Transcript Forces - Cloudfront.net

FORCE
A push or pull acting on an object
typically measured in Newtons (kg•m/s2)
is a vector (Has a magnitude And direction)
can be combined to predict motion net force
Forces can be balanced or Unbalanced
Balanced Forces cause no change in the motion
of an object.
In order to make an object move it needs to be
acted upon by an unbalanced force.
TYPES OF FORCES
Contact Forces
Friction
Tension Force
Normal Force
Compression
Buoyant Force
Air Resistance
Non-Contact Forces
Gravity
Electromagnetic
Centripetal Force
FRICTION
Friction force is the result of two surfaces bein
pressed together closely. This causes intermolecular
attractive forces between molecules of different
surfaces resulting in friction. The frictional forces
depend upon the nature of the surfaces. The more
rough the surface the more the frictional force.
Types Of Friction :
In which direction
 Kinetic: Sliding Or Rolling
is the force (friction)
vector pointing?
 Static
 Fluid Force
Motion
Friction
TENSION
Force that is transmitted through a string, rope,
cable or wire when it is PULLED tight by forces
acting from opposite ends.
It is directed along the length of the wire and
PULLS equally on the objects on the opposite ends
of the wire
NORMAL FORCE
This is also called the support force. If an object
is resting upon a table then the table is exerting
an upward force upon the object in order to
support the weight of the object. If a person
leans against a wall, the wall pushes horizontally
on the person.the support force exerted on an
object directly related to weight (gravity)
Gravity
Gravity
is always perpendicular
to the surfaces in contact
Box 900
Normal
Force
COMPRESSION
The force exerted by a compressed or stretched
spring upon any object that is attached to it
For most springs, the magnitude of the force is
directly proportional to the amount of stretch or
compression of the spring
If both springs are the same
size when not compressed, which
spring will apply more force to
the ball when released? Explain
your reasoning.
Images taken from:
http://www.lesjoforsab.com/standard-springs/compression-springs.asp
AIR RESISTANCE
It is friction due to air molecules
It acts upon objects as they travel through the air
opposes the motion of an object
It ismost noticeable for objects traveling at fast
Speeds and is Greater as the surface area increases
Example
 Sky Diver diving in the sky
BUOYANT FORCE
It is an upward
force in
a fluid(gas or liquid)
GRAVITY
natural force of attraction
between any two objects
factors:
 distance – increased distance 
less gravitational pull or vice
versa
 mass – increased mass  more
gravitational pull or vice versa
Why does the force of gravity
have more of an impact on
holding our solar system
together compared to holding
the parts of an atom together?
ELECTROMAGNETIC FORCE
force that moving charges exert on one
another
results from the repulsion of like charges
and the attraction of opposites
+ +
+
- -
Compare and contrast gravitational force and
electromagnetic force.
Notice how the
particles with the
same charge move
apart and the
particles with
different charges
move together.
FREE BODY DIAGRAMS
visuals that help show net force
 use a square and draw all forces acting on the object.
 remember size and direction of vector arrows are important!
Practice
Fnorm = 5 N
What do you think the
What is the net
symbols w/subscripts
force on this object?
represent?
Ffric = 3 N
Fapp = 3 N
Fgrav = 5 N
WHAT’S THE NET FORCE
Fnorm = 10 N
Fapp = 20 N
Ffric = 5 N
Fgrav = 10 N
You throw a baseball to your
friend who is to your left.
Ffric = 5 N
Fapp = 15 N
Fgrav = 10 N
Your dog pulls you down the
street on a skateboard in an
eastward direction.
WHAT’S THE NET FORCE
(AN INTERESTING CASE)
A skydiver is descending with a
constant velocity. Consider air
resistance.
Ffric
The same skydiver is
descending after 30 seconds.
Consider air resistance.
Ffric
Fgrav
Fgrav
What has the skydiver
reached in this scenario?
CENTRIPETAL FORCE
any force that keeps an object moving in
a circle directed toward the center of
the circle.
In this case, the force of the
ball as it accelerates around
the circle is pointing inward,
toward the center.
LAWS OF MOTION
Newton’s 1st Law
Newton’s 2nd Law
Newton’s 3rd Law
NEWTON’S 1ST LAW
objects at rest remain at rest, and
objects in motion remain in motion
with the same velocity, unless acted upon
by an unbalanced force also considered
the Law of
Inertia
INERTIA
the resistance of an object to a change
in the speed or the direction of
its motion directly related to mass
NEWTON’S 2ND LAW
the acceleration of an object increases
with increased force and decreases with
increased massthe direction in which an
object accelerates is the same as the
direction of the force
Formula: F = ma (or a = F/m)
PRACTICE PROBLEMS - FORCE
1.
What net force is needed to accelerate a 24 kg
dogsled to a rate of 3 m/s2?
F = ma
2.
2
72
kg
·
m/s
F = (24 kg) (3 m/s2) =
or 72 N
A 1.5 kg object accelerates across a smooth table
at a rate of 0.5 m/s2? What is the unbalanced
force applied to it?
F = ma
F = (1.5 kg)(0.5 m/s2) = 0.75 kg·m/s2
or 0.75 N
NEWTON’S 3RD LAW
states that every time one object exerts
a force on another object, the second
object exerts a force that is equal in
size and opposite in direction back on the
first object.
VECTOR
a quantity that has both direction and
magnitude (size)
drawn as an arrow which shows direction and
magnitude (length of arrow)
 consists of two parts: tail and head
Head
Tail
Consider the vectors above. Describe the direction and
relative magnitude (force) of each ball based on the vector.
COMBINING VECTORS
can be combined/added to help determine net force
 the overall force acting on an object when all of the forces
acting on it are combined
What is the hockey
puck’s net force?
Gravity = 14 N
Gravity = 14 N
23 N
Applied Force = 25 N
Friction = 2 N
Applied Force = 25 N
Friction = 2 N
Normal Force = 14 N
Normal Force = 14 N