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					How can NASA provide a sensation of
weightlessness without the expense and high risk
of sending astronauts into space?
                  Apparent Weight

Our perception of weightlessness is really the absence or
reduction of normal (contact) forces acting upon our body.

Conversely, we can feel heavier if the normal (contact) forces
acting on us are larger than normal. Think about the forces you
experienced on the Demon Drop or Power Tower at Cedar Point
            Apparent Weight in an Elevator
Now suppose you stand on the bathroom scale and ride an
elevator up and down. As you are accelerating upwards and
downwards, the scale reading is different than when you are at
rest and traveling at constant speed.




  Which situation does Anna Litical feel the
  heaviest? Which situation does she feel the
  lightest?
Apparent Weight in an Elevator - Graphs



               #3        #5


WT        #2        #4

     #1                       #6
     Apparent Weight in an Elevator - Graphs




FN                #3        #5


wT
             #2        #4

        #1                            #6



                                 Time (s)
     Apparent Weight in an Elevator - Concepts
     A body at rest wants to stay at rest so when the elevator starts to move
#1   down there is less force pushing on your feet.

     There is no change in motion so the upwards push on your feet must be
#2   equal and opposite to your weight


#3   Your body wants to keep moving at constant speed but the elevator is
     slowing down so more force is exerted on your feet


#4   The elevator is at rest so the force on your feet must be equal and
     opposite to your weight


#5   Your body wants to remain at rest but the elevator starts to move upwards.
     This imparts more upwards force on your feet than your weight


#6   Your body wants to move upwards at constant speed but the
     elevator slows down so there is less force on your feet
                    Apparent Weight in an Elevator
                    Coming Down - Concept FBD’s




                                                                        a                     FNET
                    a                     FNET




Elevator at rest        Elevator starts          Elevator at constant       Elevator is slowing
                        accelerating             speed downwards            down to a stop while
Zero Acceleration       downwards                                           moving down
                                                 Zero Acceleration
FNET = 0                FNET = m a (down)                                   FNET = m a (up)
                                                 FNET = 0
FN = Fg                 FN < Fg                                             FN > Fg
                                                 FN = Fg


  The normal Force (orange) is what she feels
                   Apparent Weight in an Elevator
                         Coming Down - Theory




                                                          a                     FNET
               a                    FNET




FNET = 0           FNET = m a (down)       FNET = 0           FNET = m a (up)
FN = Fg = mg       FNET = Fg - FN          FN = Fg = mg       FNET = FN - Fg

Weigh normal       mg - FN = ma            Weigh normal       FN -mg = ma

                   FN = mg - ma                               FN = ma + mg

                   FN = m(g - a)                              FN = m(g + a)

                   Weigh less                                 Weigh more
                    Apparent Weight in an Elevator
                         Going Up - Concept FBD’s




                    a               FNET                              a               FNET




Elevator at rest        Elevator starts        Elevator at constant       Elevator is slowing
                        accelerating upwards   speed upwards              down to a stop while
Zero Acceleration                                                         moving up
                        FNET = m a (up)        Zero Acceleration
FNET = 0                                                                  FNET = m a (down)
                        FN > Fg                FNET = 0
FN = Fg                                                                   FN < Fg
                                               FN = Fg
                   Apparent Weight in an Elevator
                              Going Up - Theory




               a             FNET                    a                   FNET




FNET = 0           FNET = m a (up)
                                      FNET = 0       FNET = m a (down)
                   FNET = FN - Fg
FN = Fg = mg                          FN = Fg = mg   FNET = Fg - FN
Weigh normal       FN -mg = ma
                                      Weigh normal   mg - FN = ma
                   FN = ma + mg
                                                         FN = mg - ma
                   FN = m(g + a)
                                                         FN = m(g - a)
                   Weigh more
                                                     Weigh less

				
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posted:9/13/2012
language:English
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