KD3 Linear Mechanics

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KD3 Linear Mechanics

• Chapters 4, 5 6

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Force

• Force- A push or pull which can change an
  objects state of rest or motion
• (if the force is unbalanced)
• Measured in Newtons (N)

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Force is measured in Newtons

• Newton-force that causes a mass of 1 kg to
  accelerate at a rate of 1.0 m/s²
• 1 N .225 lb or 1 lb 4.45 N

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4 Types of Force

• 1. Gravitational Force-An attractive force that
  exists between all objects
• Ex) Earth and the Moon
• 2. Electromagnetic Force-The force involved in
  electric and magnetic fields
• Ex) Friction

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4 Types of Forces Cont..

• Strong Nuclear Force-Holds particles in the
  nucleus together
• Weak Force-A form of electromagnetic force
  involved in the radio active decay of some nuclei

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Newtons First Law (Law of Inertia)

• An object at rest stays at rest, an object in
  motion stays in motion unless acted on by an
  unbalanced force

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Inertia

• Inertia - objects resistance to a change in
  motion
• NOT a force!
• All objects have inertia
• More massive objects have more inertia than less
  massive objects

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Newtons 2nd Law

• The acceleration of a body is directly
  proportional to the net force on it and inversely
  proportional to the mass

F m a
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Newtons 2nd Law
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Mass vs. Weight

• Mass-The amount of matter stuff from which a
  body is made
• Measured in kg
• Never changes regardless of gravity
• (can change if
• removed)

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Mass vs. Weight

• Weight- Mass times the acceleration due to
  gravity
• Measured in N (kg m/s²)
• Refers to the gravitational force exerted by a
  large body (Ex. Earth) on an object
• Varies depending on where you are (elevation,
  different planet)

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Weight

• Weight is a force
• Measured in Newtons

Force wt m g
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Example

• A person with a mass of 50 kg has what weight on
  Earth?
• Wt. m g
• Wt. 50 kg (9.8 m/s2)
• Wt. 500 N

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Example

• A person weighting 160 lbs, has a mass of what?
• (1 lb 4.45 N)

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Law of Universal Gravitation

• A force of attraction (gravity) exists between
  any 2 objects

F G m1 m2 r²
F Force (N) G 6.67 X 10-11 Nm²/kg² m mass
(kg) r distance (m)
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Inverse Square Law

• The gravitational force between any 2 objects
  varies inversely to the square of the distance
  between them
• (more distance less attraction)
• The gravitational force between any 2 objects
  varies directly to the mass of the objects
• (more massive more attractive)

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Law of Universal Gravitation

• What happens to the force when the mass of 1
  object doubles?
• What happens when the masses of both double?
• What happens when the distance doubles?

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Newtons 3rd Law (Action-Reaction)

• When 1 object exerts a force on a 2nd object, the
  2nd object exerts a force on the 1st that is
  equal in magnitude but opposite in direction

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Momentum

• Momentum-Inertia in motion
• Vector
• Depends on the mass AND velocity of an object
• Momentum (p) mass X velocity

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Impulse (Ft)

• Impulse Change in momentum
• Product of net force and the time interval over
  which it acts
• F ?t ?p OR F ?t m ?v

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Impulse (Ft)

• An equal change in momentum can be achieved by
  EITHER
• 1. A large force for a short period of time
• 2. A small force for a long time

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Work and Kinetic Energy

• Work-Product of the force exerted on an object
  and the distance the object moves in the
  direction of the force

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Work

• Work (W) F d
• OR
• Work (W) F d (cos T)
• Work is measured in Joules (J)
• 1 J 1 N meter

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Power

• Power- The rate of doing work
• Power Work/time
• Measured in Watts
• Watt 1 Joule of energy transferred in 1 second

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Energy

• Energy- The ability to do work!

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Kinetic Energy

• Kinetic Energy-The energy of an object due to
  motion

KE ½ m v²
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Work/Energy Theorem

• The net work done on an object is equal to its
  change in kinetic energy

F d ½ m v²
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Gravitational Potential

• Stored energy of position
• GPEmgh
• Measured in Joules