Ch. 8 Forces

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Ch. 8 Forces
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Ch. 8 Sect. 1 Newtons 1st Law

• A force is a push or pull on an object. When one
  object pushes or pulls another object, we say it
  exerts a force on the other object.
• Forces are described by how strong they are AND
  in what direction.
• Examples of forces
• Pushing a lawn mower, Lifting a book, Throwing a
  ball

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Nature of Forces

• Net Force overall force on an object (the sum
  of all the forces)
• Unbalanced Forces forces that change an objects
  motion
• start moving (speed up), stop moving (slow down),
  or change direction.
• If the net force does NOT equal zero, then it is
  unbalanced.

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Nature of Forces (cont.)

• Balanced Forces forces that will not change an
  objects motion
• Equal forces acting on an object in opposite
  directions result in a balanced force.
• If the net force equals zero, then it is balanced.

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Newtons First Law

• An object at rest will remain at rest, and an
  object in motion will remain in motion with a
  constant velocity unless acted on by an
  unbalanced force.
• This concept can be shortened into one word
• INERTIA!

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Newtons 1st Law (cont.)

• Inertia
• the tendency of an object to resist change in
  motion
• Imagine riding in a car and stopping suddenly at
  a red light. You continue moving forward in your
  seat due to inertia.
• Inertia depends on mass
• The greater an objects mass, the more inertia it
  has.

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Friction

• Friction is another type of force.
• Friction is the force that resists sliding
  between 2 touching surfaces or through air or
  water.
• Friction SLOWS DOWN an objects motion.
• Friction depends on two factors
• The types of surfaces involved
• How hard the surfaces push together

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Friction (cont.)

• There are 3 types of friction
• Static prevents objects from moving when a
  force is applied.
• Sliding due to the microscopic roughness of two
  surfaces.
• Rolling between the ground and a wheel and
  allows the wheel to roll.

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Ch. 8 Sect. 2 Newtons 2nd Law

• The net force on an object is equal to the
  product of its acceleration and its mass.
• In other words
• Force equals mass times acceleration
• F ma
• Unit of Force Newton (N)
• Named after Sir Isaac Newton
• 1N 1kg (mass) x 1m/s2 (acceleration)

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Gravity

• Gravity is a type of force.
• All objects exert a gravitational force on their
  environment, but the gravity we most often refer
  to is that of the Earth.
• The gravitational force of the Earth is what
  keeps us grounded.

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Gravity (cont.)

• The gravitational force is also called weight!
  On earth, weight is equal to the mass of an
  object times the force of gravity (9.8m/s2)!
• Terminal velocity is reached when the force of
  gravity is balanced with the force of air
  resistance.
• Size of air resistance depends on
• Shape of the object
• Speed of the object

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Other Forces

• Centripetal force when in circular motion, the
  force is always perpendicular to the motion!
• Gravitron ride at the fair! In which direction
  are you feeling the force?

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Newtons 2nd Law Practice

• A 40kg wagon is being pulled by a boy. The force
  the boy exerts on the wagon causes it to
  accelerate at 2m/s2. What is the force exerted
  on the wagon?
• Fma
• F(40kg)(2m/s2)
• F80N

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Newtons 2nd Law Practice

• A woman uses a 70N force to push a 35kg shopping
  cart. What is the resulting acceleration?
• Fma
• aF/m
• a(70N) / (35kg)
• a2m/s2

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Newtons 2nd Law Practice

• A boy throws a ball by exerting a 10N force. The
  ball accelerates at 10m/s2. What is the mass of
  the ball?
• Fma
• mF/a
• m(10N) / (10m/s2)
• m1kg

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Ch. 8 Sect. 3 Newtons 3rd Law

• If one object exerts a force on another object,
  then the second object exerts an equal and
  opposite force on the first.
• Every action has an equal and opposite
  reaction. Action-reaction
• The law of MOMENTUM!

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Newtons 3rd Law (cont.)

• If you push on a wall with 250N of force, then
  the wall pushes back with 250N of force.
• Action reaction forces act on different objects
  and
• When the mass of one object is considerably
  larger than the mass of another object, the
  action-reaction force is not noticeable.
• Air and water exert action-reaction forces with
  objects such as hands or canoe paddles.
• A rocket launches due to the equal but opposite
  forces of the burning fuel.