Newtons First Law

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Newtons First Law

• An object at rest will remain at rest, and an
  object in motion will remain in motion at
  constant velocity unless an unbalanced force acts
  on it.

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Newtons First Law

• In other words, things tend to keep doing what
  they are already doing.
• Inertiathe tendency of an object to resist a
  change in motion

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What is the effect of mass on inertia?

• The greater the mass, the greater the inertia.
• A heavier object in motion is more difficult to
  stop.
• A heavier object at rest is more difficult to
  move.

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Inertia

• A fly and an 18 wheeler are both moving at 5 mph,
  which one would you rather be hit by?
• Would you rather move an empty file cabinet, or a
  file cabinet that is full of paper?

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Faster objects are more difficult to stop

• The faster an object moves, the greater its
  momentum.
• Would you rather catch a baseball moving at 45
  mph or 300 mph?
• Thinking ahead. . . . Why do you
  think there would be greater
  pain involved in
  catching a 300 mph
  fastball versus a 45 mph fastball?

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Inertia Review

• If an object is moving, its tendency is to keep
  moving.
• If an object is at rest, its tendency is to
  remain at rest a force is needed to set it in
  motion.

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Question

• If an object is moving, its tendency is to keep
  moving in a straight line path. So why does the
  path of a baseball curve toward the ground?

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Inertia

• If you toss an object where there is no friction,
  such as in the vacuum of outer space, it will
  move forever in a straight-line path.

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Force and Motion

• A continual application of force is not needed in
  order to keep an object in motion.
• If a continual application of force is not
  needed, then why do you need to constantly keep
  your foot on the accelerator to keep your car
  moving forward?
• You need to apply a constant force so that your
  car can overcome air resistance and friction with
  the road.
• What do you feel if you stick your hand out the
  window as your car moves at a high speed? Your
  car experiences the same thing.

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Gravity and Inertia

• If the force of gravity were cut off, what would
  happen to a space shuttle that is orbiting the
  Earth?

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Newtons Second Law

• The acceleration of an object is directly
  proportional to the net force on the object and
  inversely proportional to the mass of the object.
• In other words. . . .
• The harder you push on an object, the more it
  accelerates. The heavier an object is, the
  harder you must push on it in order to cause it
  to accelerate.

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Examples
Exploding CartsEqual Mass
Exploding CartsUnequal Mass
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Newtons Second Law

• More massive objects are more difficult to
  accelerate.
• For the same force, twice as much mass will
  result in half as much acceleration.
• If the force on an object is doubled, the
  acceleration will double.
• If the force on an object is tripled, the
  acceleration will triple.

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Newtons Second Law

• Suppose you apply the same amount of force to two
  carts, one cart with a mass of 4 kg, and the
  other with a mass of 8 kg.
• Which cart will have a greater acceleration?
• How much greater will the acceleration be?

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Formulas

• Acceleration Force divided by mass
• OR a F/m
• F Force force has units of Newtons (N)
• Force is a push or a pull.

Units Force Newtons (N) Mass Kilograms
(kg) Acceleration m/s2
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• How does this explain why two objects of
  different masses accelerate toward the Earth at
  the same rate?

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Example 1

• I push a car with a force of 40N, if the cars
  mass is 3000 kg, what is its acceleration?

• F ma
• a F/m
• a 40N/3000kg
• a 0.013 m/s2

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Example 2

• I lift a box with a force of 20 N upward. If the
  box accelerates at 0.23 m/s2, what is the mass
  of the box?

• F ma
• m F/a
• m 20N/0.23m/s2
• m 86.96 kg

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Example 3

• With what force must I push on a 1000 kg car in
  order for it to accelerate at 2 m/s2?

• F ma
• F (1000kg)(2m/s2)
• F 2000 N

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Terminal Velocity

• Free fall -- objects fall and encounter no
  resistance
• Does freefall actually occur on Earth?

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Terminal Velocity

• NO. . . . Objects falling on earth are not in
  true freefall
• a lt 9/8m/s2
• In a vacuum, net force on a falling object equals
  weight.
• In air, net force is equal to the difference
  between the weight and the force of air
  resistance.

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Terminal Velocity

• Depends on
• Surface area of an object
• Density/mass of object
• Finish/surface of object
• As an objects velocity increases, air resistance
  increases.
• Velocity will increase until wind resistance is
  equal to the weight and the net force equals 0N.

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Terminal Velocity
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Mass versus Weight

• Mass
• A measure of the amount of matter in an object
• Units kilograms
• m F/a

• Weight
• The force on a body due to the gravitational
  attraction of another body.
• The force with which your body is pulled toward
  the earth.
• Units Newtons
• F ma So. . .
• Weight mg

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Example Problem

• You have a mass of 300 kg.
• What is your weight on Earth?
• What is your weight on the moon where gravity is
  1/6 that of Earth?

• W mg
• W 300kg(9.8m/s2)
• W 2940 N
• W mg
• W 300kg (1.63m/s2)
• W 489 N

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Question

• If you travel to the moon, does your weight
  change?
• If you travel to the moon does your mass change?
• How are mass and weight different?

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Newtons Third Law

• Whenever one object exerts a force on a second
  object, the second object exerts an equal and
  opposite force on the first.
• For every action, there is an equal and opposite
  reaction

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Newtons Third Law

• A force is part of a mutual interactionan
  interactionbetween one thing and another.
• When I push on the wall, what happens?
• As I exert a force on the wall, does the wall
  exert a force on me?
• What would happen if the wall did not exert an
  equal but opposite force back on me?

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• If a truck crashes into a car, the truck exerts a
  force on the car while the car exerts an equal
  force on the truck. The forces are equal in
  strength but opposite in direction.
• A car accelerates along a horizontal road.
  Strictly speaking, what is it that pushes the car?

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Forces Always Occur in Pairs
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Paired Forces

• Forces involved in any interaction are called the
  action and reaction forces.
• Action and reaction forces are equal in strength
  but opposite in direction.

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Questions

• Which exerts more force, the Earth pulling on the
  moon, or the moon pulling on the Earth?

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Question

• If an 18-wheeler hits a fly, which object feels a
  greater force?

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