# really confused

this question i cannot answer. it goes : two forces, 4N and 11N, act on a point. which one of the following cannot be the magnitude of a resultant?
1. 4N, 2.7N, 3.11N, 4.15N

 ebaines Posts: 10,055, Reputation: 5539 Expert #2 Jun 6, 2012, 05:39 AM
Quote:
 Originally Posted by nemesis94 two forces, 4N and 11N, act on a point. which one of the following cannot be the magnitude of a resultant? 1. 4N, 2.7N, 3.11N, 4.15N

The answer is: all of the above. The magnitude could vary anywhere from 7N to 15N, depending on the angle between the two forces. Are you sure you wrote down the question correctly?
 nemesis94 Posts: 3, Reputation: 1 New Member #3 Jun 8, 2012, 02:55 AM
yeah I'm sure and they say the answer is the first one.
 ebaines Posts: 10,055, Reputation: 5539 Expert #4 Jun 8, 2012, 03:49 AM
I see it now. I originally thought the four choices were 1.4, 2.7, 3.11 and 4.15. But now I see they are actually 4, 7, 11, and 15. (It would have been much clearer if the choices had been listed as 'a.', 'b.', 'c.' and 'd.' rather than '1.', '2.', '3.' and '4.' - plus the lack of a space between the numbers didn't help!) Hence the first one is the only one that is not possible, per my previous response.
 nemesis94 Posts: 3, Reputation: 1 New Member #5 Jun 12, 2012, 01:06 AM
sorry my bad, but why do you say the first one is not possible? please be clear.
 ebaines Posts: 10,055, Reputation: 5539 Expert #6 Jun 12, 2012, 04:44 AM
When you add two forces the resultant force is determine according to the rules of vector addition. It will be a minimum if the two forces are pointing in opposite directions -180 degrees apart - and at a maximum if the two forces are pointing in the same direction, or 0 degrees apart. If the angle between them is in between 0 and 180 then the combined force has a magnitude somewhere in between, . So the minimum possible value is with the 4N force and the 11N force acting in opposite directions, and that magnitude is 11N - 4N = 7N. Hence any value less than 7N is impossible. Similarly any value greater than 11N + 4N = 15N is also impossible. But all values between 7N and 15N are possible (depending on the angle between the two forces).

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