Forces and Motion

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Forces and Motion
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Three Incorrect laws of motion.

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• Why does the Earth Spin?
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• Second clip?
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• Vocabulary
• Force - A push or pull on an object.
• Net Force vector sum of all forces
• Equilibrium
• the state in which the net force on an object is
  zero or balanced
• no change in velocity

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Sir Isaac Newton

• 1643 - 1727

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2.2.4 State Newtons first law of motion. 2.2.5
Describe examples of Newtons first law.

• 1st Law of Motion (Law of Inertia)
• An object at rest will stay at rest, and an
  object in motion will stay in motion at constant
  velocity, unless acted upon by an unbalanced
  force.

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1st Law of Motion (Law of Inertia)

• Translation Objects keep doing what they are
  doing, in a straight line.

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Newtons 1st Law Inertia

• Inertia is a measurement of ______________ TO
  CHANGE
• It is measured in terms of ______. The greater
  the _____, the greater the inertia.

RESISTANCE
mass
mass
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What is Mass?

• Mass is the amount of material a body/object
  contains
• You are what you eat
• Bigger does not equal more mass

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2.2.2 - Identify the forces acting on an object
and draw free-body diagrams representing the
forces acting

• Free Body Diagrams
• FBDs are a neat way to show the forces acting on
  an object.
• The object is drawn as a dot or a box, and then
  vectors are drawn to show the forces acting on
  it.
• Ex

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FBDs

• Can you draw the FBD for you sitting on your
  chair?
• Draw the FBD for your friend pushing you in a
  rolling chair.

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FBDs

• Solve the net force for the following FBD.

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FBD

• The picture at the bottom shows a snowman pulling
  a sled. Draw a free-body diagram for this sled.
  The magnitudes of the forces acting on the sled
  are 60N by the string, 130N from gravity, and 90N
  upward by the ground.

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FBD

• Now try doing some of the free-body diagrams on
• pg 124 Practice A
• Pg 124 Section Review 2-5

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2.2.6 - State the condition for translational
equilibrium.2.2.7 - Solve problems involving
translational equilibrium.

• Equilibrium in a straight line.
• Fnet 0 so there is constant velocity

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2.2.6 - State the condition for translational
equilibrium.2.2.7 - Solve problems involving
translational equilibrium.

• Derek leaves his physics book on top of a
  drafting table that is inclined at 35º angle.
  The FBD to the right shows the forces acting on
  the book. Find the net force acting on the book.

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Vocabulary

• Force - A push or pull on an object.
• Net Force vector sum of all forces
• Equilibrium
• the state in which the net force on an object is
  zero or balanced
• no change in velocity

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2.2.8 State Newtons second law of motion.

• 2nd Law
• The net force of an object is equal to the
  product of its mass and acceleration, or Fma.

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Newtons 2nd Law Simplified

• So, a little easier to read would bea SF
  OR SF ma m
• So what are the units for force
• Forces are measured in newtons 
• N  (kg)(m/s2).

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What does F ma mean?

• Force is directly proportional to mass and
  acceleration.
• Imagine a ball of a certain mass
• moving at a certain
• acceleration. This
• ball has a certain
• force.

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What does F ma mean?
Now imagine we make the ball twice as big (double
the mass) but keep the acceleration constant. F
ma says that this new ball has twice the force
of the old ball.
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What does F ma mean?
Now imagine the original ball moving at twice the
original acceleration. F ma says that the ball
will again have twice the force of the ball at
the original acceleration.
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More about F ma

• If you double the mass, you double the force. If
  you double the acceleration, you double the
  force.
• What if you double the mass and the acceleration?
• (2m)(2a) 4F
• Doubling the mass and the acceleration quadruples
  the force.
• So . . . what if you decrease the mass by half?
  How much force would the object have now?

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What does F ma say?

• F ma basically means that the force of an
  object comes from its mass and its acceleration.

Something very massive (high mass) thats
changing speed very slowly (low acceleration),
like a glacier, can still have great force.
Something very small (low mass) thats changing
speed very quickly (high acceleration), like a
bullet, can still have a great force. Something
very small changing speed very slowly will have a
very weak force.
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Newtons 2nd Law Vectors!

• Remember, acceleration is a vector quantity, so
  Force will be too ?
• Some equations to remember
• Pythagorean Theorem
• Sin ?
• Cos ?
• Tan ?

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2.2.1 Calculate the weight of a body using the
expression W mg. Gravity causes weight

• The weight of an object is caused by the force of
  attraction between the earth and objects on the
  surface of the earth.

Gravitation force is equal to weight, therefore
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How much does a person with a mass of 70.0 kg
weigh on the earth?
Weight mass X 9.8 m/s2 Weight 70.0 kg X 9.8
m/s2 Weight 686 N Remember that 1 N (1 kg)
(1m/s2)
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Mass vs. Weight

• Your MASS is the amount of material in your body.
• Your MASS doesnt depend on where you are.
• Your WEIGHT is how much your body pushes down on
  a scale.
• Your WEIGHT depends on how much MASS you have
  and where you are.
• Weight is caused by gravity.

