Apparent weight

1. Apparent weight • If an object subject to gravity is not in free fall, then there must be a reaction force to act in opposition to gravity. • We sometimes refer to this reaction force as apparent weight.

2. Elevator rides • When you are in an elevator, your actual weight (mg) never changes. • You feel lighter or heavier during the ride because your apparent weight increases when you are accelerating up, decreases when you are accelerating down, and is equal to your weight when you are not accelerating at all.

3. v > 0 a > 0 v = 0 a = 0 v > 0 a = 0 v > 0 a < 0 Heavy feeling Normal feeling Normal feeling Light feeling Wapp Wapp Wapp Wapp W W W W Between floors Ground floor Just starting up Arriving at top floor Going Up?

4. v < 0 a > 0 v = 0 a = 0 v < 0 a = 0 v < 0 a < 0 Heavy feeling Normal feeling Normal feeling Light feeling Wapp Wapp Wapp Wapp W W W W Between floors Top floor Arriving at Ground floor Beginning descent Going Down?

5. Sample Problem An 85-kg person is standing on a bathroom scale in an elevator. What is the person’s apparent weight a) when the elevator accelerates upward at 2.0 m/s2? b) when the elevator is moving at constant velocity between floors? c) when the elevator begins to slow at the top floor at 2.0 m/s2?

6. Sample Problem A 5-kg salmon is hanging from a fish scale in an elevator. What is the salmon’s apparent weight when the elevator is a) at rest? b) moving upward and slowing at 3.2 m/s2? c) moving downward and speeding up at 3.2 m/s2? d) moving upward and speeding up at 3.2 m/s2?