Forces

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					What is a Force?

 A force is a push or a pull causing
  a change in velocity or causing
             deformation.
Dynamics
   What is dynamics?
   Dynamics is the study of the
    relationship between force and motion.
How are contact forces different
from long range forces?
   Contact forces - touching - example:
    hand on desk

   Long range forces - act at a distance -
    gravitational forces, electrostatic,
    magnetic
What is Newton’s First Law of
Motion?
   Law of Inertia: tendency to resist
    change
   An object at rest remains at rest, and an
    object in motion remains in motion
    (same direction, constant speed) unless
    acted upon by an unbalanced force.
   Forces are balanced: Equilibrium
Newton’s 1st Law - Types of
Equilibrium

   1. Static Equilibrium
       object is at rest
   2. Dynamic Equilibrium
       object is moving at constant velocity
What is Newton’s Second Law
of Motion?
   Unbalanced forces produce
    acceleration
   **Acceleration is directly proportional to
    force and inversely proportional to mass
   Fnet = ma
   a = Fnet/m in reference table
   Units 1 kgm/s2 = 1 Newton
Example:
   An artillery shell with a mass of 5.5 kg is
    fired from a gun with a velocity of 770
    m/s. The barrel of the gun is 1.5 m
    long. Find the force on the shell.
  Solution:
m = 5.5 kg
v f = 770 m/s
v i = 0 m/s
d = 1.5 m
Special Case - weight
   W = mg is a special case of F = ma,
    where weight is the force, and gravity is
    the acceleration (can be written as Fg)

   Example: What is the weight of a 5 kg
    object?
Two Kinds of Mass
   1. Gravitational mass - using a balance
    - comparing the gravitational force on
    two objects (one has a known mass).
   2. Inertial mass - using Newton’s 2nd
    law - find the force necessary to
    produce a specific acceleration for a
    given mass
   Gravitational mass = inertial mass
Graph: Inertial mass


F

                Slope is mass

       a
 Free Body Diagrams
    Used to show forces on an object
    Sketch and Equations:

                     What is Fnet?

Fnet is the sum of the forces
acting in the direction of motion
(vertical or horizontal)
Free Body Diagrams: Vertical

              Lifted object
              (Ftens is tension)
       Example:A 50 Newton force is used to lift a 2 kg
       object. What is the acceleration of the object?


Lift    50 N




        Weight
       Example:A 4500 kg helicopter accelerates
       upward at 2 m/s2. What lift force is exerted
       by the air on the propellers?

Lift   ?N




       Weight
Alternate Solution to
helicopter problem:
         What is Newton’s 3rd Law of
         Motion?
   For every action, there’s an equal and opposite reaction.
   The force on one object is equal and opposite to the
    force on the other.
   Object resting on a table: weight pushes back on table.

                       Table pushes back with an equal
                       and opposite force called Normal
                       force (FN). If no other vertical forces
                       are present: normal force equals
                       weight
Falling objects: what are the forces
on an object falling through the air?




                        Object in freefall
Object in freefall      (no air resistance)
(with air resistance)
        Falling Objects:
   What are the forces on an object falling is the
    air? What happens when they are equal?
   Drag force (Fair) - air resistance - depends on
    size/shape of the object, air density, speed.
   When drag force = weight, forces are
    balanced (Fnet=0)
   No net force means no acceleration! You
    have reached TERMINAL VELOCITY.
     Free Body Diagrams
        Free body diagram - horizontal motion
             FN

Ff
                  FA




              Fg or W
          What if….
   FA is greater than Ff
       acceleration is positive
   FA is equal to Ff then
       no acceleration (constant velocity)
   FA is less than Ff
       acceleration is negative (you can slow down, but friction
        won’t make you move backwards!)
Free Body Diagrams
   Free body diagram - horizontal motion
    with no friction:
Example:
   If you apply a 50 Newton force to a mass of 5
    kg, what is the acceleration?
Example (continued):
   What if a frictional force of 20 N opposes the applied
    force? Find a.
      What is friction?
Friction - force opposing motion between 2
  surfaces in contact and parallel to the surface
 How can you calculate frictional force?

 Equation:



        Ff = frictional force (N)
        FN = normal force (N)
        coefficient of friction (no units)
      What variables affect friction?
   Weight? YES
   Surface area? NO
   Type of surfaces in contact? YES
   Velocity? NO
   Normal force - force pushing objects together- is
    normal force always equal to weight?
   NO - they are not equal if other vertical forces are
    present.
What are two types of friction?
   1. Static - objects are NOT in relative motion
    - static friction opposes the START of motion
   2. Kinetic (sliding) - force between surfaces
    in motion - resists motion
   Which is greater?
   Static friction is greater - it is harder to start
    motion that it is to keep an object moving.
         Example #1:
   A block is place on a table. It has a weight of 50 N.
    You must exert a force of 20 N to keep the block
    moving at a constant velocity.
   1. What is coefficient of sliding friction?
   2. Place a 10 N brick on the block.
    What force is required to keep the
    block moving at constant velocity?

    Same surfaces, so  = 0.4
Example #2
   A 10kg box made of wood is pushed on
    a wood floor with a force of 100 N.
   (a.) Will the box slide?
   (b.) What force must be applied to
    move the box at a constant velocity?
   (c.) What force must be applied for the
    box to accelerate at a rate of 5 m/s2?
    Solution:
(a.) Will it move? It will move if the applied
  force exceeds the static frictional force.
              From reference table: static = 0.42




               Yes, 100N>41.2N, box will slide.
       Solution:
   (b). To move box at a constant velocity….. Use
    KINETIC friction.
From reference table: kinetic = 0.30
Since a = 0, then Fnet = ma =0
Solution:
   (c.) a = 5 m/s2, find FA

				
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