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Force and Motion
Mr. R. Gallagher
I.
Force vs. Motion
1. Force
- a push or a pull
- it is measured in the SI unit of Newton's “N”
- example – pushing a desk or pulling a sled
2. Motion
- a change in position with time
-example – the sled moves from the top of the
hill to the bottom
II.
What is a Newton?
*A Newton is what quantifies force
(gives something a value).
*One Newton is the amount of
force required to give a
1-kg mass an acceleration
of 1 m/s²
*10N means 10 Newtons of force.
III. Balance vs. Unbalanced
forces
• All objects have at least two forces acting on them,
sometimes more
• Ex. Holding a book in the air
– One force is gravity
– One force is your muscles holding the book up in the air
• When forces are equal from opposite directions there is
usually no movement and this is called balanced forces
• When one force is greater than another then the object
moves in the direction the greater force is going and this
is an unbalanced force
IV. Calculating and
Measuring Force
1. Calculating force
 Force is calculated mathematically by the
formula F=ma
 F= force
 M= the mass of the object in Kg
 A = the acceleration of the object in m/s²
Lets do some math!
This sounds complicated but is really very simple to
figure out. Example, Ken kicks a soccer ball that
is 1.5Kg and it accelerates at a rate of 6m/s².
With how much force did Ken kick the ball?
F=ma
F=?
M= 1.5Kg (is this in the correct units?)
A= 6m/s²
*replace the letters with the numbers given
Lets do some math!
When we rewrite the equation with the
numbers it should look like this.
F= (1.5Kg)(6m/s²)
*what do the brackets mean?
F= 9 Newtons
Calculating and Measuring
Force (cont.)
2. Measuring force
- Force is measured by using a force meter
- it measures in Newton's “N”
V.
Drawing a diagram to
explain the forces on an
object
• These are called free body diagrams
• Arrows are used to show the direction of the force.
• The arrows are labeled so you know what is causing the force.
Ex. A basketball dropped straight down.
F air resistance
F gravity
Lets try a few
1. A book is at rest on a table top. Diagram
the forces acting on the book.
F normal (or the force
from the table)
F gravity
VI. Friction
• is a force that resists the motion of two
objects that are in contact with each other
• Ex. A book sliding over your desk
• Ex. A Ball rolling across the floor
• Friction always works in the opposite
direction of the moving object(s).
Friction
(cont.)
• Friction occurs because every object has a
surface
• All objects have tiny “lumps and bumps” on
them that is the texture of surface
• These lumps and bumps catch as the two
objects pass over each other
Friction
(cont.)
• There is no such place as a perfectly frictionless
environment
• Even in outer space there are tiny particles
(micrometeorites) that cause friction
• The rougher the surfaces in contact the greater
the friction (texture of the surfaces)
• The greater the weight the greater the friction
• Surface area does NOT affect friction
VII. Two kinds of Friction
A. Static Friction



The friction that exists
between two surfaces
that are not moving
relative to each other.
Notice that you need to
push hard to get an object
to start moving
When the object is still
you are overcoming the
static friction
B. Kinetic Friction



The friction that exists
between two surfaces
that are moving
relative to each other
Notice that once you get
an object moving it takes
less force to keep it
moving
When the object is
moving you are pushing
against the kinetic friction
Signs of Friction
• When two surface rub they wear down
faster
• Heat
• Reduce friction by :
– Lubricant
– Replace sliding with rolling
VIII.Drag
• Drag is the mechanical
force created when a
solid object moves
through a fluid (a fluid can
be a liquid or a gas)
• Is a specific form of
friction
• Ex. A plane moving
through air
• Ex. A swimmer moving
through water
IX. Gravity
• A force that pulls anything with mass
towards another thing that has mass
– Matter – anything that takes up space and has
mass
– Mass – amount of matter in an object
A. Measuring mass and weight
Gravity (cont.)
a.




Weight
A spring scale is used to
measure weight
(Newton's)
Weight depends on
gravity
Therefore weight can
change
The moons mass is 1/6
of the earth’s and
therefore weight of
objects on the moon are
1/6 of what they would be
on earth
b. Mass
 A balance is used to
measure mass
(grams)
 Can never be altered
in an object without
adding or taking
away from the object
B. Newton’s Ideas on Gravity
Gravity
(cont.)
a. The force of gravity is acting between
every pair of objects in the universe
b. The greater the mass of the objects the
greater the force of gravity between them
c. The greater the distance between
objects, the smaller the force of gravity
between them
X.
Newton’s Laws of
Motion
Law #1
- An object in motion
tends to stay in
motion unless
outsides forces act
upon it
- Sometimes referred to
as the “Law of Inertia”
- INTERTIA
Newton’s Laws of Motion
Law #2
-The relationship between an object's
mass m, its acceleration a, and the
applied force F is F = ma.
- the harder you kick a ball the faster and
further it will go
Newton’s Laws of Motion
Law #3
- For every action
there is an equal and
opposite reaction.
XI. Inertia
• The tendency for an object to stay in the state of
motion that it is in
• Something that’s moving wants to keep moving and
something that is at rest want to stay at rest
Flight
(cont.)
a) Forces acting on a
plane:
- lift
- created by the wing
- gravity/weight
- force pulling down
- Thrust
- created by the jet engine or
propellers
- drag
- air resistance against the plane
Flight
(cont.)
b. How does a plane
create lift?



The wing moves through the
air at a great velocity
Because of the shape of the
wing the air that moves over
the top moves faster than the
air that moves under the wing
This creates an area of high
pressure under the high and
low pressure above the wing
Flight
(cont.)
b. How does a plane
create lift? (cont.)
 The area of high
pressure tries to balance
itself out and move to
the area of low pressure
 Since the wing is
between where it wants
to go it pushes up on the
wing
XIII. Helicopters
• Their blades are in the
shape of an airfoil
(wings)
• They turn fast enough
to create the same
high and low pressure
of a planes wing