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You weigh 400 N
If you are standing on a scale in an elevator, why does the scale read different numbers as you move up and down at different speeds?
AT REST. . .
The earth pulls down on you with a force of 400 N. You are not accelerating, so the net force is equal to 0 N (Newton's !st Law). Therefore, the upward force must be equal to 400 N so that Fn and Fg cancel out to equal 0 N. The scale will read 400 N.
ACCELERATING UPWARD. . .
The elevator is acclerating upward at 3 m/s^2. So, Fnet = ma. After solving the equation (to the right). The net force is equal to 120 N. Fg and Fn must cancel out to leace a net force of 120 N. Therefore, Fg = 400 N and Fn = 520 N. The scale will read 520 N.
Fg (400 N)
Fn (400 N)
Fn (520 N)
You are moving at a constant velocity, so there is no acceleration. Therefore, the net force is equal to 0 N. Also, Fn and Fg would both have to equal 400 N in order to cancel out and end up with Fnet= 0 N. So, the scale will read 400 N.
THE ELEVATOR PROBLEM
The elevator is accelerating at 3 m/s^2 downward. . Therefore, Fnet = ma. After solving the equation (to the right), Fnet= 120 N downward. The forces must cancel out and leave 120 N downward. So, Fg = 400 N, and Fn = 280 N. The scale will read 280 N.
Fn (280 N)
The velocity and acceleration of an elevator are controlled by a computer. If someone pushes Floor 3, a message is sent to the computer, and the computer is automatically set to send a message back that will move the elevator to floor 3 at a certain velocity that might have some accleration. This computer is located in the control unit located above the elevator. The pulleys pull the elevators up and down. Also, the traveling cables control the speed of the elevator.
HOW ELEVATORS WORK