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Forces

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					     Forces
•   Objectives
•   What is a force?
•   What is gravity?
•   What is friction?
•   What is pressure?
                   Forces
• Force is a push or a pull, that causes a change in
  the motion or shape of an object.

                              • Words that indicate
                                the use of a force:
                              • Pushing , Pulling,
                                Stretching, Squeezing,
                                Bending, and Falling.
                              • Force is measured in
                                units of: Newtons (N)
                  Forces
• Force is a push or a pull.
• Example:

       Electric
                  Forces
• Force is a push or a pull.
• Example:

 Gravitational
                  Forces
• Force is a push or a pull.
• Example:

    Magnetic
                    Forces
• Forces can be equal. (Balanced Forces):
• If the two girls push with the same amount of force
  on the stack of books,
• the books do not move
  due to balanced forces.
• Other examples:
• Book laying on a
  table, person sitting in
  a chair, or a light
  signal hanging from a
  power line.
                     Forces
• Forces can also be unequal. Unbalanced forces.
• When an unbalanced force is applied to an object
  motion is produced.
• Any object in motion is
  due to an unbalanced
  force.
• Motion requires a force.
• The overall amount of
  force used to move an
  object is called net
  force.
                     Forces
• Finding the net force applied to an object
  requires the sum of all applied forces.

• Forces that are
  applied in the same
  direction are added.
• Forces that are
  applied in opposite
  directions are
  subtracted.
            Forces
• Remember No motion = NO net force!
    net Force examples
• If Mary pushes with 260 newtons of force
  and Janet pushes 170 newtons of force, what
  is the net force on the books?
                                260 n
                               -170 n
                                 90 n
                               To the right
 Mary             Janet
    net Force examples
• If Mary pushes with 210 newtons of force
  and Janet pushes 235 newtons of force, what
  is the net force on the books?
                                235 n
                               -210 n
                                 25 n
                               To the left
 Mary             Janet
     net Force examples
• If a plane travels north with an engine thrust of 25N
  of force and hits a cross wind going east with 5N of
  force. What is the net force and overall direction of
  the plane?
                5N
                East               C=     a2 + b2

                                   C=     252 + 52
       25 N
       North           25.5 N
                       Northeast   C=     625 + 25

                                   C = 25.5 N
                 Gravity
• Gravity is a Universal Force.
• Gravitational Force causes every object to
  attract every other object;
• a ball falling to the ground
• the moon orbiting the Earth



  Greater Mass = greater force
                Friction
• Friction is a force that opposes motion.
• Friction acts in a direction opposite to the
  object's direction in motion.
• Without friction, the object would continue
  to move at a constant speed forever.
• There are 3 types of friction.
                Friction
• Static Friction
• In static friction the two surfaces are not
  moving past each other.

• Here the friction
  between the 2
  surfaces is so great
  that there is no
  motion.
                Friction
• Sliding Friction
• The opposing force of the two surfaces
  slides past each other.
• Friction is overcome so that there is motion.
                Friction
• Rolling Friction
• The opposing force of
  the two surfaces roll
  past each other.
• Friction is overcome
  so that there is motion.
          Force: F=ma
• Force is connected with Newton’s 2nd law of
  motion.


            F = ma
   Units: N = kg m
                 s 2
              Force: F=ma
• What is the force of impact of a 800kg car that
  is traveling at 10 m/s2 when it hits another car?
                                 • Force = ?
                                 • Mass (m) = 800kg
                                 • Acceleration (a) =
                                   10 m/s2
   Formula:       F = ma
   Substitution: F = (800kg) (10m/s2)
   Results:       F = 8,000 N
              Force: F=ma
• How much force is needed to accelerate a
  7000kg car at a rate of 1.5 m/s2 ?
                                • F= ?
                                • m = 7000kg
                                • a = 1.5 m/s2

   Formula:      F = ma
   Substitution: F = (7000kg) (1.5m/s2)
   Results:      F = 10,500 N
              Force: F=ma
• A weightlifter raises a 150kg barbell with an
  acceleration of 2.0 m/s2. How much force does
  he use to lift the weights?
                               • F= ?
                               • m = 150kg
                               • a = 2.0 m/s2

   Formula:      F = ma
   Substitution: F = (150kg) (2.0m/s2)
   Results:      F = 300 N
              Force: F=ma
• A bowler takes a 32kg ball and rolls it with an
  acceleration of 2.8 m/s2. How much force does
  the bowler use?
       • F= ?
       • m = 32kg
       • a = 2.8 m/s2
   Formula:       F = ma
   Substitution: F = (32kg) (2.8m/s2)
   Results:       F = 89.6 N
            Force: F=ma
• A dancer lifts his partner above his head with a
  force of 100N and an acceleration of 4.0 m/s2.
  What is the mass of the dancer?
       • m= ?
       • F = 100 N
       • a = 4.0 m/s2
Formula:     m = F/a
Substitution: m = 100N /4.0 m/s2
Results:     m = 25.0 kg
              Force: F=ma
• If a car with a mass of 2,500 kg hits a tree with a
  force of 500,000 N. What is the acceleration of
  the car as it hits the tree?
       • a= ?
       • m = 2,500 kg
       • F = 500,000 N
   Formula:       a = F/m
   Substitution: a = 500,000 N / 2,500 kg
   Results:       a = 200 m/s2
              Force: F=ma
• A rocket, with a mass of 20 kg, is shot into the
  air with 2,000 N of force. What is the
  acceleration of the rocket?
       • a= ?
       • m = 20 kg
       • F = 2,000 N
   Formula:       a = F/m
   Substitution: a = 2,000 N / 20 kg
   Results:       a = 100 m/s2
              Force: F=ma
• Another rocket is shot into the air with 2,000 N
  of force, but it’s acceleration was only 10 m/s2.
  What is the 2nd rocket’s mass?
       • m= ?
       • F = 2,000 N
       • a = 10 m/s2
   Formula:       m = F/a
   Substitution: m = 2,000 N / 10 m/s2
   Results:       a = 200 kg
                Pressure
• Pressure is the force per unit area.
• Pressure is the amount of force acting on an
  area.

• Each book exerts the
  same force on the table.
• However, which book
  exerts the greater
  pressure? Why?
                  Pressure
• Is it possible for a 1 pound force to exert the
  same amount of pressure as a 50 pound force?
• Yes, because P = F/A
• Notice that when a
  force acts on a
  smaller area the
  pressure is larger.
• And when a force is
  acting on a larger
  area, the pressure is
  smaller.
                 Pressure
• Pressure exerted by liquids and gases is known as
  Fluid pressure.
• 2 common forms of fluid pressure: Air & Water
  pressure

• Air pressure is
  measured with a
  barometer.
• Water pressure is
  measured with a
  manometer.
                   Pressure
• Air pressure changes with altitude.
• As the elevation increases the air pressure will decrease.
• As the elevation decreases the air pressure will increase.
                   Pressure
• Water pressure changes with depth.
• As the depth increases the water pressure will increase.
• As the depth decreases the water pressure will decrease.
  Bernoulli’s Principle
• This principle that states that as the speed of
  a fluid increases, the pressure decreases.




• This principle is what allows the wing of an airplane
  to produce lift and makes flight possible.
   Bernoulli’s Principle
• There are 4 forces on a plane.
• Gravity pulls the airplane down while lift works
  against gravity to push the plane up.

• The thrust of the
  plane’s engine
  pushes the plane
  forward, while drag
  tends to slow the
  plane down due to
  air resistance.

				
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posted:8/13/2011
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