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Elevator Forces

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100 kg Scenario: One mass hanging. What forces are acting on this mass? F g = w = mg =100 kg (10 m/s 2 ) FTFT This mass is not moving. ΣF = 0 ΣF= F T - w 0 = F T - w F T = w F T = 1000 N Remember, even if this mass was moving with a constant velocity the sum of the forces would still be zero.

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100 kg Scenario: One mass hanging, being accelerated upward at 3 m/s 2 What forces are acting on this mass? F g = w = mg = 100 kg (10 m/s 2 ) = FTFT This mass moving. ΣF = ma ΣF= F T - w ma = F T - w F T = ma + w F T = 1300 N a 1000 N TENSION IS MORE

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100 kg Scenario: One mass hanging, being accelerated downward at 3 m/s 2 What forces are acting on this mass? F g = w = mg = 100 kg (10 m/s 2 ) = FTFT This mass moving. ΣF = -ma ΣF= F T - w -ma = F T - w F T = ma - w F T = 700 N a 1000 N TENSION IS LESS

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Let’s look at what happens to forces and your apparent weight (F N ) on an elevator. FNFN This elevator is either not moving or moving with a constant velocity. ΣF = 0 ΣF= F N - w 0 = F N - w F N = w F g = w 100 kg F g = w FTFT This is just like when the mass was hanging. The scale reads your true weight.

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On an elevator that’s accelerating we experience of a change in “weight” which is actually due to a change in the normal force, not weight (mg). Because if you think about it your weight (mg) never changes (your mass remains the same so does gravity). This elevator is now accelerating upward. 100 kg F g = w FTFT This is just like when the mass was accelerating upward. F g = w FNFN What happens to your apparent “weight” when an elevator accelerates upward? The elevator is pushing up on you with more force, so you “feel” heavier.

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This elevator is now accelerating downward. This is just like when the mass was accelerating downward. What happens to your apparent “weight” when an elevator accelerates downward? The elevator is pushing up on you with less force, so you “feel” lighter. 100 kg F g = w FTFT FTFT

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This elevator has now snapped and is accelerating downward at the force of gravity. This is freefall. a = g 100 kg F g = w There is no F N, the scale is falling as fast as you are, nothing is pushing on you. This is the feeling of weightlessness. F g = w

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Astronauts in a spacecraft orbiting the Earth or out for a “spacewalk” are seen to “float” in midair. This is sometimes referred to as weightlessness or zero gravity. Are these terms correct?

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w FNFN An astronaut experiences as much as 8 g’s of force on blast-off because of the acceleration. This net force (experienced as apparent weight) is exerted on the astronaut by the seat which pushes on the astronaut with a force many times their weight. If the astronaut were sitting upright upon launch, such a force would cause the body to accelerate away from it’s own blood ( Newton’s 1 st - object at rest…). Thus the blood appears to go to the legs, where the blood vessels and capillaries would rupture. Consequently, astronauts are in a reclining position for blast- off.

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