# Tension in a String

BuBbLeS01
A 2.21 kg mass is suspended from a string which is pulled upward. The mass accelerates upwards with an acceleration of 3.50 m/s2. What is the tension in the string

I don't have anything yet because I don't know where to start or what equation to use.

## Answers and Replies

kaisxuans
it would help if you cheak your textbook and tells us what you are.eg:(pri.sec.jc.poly)
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Mentor
I don't have anything yet because I don't know where to start or what equation to use.
Identify the forces acting on the mass (draw a diagram with forces labeled) and apply Newton's 2nd law.

Homework Helper
Gold Member
Dearly Missed
Hint:
First choose whether "upwards" should be the positive direction or negative.
Then, according to this choice, determine the sign of the known force(s) and accelerations.

The correct sign for the unknown force will then appear out of thin air when you have formulated the problem correctly.

BuBbLeS01
I am using f=ma and a I am putting as positive is that not right?

Homework Helper
Gold Member
Dearly Missed
To say that a is positive is perfectly valid. Now, f is the sum of forces acting upon your mass.
What individual forces is f the sum of?

Mentor
I am using f=ma and a I am putting as positive is that not right?
Nothing wrong with that, using up = +. What forces act on the mass?

BuBbLeS01
Nothing wrong with that, using up = +. What forces act on the mass?
the tension and gravity

Mentor
the tension and gravity
Good. Which way do they act? What's the force of gravity equal to? Call the tension "T" and add them up.

BuBbLeS01
Good. Which way do they act? What's the force of gravity equal to? Call the tension "T" and add them up.
gravity acts down (g=9.8) and T acts up.

Homework Helper
Gold Member
Dearly Missed
So, weight is a "minus" force, agreed?

Set up Newton's second law!

Mentor
gravity acts down (g=9.8) and T acts up.
Good. Set up an equation per Newton's 2nd law and solve for T.

BuBbLeS01
Good. Set up an equation per Newton's 2nd law and solve for T.
Fnet=ma
T-g=ma
T=(ma)+g
is that right?

Mentor
Fnet=ma
T-g=ma
T=(ma)+g
is that right?
Almost. g is the acceleration due to gravity, but weight is mg.

BuBbLeS01
Almost. g is the acceleration due to gravity, but weight is mg.
Okay so its weight that is the force acting down? So T=(ma) + w...I am sorry this question is simple and I am just not getting it and I am so frustrated right now.

Mentor
Okay so its weight that is the force acting down? So T=(ma) + w...I am sorry this question is simple and I am just not getting it and I am so frustrated right now.
You've almost got it. Keep going!

BuBbLeS01
You've almost got it. Keep going!
I am getting an answer of 29.4151 N...

Mentor
I am getting an answer of 29.4151 N...
Sounds about right. Round off your answer to a reasonable number of significant figures.

BuBbLeS01
Thank you so much! Another question though for a problem like this...

A sphere of mass of 1.39 kg is accelerated upwards by a string to which the sphere is attached. Its speed increases from 3.01 m/s to 4.81 m/s in a time of 2.28 s. Calculate the tension in the string, assuming that the tension remains constant during that time.

How do you incorporate the speed?

BuBbLeS01
Would you use Vf = Vo + at to find the acceleration?

BuBbLeS01
Oh nevermind I got it!