Solving Dynamics Homework: Man on Elevator, 60 kg Mass

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In summary, a man standing in an elevator on a scale experiences two accelerations: the elevator moving up at 5m/s^2 and the gravitational pull. The reading on the scale, in Newtons, is found by finding the individual forces of both accelerations, F1 and F2, and subtracting them. This can also be done by simply calculating the man's weight, 60kg multiplied by the gravitational acceleration of 9.81m/s^2, or by adding the two accelerations and multiplying by the mass. Both methods can be correct depending on the context and assumptions made.
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jey1234
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Homework Statement



I'm confused w/ a dynamic question our teacher showed us today. we just started dynamics and I'm in grade 11. I made a horrible diagram on ms paint but I believe it will suffice :) A man is standing in an elevator on a scale. The elevator is moving up at 5m/s^2 and then there is the obvious gravitational pull. What will be the reading on the scale, in Newtons (so force?), if the man's mass is 60 kg?

Elevator_PhysicsQ.jpg


Homework Equations



She told us to use a= FR/m

The Attempt at a Solution



The thing is she solved it for us and I can't seem to understand her steps. She says to find the individual Forces of both accelerations. So F=ma

F1= 60kg * 5 m/s^2 [UP]
F2= 60kg * 9.81 m/s^2 [DOWN]

This is the part that confuses me: FR= F2-F1
We've never done subtraction of vectors before, and she explained that we change the direction of the second vector and just add it.

Therefore, F1= 300N [UP] , F2= 588.6N [DOWN]
which becomes: 588.6N [DOWN] + 300N [DOWN] = 888.6N [DOWN] <--- HER FINAL ANSWER

My first question is since the whole system is affected by the two accelerations doesn't "cancel" out? In other words, although he is moving up at 5m/s^2 and down at 9.81m/s^2, the scale is also experiencing those movements, therefore it will almost be like "at rest". If I was to do it, I would ignore them, and simply calculate his weight: 60kg * 9.81 m/s^2 [DOWN] = 588.6N [DOWN]. If not, I would add the two accelerations and get: 9.81m/s^2 [DOWN] + 5m/s^2 [UP] = 4.81m/s^2 [DOWN] (basically subtracting them and keeping the larger ones direction). After, I'd multiply that by mass and get: 60kg * 4.81m/s^2 [DOWN] = 288.6N [DOWN].

Are one of my methods and/or my teacher's right? If not, please explain why. Thanks!

Homework Statement


Homework Equations


The Attempt at a Solution

 
Last edited:
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1. How do you calculate the force needed to accelerate a 60 kg mass on an elevator?

To calculate the force needed, you can use the formula F = m x a, where F is the force in Newtons, m is the mass in kilograms, and a is the acceleration in meters per second squared.

2. What is the difference between static and dynamic equilibrium?

Static equilibrium occurs when an object is at rest or moving at a constant velocity, while dynamic equilibrium occurs when an object is accelerating at a constant rate.

3. How does the mass of the person on the elevator affect the force needed to move the elevator?

The mass of the person on the elevator, along with the mass of the elevator itself, will determine the total mass that needs to be accelerated. This will then affect the amount of force needed to move the elevator.

4. Can the force needed to accelerate the elevator be greater than the weight of the person on the elevator?

Yes, the force needed to accelerate the elevator can be greater than the weight of the person on the elevator. This can occur if there is additional weight on the elevator or if the elevator is accelerating at a faster rate.

5. How does the direction of acceleration affect the calculations for solving dynamics homework?

The direction of acceleration must be taken into account when solving dynamics homework, as it will affect the sign of the acceleration and thus the direction of the force. It is important to use positive and negative signs correctly in calculations to accurately represent the direction of forces and accelerations.

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