2nd Law of Motion
F = ma
A man walks into an elevator and steps on a bathroom scale. The scale reads 155 lbs.
The door closes and the elevator accelerates upward. The moment the elevator accelerates the scale shows a reading:
a) less than 155 lbs
b) greater than 155 lbs
c) equal to 155 lbs
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Riding the Elevator: How Does the Scale Reading Change?
What values would a bathroom scale have on a trip from the 1st to the fifteenth floor?
Imagine a person weighs 600. N
Start — Scale reading goes up (greater than 600. N). Inertia: object at rest tends to stay at rest.
Most of the trip up: (W = 600. N) Constant velocity. Net force = 0.
Slow Down and Stop — Scale reading less than 600. N. Object in motion tends to stay in motion.
Is this elevator going up or down?
The elevator was going down.
2nd Law — Relationship Between F, m, a
Inertia — Dr. Dave Hall of Fame →
1. Acceleration is directly proportional to the force exerted on an object.
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| a | F |
|---|---|
| x 3 | x 3 |
| x 2 | x 2 |
| divided by 3 | divided by 3 |
| divided by 2 | divided by 2 |
| x 2/3 | x 2/3 |
| quartered | quartered |
All cars are moving to the right. Which are speeding up? Which are slowing down? Which are moving at constant velocity?
1 — Speeding up 2 — Slowing down 3, 4 — Constant velocity
