Calculating Tension Force for Different Masses

[bentrinh]

Homework Statement

The following is for a lab report. We had a couple pulleys set up, with a weight on one side, and a weight on the other side. We timed how long it took for it to fall and figured out the acceleration was .891 m/s$$^{2}$$
http://img682.imageshack.us/img682/5316/uploadyt.jpg
I need to calculate the Force of tension for 50.0g mass, and the Force of tension for 60.0g mass. Gravity is 9.80 m/s$$^{2}$$

Homework Equations

F$$_{tension} + F_{weight}$$ = mass * acceleration

The Attempt at a Solution

Deriving...
F$$_{tension} + F_{weight}$$ = mass * acceleration
F$$_{tension}$$ = mass * acceleration - F$$_{weight}$$

F$$_{weight}$$ = mass * gravity
F$$_{tension}$$ = (mass * acceleration) - (mass * gravity)
F$$_{tension}$$ = (mass * .891 m/s$$_{2}$$) + (mass * -9.80 m/s$$_{2}$$)

Solving for 50.0g...
50.0g = .0500kg
F$$_{tension}$$ = (.0500kg * .891 m/s$$_{2}$$) - (0.0500kg * -9.80 m/s$$_{2}$$)
F$$_{tension}$$ = .535 N

Solving for 60.0g...
60.0g = .0600kg
F$$_{tension}$$ = (.0600kg * .891 m/s$$_{2}$$) - (0.0600kg * -9.80 m/s$$_{2}$$)
F$$_{tension}$$ = .641 N

However, apparently, one of the tension forces should be negative. I'm stumped at this point. Sorry if it seems to be a very basic mistake, but my teacher doesn't always explain things clearly, and all my classmates are equally confused at this point.

 

[diazona]

However, apparently, one of the tension forces should be negative.

You've defined the upward direction to be positive, and downward negative, right? I assume that's why you set g to a negative value. (Which is perfectly fine) In that case, both the tension forces should be positive because they're both pointing upward.

Tension is normally given as a positive value, anyway, since we consider it to be the amount of force pulling pieces of the string together. A negative tension, to my mind, would mean that the string should spontaneously blow itself apart.