Download File forces and fbds

Motion and Forces
Newton at work
Isaac Newton
• Came up with 3 Laws of Motion based on the works of
Galileo and Johannes Kepler .
• “If I have seen further, it is by standing on the shoulders of
giants.”
• Invented Calculus. (Archimedes actually had laid out the basic
principles of calculus in 3rd century BC, but work was lost in Middle Ages and
not rediscovered until late 1990s)
• Published his Laws in 1687 in the book Mathematical
Principles of Natural Philosophy.
Force?
What is a force? – Think silently for 12 s
Write your definition in your notebook and
share with 2 people
= a push or pull that one
body exerts on another
•Can exist during physical contact – CONTACT FORCES
•Can exist with NO physical contact, called FIELD
FORCES
Forces are measured
in units called
Newtons
𝑘𝑔 ∙𝑚
N=(
𝑠2)
Tool for measuring the
magnitude of a force =
SPRING SCALE
Forces and Motion
• Forces can cause an object
to accelerate…
• To speed up
• To slow down
• To change direction
Some specific types of forces to know:
CONTACT FORCES
• Generic applied forces FA
• Weight - W or mg
• Normal – FN or N
• Tension – FT or T
• Friction -- Ff
• Air resistance -- FAR
• Buoyant -- FB
FIELD FORCES
• Gravitational Force – FG
• Magnetic Force -- FM
• Electrical Forces -- FE
Force Normal(FN)
A surface force always
drawn perpendicular to a
surface.
Flat surfaces – 1 D
FN = mg for objects resting
on horizontal surfaces.
N
mg
Applied forces affect normal force.
friction
applied force
normal
weight
N = applied force
Tension(T or FT)
• A pulling force.
• Generally exists in a rope, string, or cable.
• Arises at the molecular level, when a rope, string, or
cable resists being pulled apart.
Friction(Ff)
A force that acts in a
direction opposite to
the motion of two
surfaces in contact with
each other.
Friction works to slow
things down or hold
them in place.
Friction is caused by “microwelds”.
Every surface, even smooth ones, have bumps.
These bumps rub
and lodge against
each other.
Friction
Friction can cause
an object to slow
down and stop.
It sometimes
seems as if the
object has lost
energy, but in
reality the energy
was converted
into
There are two types of friction
•
Friction and the Normal Force
• The frictional force which exists
between two surfaces is directly
proportional to the normal force.
• That’s why friction on a sloping
surface is less than friction on a flat
surface.
Friction
Lubricants can be
used to reduce
friction.
examples : oil,
grease, wax, water,
mud, etc…
Air resistance
• Friction
between an
object
moving
through the
air and the
air itself.
• When an
object falls
through the
air, air
resistance
pushes UP
on the
object,
working to
slow it down.
Buoyancy
• Is the ability of a fluid to
exert an upward force on
anything immersed in it.
• The upward force is called
the buoyant force.
• The denser the fluid, the
greater the buoyant force.
• This force determines if
things sink or
float.
• Explains how boats float.
• Water has a density of 1.0 g/mL.
• Anything below 1.0 g/mL will
float to the top.
• Anything greater than 1.0 g/mL
will sink.
• If it is exactly 1.0, it will remain
in place, neither going up or
down.
• LESS dense moves UP.
• MORE dense moves down
How is Mass Different Than Weight?
(Hold that thought!!!)
Your task
Investigate the relationship between mass and weight.
• You must gather data about 5 objects found in the classroom or lab. 3 of
known mass, 2 of random unknown mass
• After gathering the data, you must create a table and graph/chart
• Your table must be organized and clearly labeled with correct units shown
in the column headings
• Your graph must be clearly titled as “Weight vs Mass” with the
appropriate information on the correct axis. Your axis must also
include units for the quantities.
• You need to draw a line of best fit and figure out the slope of it.
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 N for force.
