Can't Figure out what I am doing wrong F=MA (easy problem)

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Homework Help Overview

The problem involves calculating the maximum force exerted by a motor on an elevator, given its mass and maximum acceleration. The subject area pertains to dynamics and the application of Newton's second law (F=MA).

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the forces acting on the elevator, including the force of gravity and the role of tension in the cable. Questions arise regarding the correct application of forces in the context of upward acceleration.

Discussion Status

Some participants have provided insights into the necessary forces to consider, while others reflect on their own learning experiences. There is an acknowledgment of missed components in the original calculations, and guidance is offered to help clarify the problem without providing direct solutions.

Contextual Notes

Participants note that the discussion is occurring in a learning environment, with some expressing uncertainty about dynamics concepts. The pace of the summer class is mentioned as a contributing factor to the confusion.

lanzjohn
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Homework Statement



An elevator (mass 1020kg) is to be designed so that the maximum acceleration is 0.062 g. What is the maximum force the motor should exert on the supporting cable?

Homework Equations



F=MA


The Attempt at a Solution



(.062)(9.8 m/s^2) = .6076 m/s^2

F = (1020)(.6076)

F = 619.752 N

Why is that wrong?! Unless g means something else I don't understand what's up?

Thanks for your time.
 
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The elevator accelerates upward. What force(s) act on it? Is it only the force of the motor?

ehild
 
You've missed out the force on the elevator due to gravity.
F=mg+ma
gives the required force 'F' that the motor must exert to accelerate upwards at acceleration 'a'
 
Well there is the force of gravity. But isn't that already incorporated into the acceleration of the motor?

I guess there is the Force of tension as well...
 
Ohh I gotcha. I guess I assumed the tension of the rope canceled out the force of gravity... So it just left the acceleration or Force rather due to the motor. We are just starting to learn dynamics so I am a little shaky on some concepts... Got to love summer classes- they move so damn fast.
 
Sorry, I realize I just gave an answer without helping lanzjohn to figure it out on his own.
But I think he just forgot, since I did that a lot when I was first doing these types of questions.
 

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