# Homework Help: Finding tension of a pulley

1. Nov 2, 2007

### missashley

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

A 957 g mass is attached to a pulley and a 8 N weight is attached to a thin massless cord.
Pulley is massless and frictionless.
Acceleration of gravity is 9.8 m/s^2

http://img474.imageshack.us/img474/2863/16gs4.jpg [Broken]

What is the tension T? Answer in N

2. Relevant equations

Fg (also weight) = ma

1 gram = 0.001 kg

3. The attempt at a solution

957 g = 0.957 kg

8 N = m * 9.81
mass of block = 0.81549 kg

w = 0.957 kg * 9.81
w = 9.38817 N

w = 0.81549 kg * 9.81
w = 7.9999569 N

Tension therefore is 9.38817 + 7.9995469 = 17.387739 N

Did I interpret and did this right? Please check work

Last edited by a moderator: May 3, 2017
2. Nov 2, 2007

### HallsofIvy

Do you notice that you got that "0.81549 kg" by dividing by 9.81 and then just multiplied by 9.81 again? Why not just use the 8 N you were given initially?

Why are you giving the answer to 8 significant figures when you were only given the info to 3 significant figures? I would have calculated that weight of the 0.957 kg mass to be 9.39 N and then said 8.00+ 9.39= 17.39 or 17.4 N.
(The fact that you measure one mass as 957 grams implies you are able to measure mass [and so weight] to three significant figures so I am interpreting "8 N" as 8.00 N.)

Are you given or are you assuming that everything is static- that neither of the masses is moving?

Last edited by a moderator: May 3, 2017