Transcript Newton`s Laws on an Elevator (print version)

Newton’s Laws in an Elevator
Elevators are great devices for the study of
Newton’s First and Second Laws as during
a single trip the elevator moves at constant
velocity and at other times it accelerates!
Consider a trip from the first floor up to
the 10th floor.
Newton’s Laws in an Elevator
Consider a trip from the first floor up to
the 10th floor.
The complete motion might look
something like…
Newton’s Laws in an Elevator
Accelerating (slowing down)
There are three parts
to the motion
Constant Velocity (no acceleration)
Accelerating (speeding up)
Newton’s Laws in an Elevator
Draw a force diagram of the rider (not the elevator!)

FN
FN  Fg

a

Fg
Accelerating (speeding up)
Since the elevator and
the rider are accelerating
upward, the net force
must act upward
How would you feel?
Heavier, lighter or normal?
Heavier!
You don’t feel the force of
gravity, rather you feel how
much the floor is supporting
you!
Newton’s Laws in an Elevator


FNet  ma

FNet  FN  Fg

FN

a

Fg
Accelerating (speeding up)

FN  Fg  ma
Don’t forget!
Fg  mg

FN  mg  ma
The acceleration is
positive in this case
because it and the net
force act upward
Newton’s Laws in an Elevator
Draw a force diagram of the rider (not the elevator!)
Constant Velocity (no acceleration)

FN
FN  Fg

Fg
Since the elevator
and the rider are not
accelerating
(constant velocity)
You would feel normal!
Newton’s Laws in an Elevator
Accelerating (slowing down)
Draw a force diagram of the rider (not the elevator!)

a

FN

Fg
FN  Fg
Since the elevator and the
rider are accelerating
downward, the net force
must act downward
How would you feel?
Heavier, lighter or normal?
Lighter!
You don’t feel the force of
gravity, rather you feel how
much the floor is supporting
you!
Newton’s Laws in an Elevator
Accelerating (slowing down)

FNet  FN  Fg

a

FN

Fg


FNet  ma

FN  Fg  ma
Don’t forget!
Fg  mg

FN  mg  ma
The acceleration is
negative in this case
because it and the net
force act downward
Newton’s Laws in an Elevator
What if you were standing on
a scale while in the elevator?
The scale supports you, so the scale
would read the Normal Force it exerts
on you!
Rest
Weight
Feel Lighter
Feel Heavier