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Martym 68 kg
What does Marty weigh?

• G(sun) 275 m/s2

18,700 N
G(pluto) 0.654 m/s2
44 N
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2.2.9 Solve problems involving Newtons second
law.

1. Determine the accelerations that result when a
   12-N net force is applied to a 3-kg object.
2. A net force of 15 N is exerted on an encyclopedia
   to cause it to accelerate at a rate of 5 m/s2.
   What is the mass of the encyclopedia?

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2.2.9 Solve problems involving Newtons second
law.

• Now try answering some problems on pg 132
  practice C 1-4

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• There is a net force of 12N acting on a desk
  which has a gravitational force of 40N. How much
  is the desk accelerating?

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• There is a net force of 12N acting on a desk
  which has a gravitational force of 40N. How much
  is the desk accelerating?
• A 50N crate experiences a horizontally applied
  force of 10N from a student and a frictional
  force of 4N. What is the acceleration of the
  crate?

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Newtons 2nd Law Lets practice

1. You have run out of gas and have to push your
   car. Your car weighs 560 N and you push with a
   force of 395 N, and your friend helps with 275 N.
   What is the acceleration of the car?
2. You and a friend push a 20kg rolling chair
   horizontally. You push with a force of 17 N. The
   chair has a frictional force of 5N. The chair
   accelerates at a rate of 1.35m/s2. How much force
   is your friend contributing?

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NET FORCE Practice

• Free-body diagrams for four situations are shown
  below. The net force is known for each situation.
  However, the magnitudes of a few of the
  individual forces are not known. Analyze each
  situation individually and determine the
  magnitude of the unknown forces. Then click the
  button to view the answers.

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Newtons 2nd Law A little bit harder

1. You have run out of gas and have to push your
   car. Your car weighs 560 N and you push with a
   force of 395 N, and your friend helps with 275 N.
   What is the acceleration of the car?
2. You push a rolling chair (m20kg) with a force of
   17 N. A friend pushes with 15 N at 30o. What is
   the net force? What is the acceleration of the
   chair?

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Quick Quiz
Consider the following free-body diagram.
Ignoring air resistance and friction, can this
object be moving in a horizontal
direction? Well come back to this question.
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HWA

1. What is the net force acting on an object that
   has a force of 40N to the right and another 30N
   downward?
2. What are the components of a 20N force acting on
   a desk at an angle of 30º above horizontal?

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• 2.2.14 State Newtons third law of motion.
• 2.2.15 Discuss examples of Newtons third law.
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3rd Law

• For every action, there is an equal and opposite
  reaction.

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What does this mean?

• For every force acting on an object, there is an
  equal force acting in the opposite direction.
• Right now, gravity is pulling you down in your
  seat, but Newtons Third Law says your seat is
  pushing up against you with equal force. This is
  why you are not moving.
• There is a balanced force acting on you gravity
  pulling down, your seat pushing up.
• Put really simply F F

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Think about it . . .
What happens if you are standing on a skateboard
or a slippery floor and push against a wall? You
slide in the opposite direction (away from the
wall), because you pushed on the wall but the
wall pushed back on you with equal and opposite
force.
Why does it hurt so much when you stub your toe?
When your toe exerts a force on a rock, the rock
exerts an equal force back on your toe. The
harder you hit your toe against it, the more
force the rock exerts back on your toe (and the
more your toe hurts).
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Newtons Third Law

• It states
• Whenever one object exerts a force on a second
  object, the second object exerts an equal and
  opposite force on the first object
• It requires two forces
• _____________ Force
• ______________ Force

Action
Reaction
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Newtons Third Law

• Does it matter which force we call the ________
  and which is the _________?
• Forces always occur in ________. Therefore, it
  ________ matter which is the action and the
  reaction, as long as you can identify both.

action
reaction
pairs
doesnt
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Review
Newtons First Law
Objects in motion tend to stay in motion and
objects at rest tend to stay at rest unless acted
upon by an unbalanced force.
Newtons Second Law
Force equals mass times acceleration (F ma).
Newtons Third Law
For every action there is an equal and opposite
reaction.
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Quick Quiz
Consider the following free-body diagram.
Ignoring air resistance and friction, can this
object be moving in a horizontal direction?
YES!!!! IT CAN!!!
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Quick Quiz
Think about this situation. What are the forces
on the boy on the sled between points B and C?
I hope you got something that looked a lot like
this.
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Quick Quiz
This means a force is not required to keep a
moving object in motion.
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Forces

• Types of forces
• Contact
• Non-Contact

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Forces

• Contact
• Normal
• Frictional
• Tension
• Air Resistance
• Spring

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Forces

• Non-Contact
• Electrical
• Magnetic
• Nuclear

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Normal Force

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The Normal Force

• The normal force (FN) is one component of the
  force that a surface exerts on an object with
  which it is in contact
• Namely, the component that is ____________ to the
  surface.

