CHAPTER 10 FORCES

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CHAPTER 10 FORCES

• By Mr. Todd

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NATURE OF FORCES

• What is Force?
• a push or pull
• can be described by an arrow (width of the arrow
  tells the strength of the force, the head of the
  arrow tells the direction of the force)

Small width weak force
Large width strong force
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Combining forces

• add forces in the same direction

• subtract forces in
• opposite directions

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• Unbalanced Forces
• when forces combine and there is a net force
  acting in one direction.
• it can cause an object to start moving, stop
  moving or change directions

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• Balanced Forces
• when forces combine and there is a net force of
  zero.
• there will be no change in motion.

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SIR ISAAC NEWTON(NEWTONS LAWS)
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NEWTONS FIRST LAW OF MOTION(THE LAW OF INERTIA)

• an object at rest will remain at rest and an
  object moving at constant velocity will continue
  to move at constant velocity unless acted upon by
  an unbalanced force.

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NEWTONS FIRST LAW OF MOTION(THE LAW OF INERTIA)

• all objects resist a change in motion. This is
  called inertia.
• Example Car stops suddenly. Person keeps
  moving forward because of the inertia.
• the greater the mass of an object the greater its
  inertia.

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NEWTONS SECOND LAW OF MOTION

• The net force on an object is equal to the
  product of its acceleration and its mass.
• FORCEMASS X ACCELERATION
• F M A
• Force is measured in Newton's
• 1 Newton force required to accelerate a one
  Kilogram mass one meter in one second
• 1 N 1Kg 1M/sec2

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Sample Problem 1

• Calculate the force for the car below.
• Show the letter equation
• Show the number equation
• Show the triangle
• Use proper units on all numbers
• Car A has an acceleration of 2m/s/s and a Mass of
  750kg.
• FMxA
• F 750kg x 2m/s/s
• F 1500 N

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Sample Problem 2

• Calculate the force for the car below.
• Show the letter equation
• Show the number equation
• Show the triangle
• Use proper units on all numbers
• Car B has an acceleration of 2m/s/s and a Mass of
  1000kg.
• FMxA
• F 1000kg x 2m/s/s
• F 2000 N
• Which car had more force?
• Car B

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Name two ways that you could increase the
acceleration of an object?

• If you want to increase acceleration, increase
  force!
• If you want to increase acceleration, decrease
  mass!

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Friction and Gravity

• What is Friction?
• force exerted when one surface rubs against
  another.
• Nature of Friction
• friction acts in the direction opposite to the
  objects direction of motion.
• the strength of friction depends on two factors
  
• 1. Types of surface (rough surfaces more
  friction)
• 2. How hard the surfaces push together
• (Rub your hands forcefully more friction)
• Is Friction Useful?
• it depends on the situation
• 1. Walking-? friction between ground and feet -
  very necessary
• 2. Driving a car-?friction between tires and
  road - very necessary
• 3. Rusty hinge on door ? friction between hinge
  parts -- not good

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Types of Friction

• Sliding friction solid surfaces sliding over
  each other.
• Example Shoe moving over the ground
• Rolling friction solid surfaces rolling over
  each other. (force needed to overcome this
  friction is less than sliding friction.)
• Example Ball bearings used in roller skates
• Fluid friction when a solid object moves through
  a fluid (force needed to overcome this friction
  is less than sliding friction.)
• Example (Oil and grease lubricants for fluid
  friction. These keep solids from making direct
  contact.)
• Motor oil in engines.

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What is Gravity?

• force of attraction between all objects
• force which pulls objects toward the center of
  the earth

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Free fall

• when gravity acts on a falling body, the body
  will accelerate
• Acceleration due to gravity 9.8m/s/s
• Velocity
• Example Stone dropped from bridge falls
• At 1 sec. velocity 9.8 m/s
• At 2 sec. velocity 19.6 m/s
• At 3 sec. velocity 29.4 m/s
• Terminal Velocity 190 km/hr. No more
  acceleration will happen. It will fall at a
  constant velocity.

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Projectile motion

• Projectile Motion
• an object that is thrown is a projectile
• an object horizontally thrown is in free fall.
  It will land on the ground at the same time an
  object dropped from the same height does.

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Air resistance

• Air Resistance
• Air pressure is a type of fluid friction
  experienced by all free falling objects. The
  more surface area the more air resistance.
• As velocity increases air resistance increases

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Which animation takes place in a vacuum?
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Weight

• measure of the force of gravity on an object
• Weight Mass x Acceleration due to gravity
• Example Calculate the weight in Newtons of a
  50kg object that has an acceleration due to
  gravity of 9.8 m/s/s here on Earth.
• Mass 50 kg
• Acceleration 9.8 m/s/s
• wt M x A
• wt 50 kg x 9.8 m/s/s/
• wt 490 Newtons

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What is the Law of Universal Gravitation?

• the force of gravity acts between all objects in
  the universe.
• Gravity increases as the mass of the object
  increases.
• Gravity increases as the distance between objects
  decreases.

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Action and Reaction

• Newton's Third Law of Motion
• if one object exerts a force on another object,
  then the second object exerts a force of equal
  strength in the opposite direction on the first
  object.
• For every action there is an equal and opposite
  reaction.
• Example A hammer hits a nail. The nail goes
  into the wood. The nail exerts a force on the
  hammer and stops it.

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Action and Reaction forces

• action and reaction forces do not cancel each
  other out because they are acting on different
  objects.
• Ex If you throw a tire (action) the tire pushes
  back on you and you fall backwards (reaction)
• Ex When you walk you push on the ground
  (action) and the ground pushes back on you
  (reaction)

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How do Rockets lift off?

• Newtons 3rd Law is in effect.
• As gasses are forced out of a rocket, they exert
  an equal and opposite force on the rocket. This
  is called THRUST!

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What is Momentum?

• momentum is the quantity of motion
• Momentum Mass x velocity
• The more momentum an object has the harder it is
  to stop or the more damage it will do.
• Example
• What has more momentum?
• Hammer A 3 kg swung at 1.8m/s 5.4 kg m/s
• Hammer B 4 kg swung at .9 m/s 3.6 kg m/s
• Hammer A (velocity is almost twice as fast
  greater momentum)

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Match the animation to the statement.

• 1. The purple and green spheres are of equal
  mass.
• 2. The purple sphere has a greater mass than the
  green sphere.
• 3. The green sphere has a greater mass than the
  purple sphere.

A.
B.
C.
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What is the Law of Conservation of Momentum?

• the total momentum of objects that interact does
  not change.
• the total momentum of any group of objects
  remains the same unless outside forces act on
  those objects.

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Calculate the total momentum in each situation
below both before and after the train cars meet.
Show your work here.
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Orbiting Satellites

• What is a Satellite?
• A satellite is any object that travels around
  another object in space.
• When a satellite is in circular motion, there is
  a force necessary to keep it accelerating. The
  force is called Centripetal Force.
• For a satellite, the centripetal force is gravity
  which tries to pull it to the center of the earth.

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• Satellite Motion
• in order for a satellite to orbit the earth, it
  must be thrown or launched around the earth at
  7900 m/s (200 x faster than a pitcher can throw a
  ball.)
• it falls around the earth.
• at this speed it will (because of its inertia)
  follow the curve of the earth, gravity will pull
  it toward the earth, inertia will keep it in a
  circular path.