Tension on a moving cord (Elevator)

[notsam]

Homework Statement

Vi=0 m/s, D=1m, Time=1.6s, m=9.2kg, g=-9.8 m/s squared

Homework Equations

T=F=ma,

The Attempt at a Solution

I thought that it would be as simple as finding the net acceleration in the system, and then multiplying it by the mass. Unforunatlly I can not come up with the right awnser.

 

[PhanthomJay]

When you find the acceleration of the system and multiply it by the mass, you get the NET force acting on the object. The NET force consists of the elevator weight and the cord tension. You want to solve for the tension. Please try again. And welcome to PF!

 

[notsam]

Thank you very much, as I said before, "I thought that it would be as simple as finding the net acceleration in the system, and then multiplying it by the mass. Unforunatlly I can not come up with the right awnser." But unfortunatly the awnser isn't cooperating with the UTEID quest online homework that I am plugging it into, an awnser or step by step process would be very much appreciated.

 

[notsam]

My awnser is 71.76. Even knowing if this is correct or not would be a big help

 

[gneill]

The problem description is lacking certain details. For example, are we to assume that there is an elevator, and inside the elevator is a mass of 9.2kg suspended by a cord affixed to the elevator ceiling, and you want the tension in that cord as the elevator accelerates? Also, is the elevator motion upwards or downwards?

 

[notsam]

"An elevator starts from rest with a constant
upward acceleration andmoves 1m in the first
1.6 s. A passenger in the elevator is holding a
9.2 kg bundle at the end of a vertical cord.
The acceleration of gravity is 9.8 m/s2 .
What is the tension in the cord as the ele-
vator accelerates? Give your awnser in N"
Again the awnser that I am getting is 71.76 and my net acceleration is -7.8 m/s squared

 

[gneill]

Okay, that's more clear. The elevator is accelerating upwards, which means that the reaction force on the bundle will be in the downwards direction, adding to the force of gravity and making the bundle heavier (more tension in the cord). Your answer seems to be a bit low.

Can you show us how you calculated the acceleration rate for the elevator?

 

[notsam]

I used basic kinimatic equations to calculate my acceleration. Using D=1 m t=1 s and Vi= 0 m/s. I calculated the Vf to be 2 m/s, using D= (Vi+Vf/2) t. After that I calculated my acceleration to be 2 m/s2, using Vf=Vi +at. So I'm guessing that I should add my acceleration to garvity instead of subtracting, it and that will give me the correct net acceleration?

 

[gneill]

Okay, your acceleration is off. I'm assuming that you're using Vi = 0, then

D = (Vf/2)*t, so that Vf = 2*D/t giving Vf = 1.25 m/s rather than your 2 m/s. That will affect your acceleration result.

You are right that you'll have to add your acceleration to the acceleration due to gravity in order to determine the net acceleration experienced by the bundle.

 

[notsam]

Ohhhhh my problem is I can't multiply apprently...that's embarasing. Thank you very much :) This was definitely the easiest problem and I couldn't figure out what I was doing wrong