Forces and Motion

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Forces and Motion

• Objective 8.7A Demonstrate how unbalanced forces
  cause changes in the speed or direction of an
  objects motion

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Force

• Any push or pull on an object
• Measured in Newtons (N)
• A spring scale is the instrument used to measure
  force

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Types of Forces

• Gravity, Friction, Electric force, Magnetic
  force, Buoyant force, Centripetal force, Lift,
  Drag, Thrust

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Net Force

• Sum of the forces acting on an object.

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Balanced Forces

• Do not cause a change in motion
• They are equal in size and opposite in direction

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Unbalanced Forces

• Always cause a change in motion
• They are not equal and opposite

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Unbalanced forces
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Newtons 1st Law of Motion

• An object at rest will stay at rest unless acted
  on by an unbalanced force.
• An object in motion will stay in motion at the
  same speed and direction unless acted on by an
  unbalanced force.

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In other words

• An object will keep doing what it is doing unless
  forces become unbalanced
• Also known as the Law of Inertia
• Inertia An objects tendency to resist change

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Law of Inertia (1st law)
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Newtons 2nd Law of Motion

• The acceleration of an object by a force in
  inversely proportional to the mass of the object
  and directly proportional to the force applied.

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In other words

• The smaller the object, the greater the
  acceleration AND the larger the object, the
  smaller the acceleration.
• The smaller the force applied, the smaller the
  acceleration AND the larger the force applied the
  larger the acceleration.

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Therefore

• Force Mass X Acceleration
• 1,000 kg X .05 m/s/s
• Force 50 Newtons
• Practice Fma

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Acceleration

• The rate of change in the velocity, speed and/or
  direction, of an object.
• The acceleration of the overall motion of an
  object that is subjected to an unbalanced
  external force is equal to that force divided by
  the mass of the object
• Acceleration Force/Mass
• Measured in meters/sec2

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Newtons 3rd Law of Motion

• For every action, there is an equal but opposite
  reaction

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In other words

• All forces act in pairs.
• When one object exerts a force on a second
  object, the second object exerts a force back
  that is equal in size but opposite in direction

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Newtons 3rd Law
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Space Shuttle Launch
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Speed

• How fast an object is moving
• Rate at which an object covers a distance
• Speed distance/time

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Calculating Speed

• While on vacation, Lisa Carr traveled a total
  distance of 440 miles. Her trip took her 8
  hours. What was her average speed?
• Speed 440 miles/8 hours
• Average speed 55 miles/hour

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Whats the difference between Speed and Velocity?

• Velocity - The rate at which an object changes
  position. It the measurement of the rate and
  direction of motion.

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Speed, Velocity and Acceleration Explained
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Work

• Occurs when a force is used to move an object
  through a distance
• Measured in Joules(J) Newtons/meter
• Work Force (N) X Distance (m)
• How much work is done if a force of 20N is used
  to displace an object 3m?
• Work (20N) X (3m)
• Work 60 Joules (J)

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8th Grade Science Formula Chart

• Work force X distance (W fd)
• Speed distance/time (s d/t)
• Force mass X acceleration (F ma)
• Weight mass X acceleration due to gravity
  Weight mg)
• Density mass/volume (D m/v)

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Constants and Conversions

• g acceleration due to gravity 9.8
  m/s2
• Speed of light 3 X 108 m/s
• 300,000,000 m/sec
• 300,000 km/sec
• Speed of sound 343 m/s at sea level and 20
  degrees celcius
• 1 cm3 1 mL

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Use Your Formula Chart!!!

• Practice Problems