Forces

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Forces Newtons Laws

• Identifying and defining forces
• The difference between mass and weight
• The property of Inertia
• The Law of Inertia (1st Law)
• The Law of Acceleration (2nd Law)
• The Law of Action-Reaction (3rd Law)
• The vector nature of forces

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So, what is a force, anyway?
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So, what is a force
Forces cause cars to move!
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So, what is a force?
Forces cause planes to fly!
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So, what is a force?
Forces cause planets to orbit the sun!
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So, what is a force?
Forces hold atoms together!
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So, what is a force

• Defn a force is a push or a pull that causes a
  change in motion, shape, or direction.

• If forces are equal and opposite they are called
  balanced forces.
• More than one force can combine to make a net
  force (total force).

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Think About It...
Think of all the different kinds of forces in the
universe... List as many of them as you can? How
many different forces are there?
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The Forces of Nature

• I. Gravitational
• attraction bet. masses
• tides, gravity, weight

• III. Weak Nuclear
• helps to explain atomic collisions

• II. Electromagnetic
• friction tension
• adhesion lift
• electrostatic drag
• buoyant magnetic

• IV. Strong Nuclear
• binds atomic nuclei

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Representing Forces...

• Forces are vectors
• Forces are drawn as arrows, the length represents
  the magnitude and the direction of the arrow is
  the direction of the force.
• forces add like vectors.
• the sum of all the forces is called the net
  force.
• A picture of a body with arrows drawn
  representing all the forces acting upon it is
  called a FREE BODY DIAGRAM.

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Try it...
Draw a picture of your book sitting on the desk.
Identify all the forces acting on it.
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Free Body Diagrams...
T (table)
Book
W (weight)
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Free Body Diagrams...
What forces are acting on a skier as she races
down a hill?
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Free Body Diagrams...
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Gravity and Weight...

• the force of gravity pulling you toward the
  ground is called your weight. W m x g
• What are the units of weight (force)?

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Forces are vectors

• Represented by arrows
• Vectors have magnitude and direction
• Drawn to scale with a ruler and protractor

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Adding Vectors

• The head to tail method

1. Draw first vector
2. Draw second vector beginning at the head of
the first
3. Connect the first tail to the last head to
get your answer (resultant)
L 4.6cm
4. Measure the length and angle
q 34
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Think about it...
What does gravity depend on?

• If youre standing on a scale and the pressure in
  the room rises what happens?
• If the Earth stopped spinning how would your
  weight change?
• So, what is gravity caused by?

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Law of Universal Gravitation

• Any two bodies with mass will attract eachother
  with a gravitational force!

• More mass gt more force
• farther away gt less force

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Examples

• Find the force of attraction between you and your
  neighbor (estimate distances and masses).
• Find the force of gravitational attraction
  between the Earth and Moon.

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Think about it...

• When a baseball is hit, what causes it to slow
  down? What prevents it from slowing down?
• When your car stops fast, why do you lunge
  forward?
• Under low friction circumstances which is harder
  to start moving, a large object or a small object?

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Inertia...
Def An objects tendency to resist a change in
motion is called its inertia.

• inertia is an intrinsic property that depends
  only on mass
• more mass --gt more inertia!

(Insert picture of someone running into a large
object)
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The Law of Inertia...
An object will stay in a state of rest or
straight line motion unless acted on by an
unbalanced force.
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You make the call...

• I.R.F.s or not?
• a car moving w/ constant speed
• the Earth
• a bus that is slowing down
• a skier going down a hill
• a person on a merry-go-round

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Think about it...
A little girl sits on a skate board and her
friend pulls her with a constant force. Describe
the motion of the girl.
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The Law of Acceleration
When an unbalanced force is applied to an object
it will accelerate in the direction of the net
force with an acceleration proportional to the
force applied.
F m x a
a F/m
Forces cause accelerations!
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Examples

• A jet thruster applies a force of 20,000N at
  maximum burn. If the jet has a mass of 15,500kg
  what is the acceleration of the jet? How long
  will it take to increase speed from 100 m/s to
  150 m/s?
• What force is needed to decelerate a2,000 lb car
  from 35 mph to rest in 5 seconds?

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Think About it...

• A textbook rests on a table. What forces act on
  the book? On the table?
• True or False. When you jump the Earth
  accelerates.
• An 8 ton bus crashes into a 1200 lb VW Rabbit.
  If the bus applies a force of 20,000 lb on the
  car, what is the force of the car on the bus?

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The Law of Force-Counterforce
When one body exerts a force on another body the
second one exerts an equal and opposite force on
the first.

• unaccompanied forces do not exist in nature.
• action-reaction forces are not the same as
  balanced forces.

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Think About It...

• A feather rests on a strong, solid oak table.
  Does the table exert any forces on the feather?
• I lean on a solid cement wall. Does it exert
  forces on me?
• What force causes a ball to bounce?

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Normal Forces...

• when you stand on the ground your weight pushes
  down on it. It pushes up on you with the same
  force! Thats called a normal force.
• a normal force is always perpendicular to the
  surface in contact.
• Take a minute and identify all the normal forces
  in the room.

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Think about it...

• What is the reading on the scales in the two
  diagrams below?

fig. 2
fig. 1
(weights are 200 N each)
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Tension...

• Tension is a force that pulls things apart.
• Tension is an electromagnetic force.
• Tension is the force that pulls in BOTH directions

(insert movie clip of a tug of war PT video
here)
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Think about it...

• an ice skater coasts to a stop...
• a marble rolls across the table and come to
  rest...
• a skydiver reaches terminal velocity...
• What force is responsible for the above?
• If you said FRICTION, you are correct!

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Brainstorm...

• So what exactly is friction and what does it
  depend on?
• Friction is a resistive force
• surface locking
• air drag
• fluid viscosity
• electrostatic attraction

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Friction...

• In mechanics, f µ x N
• µ (Greek mu) is a constant which is different
  for each situation.
• N is the normal force
• What is N for a book sliding across the floor?

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Examples

• What is the force of friction on a150 lb boy
  coasting down the street on his bike? How would
  it change if he locked up his brakes?
• How many meters will the boy above take to come
  to rest in each situation if his initial speed is
  26 mph?

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Types of friction...

• Rolling Friction
• Sliding Friction (kinetic)
• Static Friction
• In each case the only difference is the
  coefficient of friction, µ.

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Lawnmower Problem
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Summary...

• So, Sir Isaac Newtons three laws tell us all
  about how forces on every object in the universe
  act and interact. Can you state them all in your
  own words?
• The Law of Inertia...
• The Law of Acceleration...
• The Law of Action-Reaction...

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Wrap-up Questions...

• 1. Which of Newtons laws is the reason for
  wearing seat belts?
• 2. Inertia depends on ? .
• 3. When a compact car hits a large truck which
  applies the greater force?
• 4. What force(s) is/are responsible for slowing
  down motion on Earth?
• 5. On Jupiter a rolling skateboard would come to
  rest (faster/slower) than on Earth.