Weight(mg)
= Mass (m) X gravity (g)
• Unit of mass = kg
• Unit of acceleration = m/s2
• Unit of weight = Newton
• 1 Newton= about ¼ pound
Balanced Forces
• A force that
produces no change
in an object’s
motion because it is
balanced by an
equal, opposite net
force.
Unbalanced Forces
Are net
forces that
results in
an object’s
motion
being
changed.
+
Consider
onaaforce
table.isThe
forces
Now,
considerthis
ballball
b. If
applied
tondit,
It will act the same way. However, what if a 2 force
Now
consider
ball is
c which
hasinthe
force
on
it are balanced.
it will
move.
This
a change
itssame
motion
due
is applied to it at another point in its path?
applied
to it.
to an unbalanced
force
Its motion will change AGAIN due to the
UNBALANCED FORCE on it
Free-body diagrams
Free-body diagrams are used to show the
relative magnitude and direction of all forces
acting on an object.
This diagram shows
four forces acting upon
an object. There aren’t
always four forces, For
example, there could be
one, two, or three
forces.
Problem 1
A book is at rest on a table top. Diagram the
forces acting on the book.
Problem 1 Solution
In this diagram, there
are normal and
gravitational forces on
the book.
Problem 2
An egg is free-falling from a nest in a tree. Neglect
air resistance. Draw a free-body diagram showing
the forces involved.
Problem 2 Solution
Gravity is the only force
acting on the egg as it
falls.
Problem 3
A flying squirrel is gliding (no wing flaps) from a
tree to the ground at constant velocity. Consider
air resistance. A free body diagram for this
situation looks like…
Problem 3 Solution
Gravity pulls down on
the squirrel while air
resistance keeps the
squirrel in the air for a
while.
Problem 4
A rightward force is applied to a book in order to
move it across a desk. Consider frictional forces.
Neglect air resistance. Construct a free-body
diagram. Let’s see what this one looks like.
Problem 4 Solution
Note the applied force
arrow pointing to the
right. Notice how
friction force points in
the opposite direction.
Finally, there is still
gravity and normal
forces involved.
Problem 5
A skydiver is descending with a constant velocity.
Consider air resistance. Draw a free-body
diagram.
Problem 5 Solution
Gravity pulls down on
the skydiver, while air
resistance pushes up as
he falls.
Problem 6
A man drags a sled across loosely packed snow
with a rightward acceleration. Draw a free-body
diagram.
Problem 6 Solution
The rightward force arrow points
to the right. Friction slows his
progress and pulls in the opposite
direction. Since there is not
information that we are in a
blizzard, normal forces still apply
as does gravitational force since
we are on planet Earth.
Problem 7
A football is moving upwards toward its peak after having been booted
by the punter. Draw a free-body diagram.
Problem 7 Solution
The force of gravity is
the only force
described. It is not a
windy day (no air
resistance).
Problem 8
A car runs out of gas and is coasting to the right
down a hill and slowing down.
Problem 8 Solution
Even though the car is
coasting down the hill,
there is still the
dragging friction of the
road (left pointing
arrow) as well as gravity
and normal forces.
Net Force
Now let’s take a look at what happens when
unbalanced forces do not become completely
balanced (or cancelled) by other individual forces.
An unbalanced forces exists when the vertical and
horizontal forces do not cancel each other out.
Example 1
Notice the upward
force of 1200 Newtons
(N) is more than gravity
(800 N). The net force is
400 N up.
Example 2
Notice that while the
normal force and
gravitation forces are
balanced (each are 50 N)
the force of friction
results in unbalanced
force on the horizontal
axis. The net force is 20 N
left.
Another way to look at balances and
unbalanced forces
A force diagram illustrating Static Friction
Normal Force
Frictional
Force
Applied Force
Gravity
A force diagram illustrating Static Friction
Normal Force
Bigger Applied
Force
Bigger
Frictional
Force
Gravity
A force diagram illustrating Static Friction
The forces on the book are now UNBALANCED!
Normal Force
Frictional
Force
Gravity
Static friction cannot get any larger, and can
no longer completely oppose the applied
force.
Even Bigger
Applied Force