Calculate Tension in Cord: Elevator Accelerates

[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

 

[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.

 

[anglum]

and gravity is 9.8m/s^2 downward

 

[anglum]

so those are the 2 forces acting on the object

 

[learningphysics]

so those are the 2 forces acting on the object

gravity and?

 

[anglum]

weight of the object

 

[anglum]

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

 

[learningphysics]

weight of the object

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?

 

[learningphysics]

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

sum of all forces = ma

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

 

[anglum]

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

 

[anglum]

ok i think i got it...

the upward is .555555* 9.7

and the downward is 9.8*9.7

?

 

[learningphysics]

ok i think i got it...

the upward is .555555* 9.7

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

and the downward is 9.8*9.7

?

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?

 

[anglum]

the vector sum equals 0?

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

 

[learningphysics]

the vector sum equals 0?

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

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...

 

[anglum]

vector sum = T + MG

 

[learningphysics]

vector sum = T + MG

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...

 

[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

 

[anglum]

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

 

[learningphysics]

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

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

 

[learningphysics]

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

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

 

[anglum]

oooo it is not balanced since the 35N is not the vertical... we arent able to determine it without the angle then correct? its either that or downward acceleration

since it can't be balance and can't be upward

 

[anglum]

i think it has to be downward becuz there is no way the vertical component can equal 70 if the angles comin of the Y are bothe givin 35N in an angle and not straight up

 

[learningphysics]

oooo it is not balanced since the 35N is not the vertical... we arent able to determine it without the angle then correct? its either that or downward acceleration

since it can't be balance and can't be upward

yeah, I think it is unbalanced downward. I think that although they haven't given the angle, we can presume it is not vertical...