Homework Help: What is the resultant of the vector from this impact?

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1. Feb 14, 2016

j doe

1. The problem statement, all variables and given/known data
two people kick a ball. one of them kicks it with a force of 215.0 newtons. the other one with 189.0 newtons. their approach creates a 210.0 degree angle at the point of impact and causes the ball to go in a different direction. what is the resultant of the vector from this impact?

2. Relevant equations

3. The attempt at a solution
215.0^2 = 46,225.0

189.0^2 = 35,721.0

46,225.0 + 35,721.0 = 81,946.0

2 x 189.0 = 378.0 + 215.0 = 81,270cos(30) = 70,381.88457

81,946.0 - 70,381.88457 = 11,564.11543

√11,564.11543 = 107.5365772 newtons

2. Feb 14, 2016

haruspex

It's very hard to reverse engineer your logic from a string of numbers. Please write your steps algebraically.

3. Feb 14, 2016

j doe

• magnitude^2=215^2+189^2-2*215*210*cos(30degrees)=61^2N

4. Feb 14, 2016

haruspex

That was easier to follow (though still not exactly algebraic) but now you seem to have plugged in a wrong number at one point and got a wrong answer.
I agree with your original 107, but you quote too many significant digits. Never quote more in the answer than the form of the input data implies.

5. Feb 14, 2016

j doe

how many significant figures am i suppose to have and how can i tell?

6. Feb 14, 2016

haruspex

Each number you were given was quoted to four significant figures, so use the same in the answer. Usually, you should use the same as the least precise input number, but in reality it is more complicated than that.
If you care to, you can experiment with this. For each of the three numbers given, consider the highest and lowest exact number that is consistent with it. E.g. 210.0 degrees is anything from 209.95 to 210.05. That gives you eight combinations of extreme values, each combination giving a slightly different answer. What is the range of answers that results?
(But this is probably well beyond what you are expected to do here.)