Perpendicular
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The Normal Force

• How do I find the normal force?
• Easy, it is equal and opposite to the weight of
  an object if the object is on a horizontal
  surface.
• The Fn is not always horizontal.
• Lets look at Fig 12 (pg136)
• The Fn can be found by
• Fnmg cosT (where T is the angle between the
  contact surface and horizontal)

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The Normal Force

• Where is the normal force?

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What is Friction?

• is a force resisting the relative motion of two
  surfaces in contact with each other.
• is the parallel component of the force on an
  object that is in contact with a surface.
• is a force that prevents motion from occurring.
• Types
• Static
• Kinetic
• Fluid

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What causes friction?

• There are two factors which affect friction
  between two surfaces
• Kind of surfaces in contact (rough or smooth)
• Amount of force pressing the surfaces together.
• The rougher the surface and the stronger the
  force between the surfaces, the greater the
  amount of friction.

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Static and Kinetic Friction
Static Friction- Friction force that acts on
objects that are not moving Kinetic Friction -
Friction force that opposes the direction of
motion of an object as it slides over a surface.
I better be safe Ump!!
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Static Friction

• Static Friction is the force that prevents the
  start of motion.
• With no movement, Fs Fapplied
• Has a range of 0 Fs,max

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Kinetic Friction

• Once motion has begun, the static friction force
  is no longer a concern.
• Instead, kinetic friction is important.
• Kinetic friction is resistance to movement of an
  already moving object.

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Fluid Friction
- Friction force that opposes the motion of an
object through a fluid.
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Frictional Forces

• The frictional force will vary depending on the
  surface interactions.
• Ex. Carpet vs tile
• Friction can be calculated by using the
  coefficient of friction.
• Use the basic formula of
• Ff µFn
• It can be rearranged to
• µk (Fk/Fn) and µs (Fs/Fn)

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Frictional Forces

• Use the formula of
• Ff µFn
• Fk µkFn and Fs µsFn
• A 24kg crate initially at rest on a horizontal
  floor requires a 75N horizontal force to set it
  in motion. Find the coefficient of static
  friction between the crate and the floor.

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Force of Tension

• Tension commonly means the tendency of a rope to
  be pulled apart.
• Tension is typically the same everywhere in a
  rope because the rope is treated as massless. If
  it isnt (and is vertical) you must account for
  the mass of the rope.

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Finally, AIR RESISTANCE!!!
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2.2.6 State the condition for translational
equilibrium

• Translational equilibrium is equilibrium in a
  straight line.
• The net force acting on a free falling object at
  terminal velocity is zero, so it falls to the
  ground at constant speed.

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Terminal Velocity
The velocity at which a free falling object
ceases to accelerate.
T.V. is the constant velocity of a falling object
when the force of gravity equals the force of air
resistance
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Spring Force

• We will dive deep into spring force when we get
  to elastic energy.
• Until then, play safely!!

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Electric Forces
- forces that act between charged objects or
particles such as protons and electrons
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Magnetic Forces
-Forces that act on certain metals, poles of
magnets, and on moving charges
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Nuclear Forces
- Nuclear forces act over very short distances
and are what hold the particles of an atomic
nucleus together.
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• Use the formula of
• Fk µkFn and Fs µsFn
• A hockey puck has a coefficient of kinetic
  friction of µk .10. If the puck feels a normal
  force (FN) of 5 N, what is the frictional force
  that acts on the puck? What is the mass of the
  puck?
• A desk has a mass of 71.25 kilograms. If the
  coefficient of static friction between the desk
  and the floor is 1.14, what force must be used to
  move the desk from rest?

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Quick Review (Net Force)

• There is a 15kg book sliding down a desk that has
  an incline of 25º. If there is a frictional force
  of 20N, what is the acceleration of the book?
• Derek leaves his physics book on top of a
  drafting table that is inclined at a 35 angle.
  There is a frictional force of 11N, a normal
  force of 18N and gravitational force of 22N. What
  is the net force?

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On your own!! (Net Force)

• A student is designing a support to keep a tree
  upright. Two wires have been attached to the tree
  and placed at right angles to each other. One
  wire exerts a force of 30N on the tree the other
  wire exerts a 40N force. Determine where the
  third wire should be placed and how much force is
  should have so that the net force acting on the
  tree is zero.
• A stationary, flying kite is acted on by a force
  of 9.8N downward. The wind exerts a force of 45N
  at an angle of 50 above the horizontal. Find the
  force that the string exerts on the kite.

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Putting it all together!!

• A student attaches a rope to a 20kg box of books.
  He pulls with a force of 90N at an angle of 30
  with the horizontal. The coefficient of kinetic
  friction between the box and the sidewalk is
  0.50. Find the acceleration of the box.
• Slow down, take a deep breath and lets take this
  one step at a time.

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Putting it all together!!

• Now lets try pg 141 Practice F, 1-4
• Dont get bogged down with 2

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What is momentum?

• The momentum of an object is equal to the product
  of its mass and its velocity. The unit for
  momentum is kg m/s

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How some things react without forces