# Tension in the cord?

1. Sep 25, 2007

### anglum

Tension in the cord???

An elevator starts from rest with a constant acceleration upward and moves 1m in 1.8s
A passenger in the elevator is holding a 9.7kg bundle at the end of a vertical cord
Acceleration of gravity is 9.8m/s squared
What is the tension in the cord as the elevator accelerates? answer in units of N

so i calculated the acceleration to be .55555m/s squared upward

and the downward acceleration is 9.8m/s squared

and the mass is 9.7 kg

so where do i go from here

2. Sep 25, 2007

### learningphysics

What are the two forces acting on the object hanging by the cord?

you know the acceleration of this object is 0.5555m/s^2 upward.

3. Sep 25, 2007

### anglum

and gravity is 9.8m/s^2 downward

4. Sep 25, 2007

### anglum

so those are the 2 forces acting on the object

5. Sep 25, 2007

### learningphysics

gravity and???

6. Sep 25, 2007

### anglum

weight of the object

7. Sep 25, 2007

### anglum

so the tension on the rope is the difference between .5555 and 9.8*9.7kg????

8. Sep 25, 2007

### learningphysics

that's gravity... you have the weight of the object, and the tension in the cord... so what is the $$\Sigma{F} = ma$$ equation for the bundle?

9. Sep 25, 2007

### learningphysics

sum of all forces = ma

What goes in the left side of the above equation? What goes in the right side?

10. Sep 25, 2007

### anglum

the bundle force is 95.06N??? and then what do i do with the acceleration?

11. Sep 25, 2007

### anglum

ok i think i got it...

the upward is .555555* 9.7

and the downward is 9.8*9.7

?

12. Sep 25, 2007

### learningphysics

That is not one of the individual forces acting on the cord...

Remember, the vector sum of all the individual forces = ma

There are two individual forces here. Tension and gravity. Let tension = T. So what is the vector sum of tension and gravity?

13. Sep 25, 2007

### anglum

the vector sum equals 0???

so the gravity is 95.06 and the tension on the cord is -95.06

14. Sep 25, 2007

### learningphysics

no.

just using T and mg... write out the vector sum arithmetically. don't worry about the ma for now... what is the vector sum of the tension and mg, written in terms of T and mg...

15. Sep 25, 2007

### anglum

vector sum = T + MG

16. Sep 25, 2007

### learningphysics

it should be T - mg. because tension acts upward... mg acts downwards...

So T - mg = ma

remember... all force problems are like this... the vector sum of all the individual forces = ma... you shouldn't think of ma as another force. All the individual forces add up to ma...

17. Sep 25, 2007

### anglum

ok so .... then T = MA + MG

and in this problem T = (9.7)(.5555)+(9.7)(9.8)

real fast problem that is multiple choice

imagine a Y

with the intersecting point being a box
and the downward part bein 70N and the two upward ones bein 35N

will the box experience acceleration?
a - unable to determine without the angle
b- Yes; upwards
c-Yes;downwards
d- no it is balanced

i say the answer is D they are balanced

Last edited: Sep 25, 2007
18. Sep 25, 2007

### anglum

after taht multiple choice one i have 3 left... mind helpin me?

19. Sep 25, 2007

### learningphysics

why do you say it is balanced? What is the net force in the vertical direction?

20. Sep 25, 2007

### learningphysics

sure. np. my computer keeps freezing, so I may be gone for short periods of time... but I'll do my best.