Transcript Apparent Weight (PowerPoint)

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
WT
#2
#1
#5
#4
#6
Apparent Weight in an Elevator - Graphs
FN
#3
#5
wT
#2
#1
#4
#6
Time (s)
Apparent Weight in an Elevator - Concepts
#1
A body at rest wants to stay at rest so when the elevator starts to move
down there is less force pushing on your feet.
#2
There is no change in motion so the upwards push on your feet must be
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
Zero Acceleration
Elevator starts
accelerating
downwards
FNET = 0
FNET = m a (down)
Elevator at rest
FN = Fg
FN < Fg
FNET
a
Elevator at constant
speed downwards
Zero Acceleration
FNET = 0
FN = Fg
The normal Force (orange) is what she feels
Elevator is slowing
down to a stop while
moving down
FNET = m a (up)
FN > Fg
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
Elevator at rest
Zero Acceleration
FNET = 0
FN = Fg
a
FNET
Elevator starts
accelerating upwards
Elevator at constant
speed upwards
FNET = m a (up)
Zero Acceleration
FN > Fg
FNET = 0
FN = Fg
FNET
Elevator is slowing
down to a stop while
moving up
FNET = m a (down)
FN < Fg
Apparent Weight in an Elevator
Going Up - Theory
a
FNET = 0
FN = Fg = mg
Weigh normal
a
FNET
FNET = m a (up)
FNET = FN - Fg
FNET = 0
FNET = m a (down)
FN = Fg = mg
FNET = Fg - FN
Weigh normal
mg - FN = ma
FN -mg = ma
FN = ma + mg
FN = m(g + a)
FNET
FN = mg - ma
FN = m(g - a)
Weigh more
Weigh less