Transcript Aim: How can we explain the motion of elevators using

Aim: How can we explain the motion
of elevators using Newton’s 2nd law?
F = 150 N
Do Now:
What is the
acceleration of this
object?
m = 20 kg
FNet = ma
F = 100 N
150 N – 100 N = (20 kg)a
50 = 20a
2.5 m/s2 up = a
Elevator Problems
FN = Normal Force (scale reading)
A man is standing on
a scale while in an
elevator
What forces exist?
Fg = Weight = mg
While the elevator is at rest
Objects at rest: FNet = 0
Forces are balanced
Fg = FN
While the elevator is moving with a constant
velocity
Objects moving with a constant velocity:
FNet = 0
Forces are balanced
Fg = FN
A 50 kg man is standing on a scale in an
elevator while it is at rest.
How much does he weigh?
Fg = mg
Fg = (50 kg)(9.8 m/s2)
Fg = 490 N
What does the scale read?
FNet = 0
FN = F g
FN = 490 N
The elevator now accelerates up at 2 m/s2
What does the scale read?
I feel
heavier!!!
FN = ?
FNet = ma
The object will always accelerate
in the direction of the larger
force
FN is larger
FN – Fg = ma
FN – 490 N = (50 kg)(2 m/s2)
FN – 490 = 100
Fg = 490 N
FN = 590 N
This is your
apparent weight
The elevator now accelerates down at 1 m/s2
What does the scale read?
I feel
lighter!!!
FN = ?
FNet = ma
The object will always accelerate
in the direction of the larger
force
Fg is larger
FN - Fg = ma
FN - 490 N = -(50 kg)(1 m/s2)
FN - 490 N = -50 N
Fg = 490 N
FN = 440 N
The elevator now moves with a constant velocity of
3 m/s up.
What does the scale read?
I feel
normal!!!
FN = ?
FNet = 0
FN = F g
FN = 490 N
Fg = 490 N
The elevator now is in free fall.
What does the scale read?
FN = ?
FNet = ma
The object will always accelerate
in the direction of the larger
force
Fg is larger
Fg – FN = ma
490 N – FN = (50 kg)(9.8 m/s2)
490 – FN = 490
Fg = 490 N
FN = 0 N
The person
seems weightless
Spinning in Zero Gravity
What is the acceleration of the
elevator?
FNet = ma
FN = 980 N
The object will always accelerate
in the direction of the larger
force
F = mg
g
FN is larger
630 N = m(9.8 m/s2)
FN – Fg = ma
m = 64.3 kg
980 N – 630 N = ma
350 = 64.3a
Fg = 630 N
a = 5.4 m/s2 up