Balanced and Unbalanced Forces Chp 5 Section 2

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Balanced and Unbalanced ForcesChp 5 Section 2
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Heres Tim Mobey on Force

• Brainpop

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• Suppose I asked you to move a heavy desk in the
  classroom. How would you move it?
• hmmmmmmmmmmm.....

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• You might get on one side of the desk and start
  pushing, Or you might grab the legs and start
  pulling.
• Either way, you would be using a force.
• Any push or pull is a force.
• The force you use in pushing or pulling the desk
  has a size.
• You could use a small amount of force or a lot of
  force. Forces differ in size.
• Another way forces differ is in direction.
• A force can be left or right, up or down.

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• The desk will not move by itself.
• We need to remember an object at rest tends to
  stay at rest and an object in motion tends to
  stay in motion with the same speed and in the
  same direction unless acted upon by an unbalanced
  force.
• A force is needed to put the desk in motion.

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• But what exactly is meant by the phrase
  unbalanced force?
• What is an unbalanced force?
• In pursuit of an answer, we will first consider
  a physics book at rest on a table top.

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• There are two forces acting upon the book.
• One force - the Earth's gravitational pull -
  exerts a downward force.
• The other force - the push of the table on the
  book (sometimes referred to as
• a normal force) - pushes upward on the book.

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• Since these two forces are of equal magnitude and
  in opposite directions, they balance each other.
• The book is said to be at equilibrium.
• There is no unbalanced force acting upon the book
  and thus the book maintains its state of motion.
• When all the forces acting upon an object balance
  each other, the object will be at equilibrium
• it will not accelerate.

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• Consider another example of a balanced force - a
  person standing upon the ground.
• There are two forces acting upon the person.
• The force of gravity exerts a downward force.
• The force of the floor exerts an upward force.

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• Since these two forces are of equal magnitude and
  in opposite directions, they balance each other.
• The person is at equilibrium.
• There is no unbalanced force acting upon the
  person and thus the person maintains its state of
  motion.

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• Now consider a book sliding from left to right
  across a table top.
• Sometime in the prior history of the book, it may
  have been given a shove and set in motion from a
  rest position.
• Or perhaps it acquired its motion by sliding down
  an incline from an elevated position.

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• Whatever the case, our focus is not upon the
  history of the book but rather upon the current
  situation of a book sliding across a table top.
• The book is in motion and at the moment there is
  no one pushing it to the right.

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• http//www.engineeringinteract.org/resources/parkw
  orldplot/flash/concepts/balancedandun.htm

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• (Remember a force is not needed to keep a moving
  object moving to the right.)

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• The force of gravity pulling downward and the
  force of the table pushing upwards on the book
  are of equal magnitude and opposite directions.
• These two forces balance each other.

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• Yet there is no force present to balance the
  force of friction.

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• As the book moves to the right, friction acts to
  the left to slow the book down.
• There is an unbalanced force and as such, the
  book changes its state of motion.

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• The book is not at equilibrium and subsequently
  accelerates.

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• Unbalanced forces cause accelerations.
• In this case, since the unbalanced force is
  directed opposite the object's motion, it will
  cause a deceleration (a slowing down of the
  object).
• To determine if the forces acting upon an object
  is balanced or unbalanced, an analysis must first
  be conducted to determine what forces are acting
  upon the object and in what direction.

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• If two individual forces are of equal magnitude
  and opposite direction, then the forces are said
  to be balanced.
• An object is said to be "acted upon by an
  unbalanced force" only when there is an
  individual force which is not being balanced by a
  force of equal magnitude and in the opposite
  direction .

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• So remember, even when the desk is not moving,
  forces are acting on it.
• The floor is actually pushing up on the desk.
• At the same time, the force of gravity is
  pulling down on it.
• If the desk is not in motion, the upward push is
  the same as the downward pull.
• The size of the forces is the same, but the
  directions of the forces are different

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• . Whenever forces are the same in size but
  opposite in direction,
• they are balanced forces.
• An object that is NOT in motion will never move
  if balanced forces act on it.

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• How can you move the desk?
• You can move it because you unbalance the forces.
• The size of your force is greater than the
  forces of air pushing on the other side of the
  desk.
• Unbalanced forces act on an object,
• the object will always move in the direction of
  the greater force.

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Forces Motion
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