
to “just miss” entering
into the Earth’s
atmosphere.
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The moon does
the same thing.
17 Does the moon’s gravitational pull affect 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.
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Newton is even credited with inventing
CALCULUS.
Needless to say, Newton was very
important in shaping our
understanding of science.
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These laws explain why objects move (or don't move).
Let’s look at Newton's three laws of motion...
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An object will remain at rest unless acted upon by
an “unbalanced” force. An object in motion will
continue with constant speed and direction, unless
acted on by an unbalanced force.
This law shows how force, mass and acceleration
are related as shown in the equation below:
Force = mass x acceleration
For every action there is an equal and opposite
reaction.
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Newton’s First Law: Balanced Forces
An object will maintain a constant state of motion (balanced).
This means an object at rest tends to stay at rest
and
an object in motion tends to stay in motion.
Forces that are balanced can be:
Not in motion
Stay at rest:
a book on a table
IP&C
This is known as
Inertia
Table pushes
up on book
Gravity pulls
down on book
In Motion
Stay in motion:
a bowling ball tossed
in space
It will go on
and on forever.
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Newton’s First Law: Unbalanced Forces
An unbalanced force is a force that changes the motion.
The book below slides and
then stops because of a
resistant force called friction.
Friction is force that
opposes motion.
In space there is no
resistance to cause friction,
so a bowling ball would stay
in motion
...unless another
It kind of
object got in
reminds me of
the way…
a few movies
I’ve seen
lately...
Force of friction
stops the book
IP&C
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Of course “frictional”
force happens on earth...
but, if you weren’t wearing a safety belt
you would continue your motion…
Do not let this happen to you; buckle up!
Newton’s Second Law:
Force = Mass X Acceleration
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Force and acceleration are related.
You are going down the road on your inline skates. Suddenly you look to
your right and see a car accelerating fast coming right for you! You look
for an escape to your left… Oh no! There’s a rock at the edge of neatly
manicured lawn. Which will you choose...
A massive car accelerating fast?
...or a medium size rock not in motion?
Good
Choice
Force (car) = mass x acceleration
is greater than
Force (rock) = mass x acceleration
You still might get hurt, but would be much worse off
choosing the force of the car over the force of the rock.
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Solving Force Problems
Step 1
Read the problem.
Draw a picture.
Step 2
Write down what you know,
What are you trying to find?
Step 3
Set up the formula.
Step 4
Plug-in the numbers.
Solve.
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Consider the problem…
“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”
Step 1
1400-kg
car
Read the problem.
Draw a picture.
How much force?
2 m/s2
F=
m=
a=
Formula
Plug-in
Units, units, units!
Answer
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“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”
Step 2
Write down what you know,
What are you trying to find?
1400-kg
car
2 m/s2
F=
m =1400 kg
a = 2 m/s2
Formula
Plug-in
Units, units, units!
Answer
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“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”
Step 3
Set up the formula.
Solve.
F=mxa
F=
m =1400 kg
a = 2 m/s2
Formula
Plug-in
mxa
Units, units, units!
Answer
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“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”
Step 4
Plug-in the numbers.
Solve.
2800kg x m/s2
F=
m =1400 kg
a = 2 m/s2
Formula
mxa
Plug-in
1400 kg x 2 m/s2
Units, units, units!
Answer
2800 kg x m/s2
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Be sure to do the problems.
• Helpful Hints:
a=F/m
F=mxa
m=F/a
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The unit for force:
1 kg x
2
m/s
= 1 newton or 1N
Which one would you rather write for your answer?
2800 N
…I thought so.
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Newton’s Third Law:
Action / Reaction
For every action there is an equal and opposite reaction.
Look at the picture below:
What is the action?
As the gases push
downward out of the rocket,
What is the reaction?
The rocket is pushed upward
by gases.
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An object will remain at rest unless acted upon by
an “unbalanced” force. An object in motion will
continue with constant speed and direction, unless
acted on by an unbalanced force.
This law shows how force, mass and acceleration
are related as shown in the equation below:
Force = mass x acceleration
For every action there is an equal and
opposite reaction.
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Newton’s laws explain how earthquakes
create such devastation.
Most of the time the Earth’s massive plates push on
each to move very slowly.
But, sometimes a massive plate exerts a greater amount
of force that accelerates another plate.
This results in an earthquake.
To learn more go to:
http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeIntro.html