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.