Chapter 8 Section 3 Notes

1
Chapter 8 Section 3 Notes

• Motion and Force

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What is Force?

• A force is the cause of acceleration or change in
  an objects velocity

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Fundamental Forces

• There are 4 fundamental forces in nature
• Gravity
• Electromagnetic force
• Strong nuclear force
• Weak nuclear force

4
Fundamental Forces

• Gravity very, very weak force this force acts
  over a long distance much weaker than the
  electromagnetic force field force
• Electromagnetic force moderate force acts over
  long distances about 1/100 the strength of the
  strong force field force

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Fundamental Forces

• Strong nuclear force strongest of all forces,
  but acts over a very short distance only acts
  over distances the size of the atomic nucleus
• Holds together protons and neutrons in the nuclei
  of atoms
• Weak nuclear force associated with nuclear decay

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Fundamental Forces

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Types of Forces

• Contact forces caused when objects touch
• Examples when you push a shopping cart or catch
  a baseball and it stops moving
• Field forces do not require that objects touch
  each other
• Examples attraction of gravity or repulsion
  between 2 north poles of a magnet
• Both contact and field forces can cause an object
  to move or stop moving!

8
Balanced and Unbalanced Forces

• Net force the sum of all the forces acting on an
  object
• Whenever there is a net force acting on an
  object, the object accelerates in the direction
  of the net force.

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Balanced and Unbalanced Forces

• An object will not accelerate if the net force is
  equal to zero.
• Balanced forces net force of zero
• Do not cause a change in motion of an object.

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Balanced and Unbalanced Forces

• Example of Balanced Forces
• When 2 teams are playing tug of war and both
  teams are exerting equal amounts of pressure,
  there is no acceleration, and neither team moves.
  

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Balanced and Unbalanced Forces

• Unbalanced forces occurs when forces acting on
  an object dont cancel out there is a greater
  force acting on 1 side of the object
• Example When 2 teams are playing tug of war and
  one team exerts a greater force and pulls the
  other team forward

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Balanced and Unbalanced Forces

• When an object is at rest, are there still forces
  acting on it?
• Yes! It is just not moving because the forces
  are balanced.
• What happens if forces act in different
  directions that are not opposite one another?
• The combination of forces acts like a single
  force and cause acceleration in a combined
  direction
• Example You push your book east and your friend
  pushes the book north the book will move
  northeast

13
Friction

• Friction force between 2 objects that opposes
  the motion of either object
• Example Imagine a car rolling on a flat, evenly
  paved street. Eventually, the car will stop due
  to the force of friction.
• Friction depends on surfaces in contact
• Frictional forces are greater when both surfaces
  are rough
• Example new tennis shoes walking along a
  carpeted floor will keep you from falling down.

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Friction

• Two types of friction
• Static Friction when 2 objects touch but do not
  move
• Example A filing cabinet sitting on the floor

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Friction

• Kinetic Friction friction between moving
  surfaces. Two types of kinetic friction
• Sliding friction when 1 object slides over
  another
• Example Sliding a box across the counter
• Rolling friction when 1 object rolls across
  another
• Example Rollerblading

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Friction

• The force necessary to make a stationary object
  start moving is usually more than the force
  necessary to keep it moving therefore, static
  friction is usually greater than kinetic
  friction.
• Rolling friction is usually less than sliding
  friction.

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Friction and Motion

• Friction is not all bad it is actually required
  for many everyday tasks to work correctly.
• Examples
• Cars could not move without friction as a cars
  wheels turn, they push against the road and as a
  reaction, the road pushes forward on the car.
• You would not be able to write down these notes
  either!

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Friction and Acceleration

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Air Resistance

• Air Resistance a type of friction
• Air Resistance is caused by the interaction
  between the surface of a moving object and the
  air molecules
• Air Resistance depends on 3 factors
• Size of the object
• Shape of the object
• Speed of the object

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Gravity

• Gravity force of attraction between 2 particles
  of matter due to their mass
• Gravity is different from other forces because
  gravity acts on an object even when the objects
  dont touch
• All objects in the universe attract each other
  through the force of gravity!

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Gravity

• Gravity depends on two factors
• The mass of the objects the greater the mass of
  the object, the larger the gravitational force
• The distance between the objects as the distance
  between 2 objects increases, the gravitational
  force decreases.