Transcript Forces

Forces
Force

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

a push or pull acting on an object
typically measured in Newtons (kg•m/s2)
is a vector
can be combined to predict motion net force
 Soccer Simulation
Types of Forces
 Contact Forces
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Applied
Normal
Friction
Air Resistance
Tension
Spring
 Non-Contact Forces
 Gravity
 Electromagnetic
Applied Force
 any push or pull on an object created
from another source (person, animal,
another object, etc.)
Normal Force
 the support force exerted on an object directly
related to weight (gravity)
 consequence of Newton’s 3rd Law
 is always perpendicular to the surfaces in contact
Gravity
Box 900
Normal
Force
Gravity
Friction
 exerted by a surface as an object moves across it or
attempts to move across it
 opposes the motion of an object
 depends on the type of surfaces and the normal
force (weight)
In which direction
 Types
is the force (friction)
 Kinetic
vector pointing?
 Static
Motion
Friction
Air Resistance
friction due to air molecules
acts upon objects as they travel through the air
opposes the motion of an object
most noticeable for objects traveling at fast
speeds
 Examples
 Space shuttle re-entry
 Meteorite in Freehold
 Meteor over Russia
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Minimizing Air Resistance
(Drafting)
 used in variety of competitive events
(bicycle and car racing, swimming, etc.) to
reduce air resistance
Notice how the second biker
experiences less air
resistance because he is
shielded by the first biker.
Image taken from:
http://www.bbc.com/news/magazine-19166035
Although it does not work
exactly the same way, where
is these seen in nature?
Tension
 force that is transmitted through a string, rope,
cable or wire when it is PULLED tight by forces
acting from opposite ends
 directed along the length of the wire and PULLS
equally on the objects on the opposite ends of the
wire
Spring
 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
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
What do you think the
What is the net
symbols w/subscripts
force on this object?
represent?
Fnorm = 5 N
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?
Laws
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
How is this illustrated when riding in a
car? Can you think of other
experiences where this is illustrated?
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 mass
 the direction in which an object
accelerates is the same as the direction
of the force
 Formula: F = ma (or a = F/m)
 Shopping Cart Simulation
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.
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.
How was this illustrated during the
Scooter Games competition? Can you
think of other experiences where this
is illustrated?
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
Gravity = 14 N
puck’s net force?
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