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Question 3: The resultant of two forces, one is double than the other, is perpendicular to the smaller force. What is angle between two forces?


Let two forces vecf1and vecf2are acting with one another according to the conditions of the problem, i-e,

(1) The magnitude of vecf2 is double the magnitude of vecf1.

(2) The resultant of both forces is perpendicular to the smaller force, i-e, vecf1.

(3) These situations are shown in the diagram.

Now, to the right-angled triangle, apply the Pythagoras theorem,


So Θ = 60°

As the angle between two vectors is determined by putting them tail to tail, therefore, looking at the figure above and using geometry, the angle between the two forces must be 90 + 30 = 120 degrees.


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