Motion and Forces

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Motion and Forces
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LETS START WITH MOTION

• How can you prove that an object is moving?

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MOTION

• Motion- An object is in motion when the distance
  from another object is changing.
• Reference Point- A place or object used for
  comparison to determine if something is in
  motion.
• An object is in motion if it changes position
  relative to a reference point.

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CALCULATING SPEED

• Speed Distance / Time
• If you measure distance in meters and time in
  seconds you express speed in m/s
• Give an example of the speed a snail could
  travel.
• Constant speed-When the speed of an object does
  not change
• Average speed- Total distance / total time

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MOTION

• Velocity- Speed and direction
• Acceleration- Rate at which velocity changes
  (increasing speed, decreasing speed, changing
  direction) m/s/s
• EXAMPLES OF ACCELERATION???
• Accelerationfinal velocity- initial velocity /
  time

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Acceleration problems

• Suppose a car moving in a straight line steadily
  increases its speed each second, first from 35 to
  40 km/hr, then from 40 to 45 km/hr, then from 45
  to 50 km/hr. What is its acceleration?

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Acceleration problems continued

• In 5 seconds a car moving in a straight line
  increases its speed from 50 km/hr to 65 km/hr
  while a truck goes from rest to 15 km/hr in one
  second. Which undergoes the greatest
  acceleration? What is the acceleration of each
  vehicle?

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Think about this

• The Cat bus is approaching Lone Mountain at 144
  cm/s. A person on the bus is facing forward and
  walking 36 cm/s relative to everything in the
  bus. The person is eating a hotdog which is
  entering his mouth at 2 cm/s. An ant on the
  hotdog is running away from the persons mouth at
  1cm/s. How fast is the ant approaching Lone
  Mountain?

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NEWTONS FIRST LAW OF MOTION

• Law of Inertia
• An object at rest will remain at rest and an
  object that is moving will continue moving unless
  acted upon by an unbalanced force.
• What do you think an unbalanced force is?

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Unbalanced Force
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UNBALANCED FORCE

• Unbalanced forces cause an object to start
  moving, stop moving, or change direction. (an
  unbalanced force will cause an object to
  accelerate)
• Net force- The overall force on an object after
  all the forces are added together

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BALANCED FORCES

• Balanced forces are equal forces acting in an
  opposite direction.
• Balanced forces do not cause acceleration.

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A force is just a push or a pull
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MOMENTUM!!!

• Mass x Velocity Momentum (P)
• Increase the mass or the velocity to get more
  momentum.

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Conservation of Momentum

• Newtons Balls (Newtonian Demonstration)
• Pool Table

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NEWTONS SECOND LAW OF MOTION

• Force, Mass, and Acceleration

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UNITS

• Mass- kg which equals 1000 grams (g)
• Acceleration- m/s/s
• Force- kg x m/s/s or Newton (N)
• SAMPLE PROBLEM
• A 52 kg water-skier is being pulled by a
  speedboat. The force causes her to accelerate at
  2m/s/s. Calculate the net force that causes this
  acceleration.

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FRICTION

• Friction- The force that one surface exerts on
  another when the two rub against each other.
• Two factors- Type of surface and how hard they
  rub together.
• Sliding friction- dry solid surfaces
• Rolling friction- rolling over a surface
• Fluid friction- object moves through a liquid or
  gas.

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LAW OF UNIVERSAL GRAVITATION

• Law of universal gravitation states that the
  force of gravity acts between all objects in the
  universe.
• The strength of the force depends on the masses
  of the objects and the distance from them

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GRAVITY

• Free fall- when the only force acting on a
  falling object is gravity. (9.8m/s/s)
• Do all objects regardless of mass fall at this
  rate? (water cup demo)
• What about projectiles?

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GRAVITY

• Air resistance- a type of fluid friction that
  objects falling through air experience.
• Paper vs.. Book
• Weight- The measure of the force of gravity on an
  object (scale)
• Mass- is a measure of the amount of matter in
  that object.

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WEIGHT DUE TO GRAVITY

• Remember Fma???
• Weight mass x acceleration due to gravity
• Weight mass(kg) x 9.8 m/s/s
• The amount of force you apply to the ground in
  Newtons

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NEWTONS THIRD LAW OF MOTION

• ACTION AND REACTION

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EVERY ACTION HAS AN EQUAL AND OPPOSITE REACTION!!!
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3rd law only refers to forces on 2 different
objects
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What about circular motion???

• Bucket Demonstration
• What is the centripetal force for a satellite?
• Centripetal- center seeking
• Centripetal force- any force that causes an
  object to move in a circle.

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