1). If you're standing on a scale in an elevator, and the scale reads much less than your real weight, then the elevator must be<em> accelerating downward.</em>
(Think of the extreme limit: If the elevator is in free-fall, accelerating down at 9.8 m/s², then you'll apear to be weightless ... the scale will read zero.)
2). A 100 kg man is standing in an elevator, accelerating up at 2.0 m/s².
The net force acting on him is F = m·a = (100 kg) x (2.0 m/s²) = 200 N up.
The forces acting on him are:
-- gravity, m·g = (100 kg) x (9.8 m/s²) = 980 N down
-- elevator floor force . . . EF up
Net force = (980N down) + (EF up)
(200N up) = (980N down) + (EF up)
Add (980N up) to each side:
(200N up + 980N up) = EF up
<em>Elevator force = 1180N up</em>.
Pick the second choice: <em>1.2 x 10³ N up</em>
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3). My computer is resting on my desk.
<em>There are many forces acting on my computer, and they are all balanced.</em>
That means the NET force acting on my computer is zero.
If the net force were NOT zero, then my computer would be accelerating in the direction of the net force. But it's at rest. So the net force must be zero.
