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The resultant of two forces $3P$ and $2P$ is $R$. If the first force is doubled, the resultant is also doubled. Then the angle between the forces is:

1. 30º
2. 60º
3. 120º
4. 150º

Correct answer: 120º

Solution

Let the angle between the forces be $\theta$. Using the resultant formula before and after doubling the first force and setting the second resultant equal to twice the first, the condition simplifies to $\cos\theta=-\frac{1}{2}$, so $\theta=120^\circ$.

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