Ok I am pretty good at Physics but my teacher is sorry, With this problem rational to me the answer should be positive, and i know when you work it out the numerical answer is negative but opposite forces attract right? I need to know the correct answer.
F=(K)(Q1)(Q2)/r^2
There are two balloons of charges +3.37x10^-6C and -8.21x10^-6C. The distance between the 2 balloons is 2m. Find the force between the balloons.
Forget to mention i have the answer 6.225x10^-2 but is it positive or negative.
Yes.
You've forgotten that there are two forces. One is the force that 1 induces on 2 and the other is the force 2 induces on 1. We know the magnitude of the two forces is equal. If balloon 1 is pulling balloon 2 in the positive direction then balloon 2 is pulling balloon 1 in the negative direction. Or you could orient your coordinate system in the opposite direction. Then balloon 1 is pulling balloon 2 in the negative direction and balloon 2 is pulling balloon 1 in the positive direction. One force is negative and one is positive no matter which direction you orient your coordinate system or even whether the balloons have charges of the same sign or of opposite signs.
Your teacher should be sorry and ought to be more careful about wording questions. What he/she should have asked for is the magnitude of the force(s). In that case the answer would be positive.
You've forgotten that there are two forces. One is the force that 1 induces on 2 and the other is the force 2 induces on 1. We know the magnitude of the two forces is equal. If balloon 1 is pulling balloon 2 in the positive direction then balloon 2 is pulling balloon 1 in the negative direction. Or you could orient your coordinate system in the opposite direction. Then balloon 1 is pulling balloon 2 in the negative direction and balloon 2 is pulling balloon 1 in the positive direction. One force is negative and one is positive no matter which direction you orient your coordinate system or even whether the balloons have charges of the same sign or of opposite signs.
Your teacher should be sorry and ought to be more careful about wording questions. What he/she should have asked for is the magnitude of the force(s). In that case the answer would be positive.
You're welcome.
QUOTE (barakn+May 15 2008, 09:49 PM)
Yes.
You've forgotten that there are two forces. One is the force that 1 induces on 2 and the other is the force 2 induces on 1. We know the magnitude of the two forces is equal. If balloon 1 is pulling balloon 2 in the positive direction then balloon 2 is pulling balloon 1 in the negative direction. Or you could orient your coordinate system in the opposite direction. Then balloon 1 is pulling balloon 2 in the negative direction and balloon 2 is pulling balloon 1 in the positive direction. One force is negative and one is positive no matter which direction you orient your coordinate system or even whether the balloons have charges of the same sign or of opposite signs.
The sign convention for electromagnetic forces is taught for a reason.
Attractive forces = +
Repulsive forces = -
There is no reason for the teacher to be sorry.
You've forgotten that there are two forces. One is the force that 1 induces on 2 and the other is the force 2 induces on 1. We know the magnitude of the two forces is equal. If balloon 1 is pulling balloon 2 in the positive direction then balloon 2 is pulling balloon 1 in the negative direction. Or you could orient your coordinate system in the opposite direction. Then balloon 1 is pulling balloon 2 in the negative direction and balloon 2 is pulling balloon 1 in the positive direction. One force is negative and one is positive no matter which direction you orient your coordinate system or even whether the balloons have charges of the same sign or of opposite signs.
The sign convention for electromagnetic forces is taught for a reason.
Attractive forces = +
Repulsive forces = -
There is no reason for the teacher to be sorry.
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