Download What is a Force?

Isaac Newton
(1642 – 1727)
Newton’s
Laws of
Motion
What is a Force?
Force can be defined as a
or a
or anything else that has the ability to change motion.
In SI units, a
force is
measured in
newtons (N).
One newton is
about the
amount of force
it takes to lift a
quarter-pound
cheeseburger.
How do you figure force?
F=m•a
Force = mass x acceleration
Newton (N)
(kg)
(m/s2 )
Easier than saying kilogram  m/sec2
To Understand Force…
You must understand the difference between
AND
2 Kg Mass
What is Mass?
The amount of “matter” in an object.
1 Kg
Mass
1.5 Kg
Mass
Mass is measured in kilograms.
2 Kg
Mass
How is Mass Different Than Weight?
Weight is mass times the force of gravity
Weight is a measurement of FORCE.
9.8 N
(1.5 kg x 9.8
Acceleration
due to gravity
(9.8 m/s2)
1.0 Kg
Mass
19.7 N
14.8 N
(1 kg x 9.86 m/s2)
F=ma
(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
How can you measure force?
Calculate mass x acceleration.
8
100 Kg
Mass is
220 lbs
Being pulled by different gravity?
Your weight may be different.
However, your mass never changes.
On the moon 220 lbs. would weigh 35 lbs.
On the sun 220 lbs. would weigh 5,956 lbs.
On Mars 220 lbs. would weigh 82.9 lbs.
What’s “Your Weight On Other Worlds?”
http://www.exploratorium.edu/ronh/weight/
Isaac Newton is the first scientist responsible for
formulating laws regarding motion.
These laws explain why objects move (or don't move).
Let’s look at Newton's three laws of motion...
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 force there is an equal and
opposite reaction force.
Newton’s First Law: Balanced Forces
An object will maintain a constant state of motion.
(Speed and direction remain unchanged)
This means an object at rest tends to stay at rest
and
an object in motion continues in the same direction with the same speed.
Balanced Forces
This is known as
Stay at rest:
a book on a table
Science
Inertia
Table pushes
up on book
Gravity pulls
down on book
Stay in motion:
a bowling ball tossed
in space
It will go on
and on forever.
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.
Force of friction
stops the book
Science
In space there is no
resistance to cause friction,
so a bowling ball would stay
in motion…
unless another
object got in
the way
What causes friction?
There are two factors which affect
friction between two surfaces:
– Kind of surfaces in contact (rough or
smooth)
– Amount of force pressing the surfaces
together
The rougher the surface and the
stronger the force between the
surfaces, the greater the amount of
friction.
Of course “frictional”
force happens on earth...
but, if you weren’t wearing a safety belt
you would continue your motion…
Don’t let this happen to you! BUCKLE UP!!!!
There are 4 types of
friction
1)
Static friction:
friction
between two
surfaces which
are not moving
past each
other
Example: trying to move a
box and it will not move
There are 4 types of
friction
2)
Sliding friction:
occurs when a
force is great
enough to
overcome the
static friction
Example: two people create
enough force to make the box
move
There are 4 types of
friction
3)
Rolling friction:
the friction
which enables
wheels to turn
and objects to
move
There are 4 types of
friction
4)
Fluid friction:
occurs when
an object
moves through
a gas or liquid
Newton’s Second Law:
Force = Mass X Acceleration
Force and acceleration are related.
120 kg Lineman
F=
ma
100 kg Center
a
F= m
Scenario # 1
Look at the situation above. If the
acceleration 3 m/s2 from the snap
was equal between these players,
then who would have the
advantage of force?
Scenario # 2
Now, let’s say the Center has a
1 m/s2 advantage over his
opponent at an acceleration of
4 m/s2 from the snap. Now who has
the advantage of force?
Lineman = 3 m/s2 x 120 kg is 360 N
Lineman = 3 m/s2 x 120 kg is 360 N
Center = 3 m/s2 x 100 kg is 300 N
Center = 4 m/s2 x 100 kg is 400 N
The Lineman has the advantage.
Now, the Center has the advantage.
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.
Consider the problem…
“How much force is needed to accelerate a 1400 kg car 2 m/s2? ”
Step 1
Read the problem.
Draw a picture.
1400 kg
car
2 m/s2
F=
m=
a=
Formula
Plug-in
Units, units, units!
Answer
“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
How much force?
2 m/s2
F=
m =1400 kg
a = 2 m/s2
Formula
Plug-in
Units, units, units!
Answer
“How much force is needed to accelerate a 1400 kg car 2 m/s2? ”
Step 3
Set up the formula.
F=mxa
F=
m =1400 kg
a = 2 m/s2
Formula
mxa
Plug-in
Answer
“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 or N for
newtons…it’s easier to
write!
F=
m =1400 kg
a = 2 m/s2
Formula
F = ma
Plug-in
1400 kg x 2 m/s2
Units, units, units!
Answer
2800 kg x m/s2
HELPFUL HINTS:
a=F/m
F=mxa
m= F/a
F
M
A
There are 2 kinds of
forces
1) Normal force:
the force a
surface
exerts on an
object that is
pressing on it
There are 2 kinds of
forces
2) Net force:
the sum of
two or more
forces on an
object
Newton’s Third Law:
Action Forces = Reaction Forces
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.
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.
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.
Gravity is the force that ALL objects in the
UNIVERSE exert between 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 objects have
more gravitational force
between them than the
green objects. This is
because the blue objects
have more MASS.