Math Problem Statement

Former Governor Phil Bredesen takes the elevator down from the Neyland Stadium skyboxes after watching the home opener. Having earned a physics degree from Harvard, he makes some measurements while riding the elevator down. He stands on a scale and notes various readings. The former governor has a mass of 71 kg. The mass of the elevator and the other occupants is 810kg. Let the direction the elevator is travelling be positive. As the elevator starts down, the scale Gov. Bredesen is standing on reads 444 N . Determine the acceleration of the elevator.

Solution

To determine the acceleration of the elevator, we can use Newton's second law of motion, which states:

Fnet=maF_{\text{net}} = m \cdot a

Where:

  • FnetF_{\text{net}} is the net force acting on Governor Bredesen.
  • mm is his mass.
  • aa is the acceleration of the elevator.

Let's break this down into steps:

1. Forces acting on the Governor

There are two forces acting on Governor Bredesen:

  • His weight, which is the force due to gravity: Fgravity=mg=71kg9.8m/s2=695.8NF_{\text{gravity}} = m \cdot g = 71 \, \text{kg} \cdot 9.8 \, \text{m/s}^2 = 695.8 \, \text{N} This force points downward.

  • The normal force (the force exerted by the scale), which is the scale reading: Fnormal=444NF_{\text{normal}} = 444 \, \text{N} This force points upward (because the scale pushes upward on him).

2. Net force on the Governor

The net force is the difference between the force of gravity and the normal force: Fnet=FnormalFgravity=444N695.8N=251.8NF_{\text{net}} = F_{\text{normal}} - F_{\text{gravity}} = 444 \, \text{N} - 695.8 \, \text{N} = -251.8 \, \text{N} Since the result is negative, it indicates the elevator is accelerating downward.

3. Acceleration of the elevator

Using Newton's second law, we can solve for the acceleration of the elevator: Fnet=maF_{\text{net}} = m \cdot a 251.8N=71kga-251.8 \, \text{N} = 71 \, \text{kg} \cdot a a=251.8N71kg=3.55m/s2a = \frac{-251.8 \, \text{N}}{71 \, \text{kg}} = -3.55 \, \text{m/s}^2

Conclusion

The acceleration of the elevator is 3.55m/s2-3.55 \, \text{m/s}^2, indicating that the elevator is accelerating downward.


Would you like more details on any part of this solution? Here are some related questions you might be interested in:

  1. How does the direction of acceleration affect the forces in an elevator?
  2. What happens if the elevator accelerates upward instead?
  3. How can we calculate the tension in the elevator cable?
  4. What would the scale read if the elevator were in free fall?
  5. How does the total mass of the elevator system affect its acceleration?

Tip: Remember, a negative acceleration in this context means the elevator is either speeding up while going down or slowing down while going up.

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Math Problem Analysis

Mathematical Concepts

Newton's Second Law
Forces
Acceleration
Weight and Normal Force

Formulas

F_net = m * a
F_gravity = m * g
F_net = F_normal - F_gravity
a = F_net / m

Theorems

Newton's Second Law of Motion

Suitable Grade Level

Grades 11-12 (High School Physics)