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
Check out our YouTube Channel called
Show Me The Physics
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.
.gif)
| 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
NYSSLS Reading Passage · Dynamics
The Fastest Man on Earth: Physics of Crash Survival
Open Reading Passage → Regents Practice Version →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…
Testing an Electric Scooter
Engineers measured the acceleration of an electric scooter and rider for different net forces. The combined mass of the scooter and rider stayed the same. The data are shown in Table 1.
Table 1: Net Force and Acceleration
| Net Force (N) | 0 | 100. | 200. | 300. |
|---|---|---|---|---|
| Acceleration (m/s²) | 0 | 2.0 | 4.0 | 6.0 |
Which statement describes the relationship shown by the data?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. Using the data in Table 1, determine the combined mass of the scooter and rider.
C2. Which graph best represents acceleration versus net force for these data?
C3. A lighter rider uses the scooter, making the combined mass 65 kg. The net force is 260 N. Substitute numerical values into a mathematical representation that can be used to determine the acceleration. Then calculate the acceleration.
C4. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
C5. The motor pushes forward with 240 N while friction and air resistance total 40. N. What is the acceleration?
C6. The company plans to give the scooter a larger battery so the motor can provide more force, but the larger battery adds mass. Identify one benefit and one drawback of this design change, referring to Fnet = ma.
.gif)