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