What is Force

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What is Force?

• Chapter 5 Section 2

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What Is a Force?

• You often hear the word force in everyday
  conversation
• "That storm had a lot of force!"
• "Our basketball team is a force to be reckoned
  with."
• " A flat tire forced me to stop riding my
  bicycle."
• "The inning ended with a force-out at second
  base."

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What is a Force?
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What Is a Force?

• But what exactly is a force?
• In science, a force is simply a push or a pull.
• All forces have both size and direction.
• Forces are everywhere.

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What Is a Force?

• In fact, any time you see something moving, you
  can be sure that its motion was created by a
  force.
• Scientists express force using a unit called the
  Newton (N).
• The more Newtons, the greater the force.

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Forces Act on Objects

• All forces are exerted by one object on another
  object.
• For any push to occur, something has to receive
  the push.
• You can't push anything!

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Forces Act on Objects

• The same is true for any pull.
• When doing finger painting, you use your fingers
  to pull and push the paint around on the paper.
• Your fingers are exerting forces on the paint and
  the paper.

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Forces Act on Objects

• Just because a force is being exerted by one
  object on another doesn't mean that motion will
  occur.
• For example, you are sitting on a chair, but the
  force you are exerting on the chair does not
  cause the chair to move.

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Forces Act on Objects

• That's because the Earth is also exerting a force
  on the chair.
• In most cases, it is easy to determine where the
  push or pull is coming from.

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Forces Act on Objects

• 'It is not always so easy to tell what is
  exerting a force or what is receiving a force.
• You cannot see what exerts the force that pulls
  magnets to refrigerators, and the air you breathe
  is an unseen receiver of a force called gravity.

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Forces in Combination

• Often more than one force is exerted on an object
  at the same time.
• The net force is the force that results from
  combining all the forces exerted on an object.
• So how do you determine the net force?

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Forces at an Amusement Park
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Forces in the Same Direction

• Suppose you and a friend are asked to move a
  piano for the music teacher.
• To do this, you pull on one end of the piano, and
  your friend pushes on the other end.
• Together, your forces add up to enough force to
  move the piano.

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Forces in the Same Direction

• This is because your forces are in the same
  direction.
• Because the forces are in the same direction,
  they can be added together to determine the net
  force.
• In this case, the net force is 45 N, which is
  plenty to move a piano-if it is on wheels, that
  is!

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Forces in Different Directions

• Consider two dogs playing tug of war with a short
  piece of rope.
• Each is exerting a force, but in opposite
  directions.
• Notice that the dog on the left is pulling with a
  force of 10 N and the dog on the right is pulling
  with a force of 12 N.
• Which dog do you think will win the tug of war?

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Forces in Different Directions

• Because the forces are in opposite directions,
  the net force is determined by subtracting the
  smaller force from the larger one.
• In this case, the net force is 2 N in the
  direction of the dog on the right.
• Give that dog a dog biscuit!

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Unbalanced and Balanced Forces

• If you know the net force on an object, you can
  determine the effect the force will have on the
  object's motion. Why?
• The net force tells you whether the forces on the
  object are balanced or unbalanced.

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Unbalanced Forces Produce a Change in Motion

• When the net force on an object is not zero, the
  forces on the object are unbalanced.
• Unbalanced forces produce a change in motion
  (acceleration).

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Unbalanced Forces Produce a Change in Motion

• Unbalanced forces are also necessary to change
  the motion of moving objects.
• For example, consider a soccer game.

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Unbalanced Forces Produce a Change in Motion

• The soccer ball is already moving when it is
  passed from one player to another.
• When the ball reaches the second player, the
  player exerts an unbalanced force-a kick-on the
  ball.
• After the kick, the ball moves in a new
  direction and with a new speed.

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Unbalanced Forces Produce a Change in Motion

• An object can continue to move even when the
  unbalanced forces are removed.
• A soccer ball receives an unbalanced force when
  it is kicked.
• However, the ball continues to roll along the
  ground long after the force of the kick has
  ended.

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Forces During a Crash
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Balanced Forces Produce No Change in Motion

• When the forces applied to an object produce a
  net force of zero, the forces are balanced.
• Balanced forces do not cause a nonmoving object
  to start moving.
• Balanced forces will not cause a change in the
  motion of a moving object.

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Balanced Forces Produce No Change in Motion

• A light hanging from the ceiling does not move
  because the force of gravity pulling down on the
  light is balanced by an elastic force due to
  tension that pulls the light up.

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Balanced Forces Produce No Change in Motion

• A bird's nest in a tree and a hat resting on your
  head are also examples of objects with only
  balanced forces acting on them.

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