# Forces experiment. Needing to find vector sum of the tensions in string.

#### tatsalez

1. The problem statement, all variables and given/known data

The question is: find the vector sum of the tensions in the strings; remember to multiply each mass by g. In theory, this sum should be the zero vector.

We had to use pulleys and weights and find the angle and mass of a third hanger to make a ring in the middle of a plate in equilibrium.

For one example: We placed .200kg on a hanger at 30 degrees and 120 degrees. For equilibrium we needed a hanger at 255 degrees with .275kg It says we need to find the vector sum of the tensions.

Our teacher didn't tell us how to go about doing so. We thought we would use fnetx and fnety Something like: .200(9.8)cos(30) + .200(9.8)cos(120) + .275(9.8)cos(255) = Fnetx

Then do it for Fnety with sin.

Is this correct? A lab partner thought this was right. I'm completely lost in my physics 151 class. I know vector sum is when you add the vectors, but are these considered vectors?

Any help I would be grateful for!!!

Thanks,
Matt

#### CentreShifter

As for vectors: Vectors have magnitude and direction. This is opposed to scalars, which have only magnitude. For example, tension, a force, is a vector. Mass, however, has no direction and is therefore scalar.

You have the right idea using trig to find the x and y components. Did you take into consideration all of the tension on the string (tension you created with the mass and tension from the object you're lifting)? Are you using a good reference angle? I like to use the positive x-direction for 0 degrees. Did you draw a free-body diagram?

I hope this helps.

#### tatsalez

I dont believe we need to use tensions. we used a force plate and just had to make the third pulley and its hanger/mass into equilibrium. Thats gives 3 degree's and 3 masses's. for each problem which there are 5 different sets. So we just with fnetx and fnety for the 3 degree's and 3 masses. I believe the result is suppose to come out near 0 right? we all got close like .02 -.06 etc

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving