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
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Open Reading Passage →Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. A person stands on a scale in an elevator. As the elevator starts accelerating upward from rest, the scale reading is…
2. A 600 N person rides an elevator moving upward at constant velocity. What does the scale read?
3. A net force acts on an object. If the net force is tripled and the mass stays the same, the acceleration…
4. A car is moving to the right, but the net force on it points to the left. The car is…
5. An elevator moving upward slows down to stop at the 15th floor. For a 600 N rider, the scale reads…
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