Forces

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Forces
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Force

• a push or pull acting on an object
• typically measured in Newtons (kgm/s2)
• is a vector
• can be combined to predict motion? net force
• Soccer Simulation

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Types of Forces

• Contact Forces
• Applied
• Normal
• Friction
• Air Resistance
• Tension
• Spring

• Non-Contact Forces
• Gravity
• Electromagnetic

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Applied Force

• any push or pull on an object created from
  another source (person, animal, another object,
  etc.)

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Normal Force

• the support force exerted on an object directly
  related to weight (gravity)
• consequence of Newtons 3rd Law
• is always perpendicular to the surfaces in
  contact

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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)
• Types
• Kinetic
• Static

In which direction is the force (friction)
vector pointing?
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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/magazin
e-19166035
Although it does not work exactly the same way,
where is these seen in nature?
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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

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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.
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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?
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Electromagnetic Force

• force that moving charges exert on one another
• results from the repulsion of like charges and
  the attraction of opposites

Notice how the particles with the same charge
move apart and the particles with different
charges move together.



-
-
-
Compare and contrast gravitational force and
electromagnetic force.
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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 symbols w/subscripts
represent?
What is the net force on this object?
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Whats the Net Force
Fnorm 10 N
Ffric 5 N
Fapp 15 N
Fapp 20 N
Ffric 5 N
Fgrav 10 N
Fgrav 10 N
You throw a baseball to your friend who is to
your left.
Your dog pulls you down the street on a
skateboard in an eastward direction.
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Whats the Net Force (An Interesting Case)
A skydiver is descending with a constant
velocity. Consider air resistance.
The same skydiver is descending after 30 seconds.
Consider air resistance.
What has the skydiver reached in this scenario?
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Laws

• Newtons 1st Law
• Newtons 2nd Law
• Newtons 3rd Law

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Newtons 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?
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Inertia

• the resistance of an object to a change in the
  speed or the direction of its motion
• directly related to mass

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Newtons 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

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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.
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Practice Problems - Force

• What net force is needed to accelerate a 24 kg
  dogsled to a rate of 3 m/s2?
• 2. 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?

72 kgm/s2 or 72 N
F ma
F
(24 kg)
(3 m/s2)

F
(1.5 kg)
(0.5 m/s2)
0.75 kgm/s2 or 0.75 N

F ma
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Newtons 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?
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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.
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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 pucks net force?
Gravity 14 N
23 N
Applied Force 25 N
Friction 2 N
Normal Force 14 N