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Work Power and Energy question

2021 · 27 Aug · Shift 1 · Q65
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Work Power and Energy question

2021 · 27 Aug · Shift 1 · Q65

JEE MainPhysicsWork Power and EnergyNumerical+4 / −1
Two persons A and B perform same amount of work in moving a body through a certain distance d with application of forces acting at angle 45 ∘^\circ∘ and 60 ∘^\circ∘ with the direction of displacement respectively. The ratio of force applied by person A to the force applied by person B is 1x{1 \over {\sqrt x }}x​1​. The value of x is .................... .
Numerical answer
View written solutionFree

Correct answer: 2

  1. Use the formula for work done

For a force FFF acting at an angle θ\thetaθ to the displacement ddd,

W=Fdcos⁡θW = Fd\cos\thetaW=Fdcosθ

  1. Write work done by A and B

Since both do the same work over the same displacement ddd:

  • For person A, angle =45∘=45^\circ=45∘ WA=FAdcos⁡45∘W_A = F_A d \cos 45^\circWA​=FA​dcos45∘

  • For person B, angle =60∘=60^\circ=60∘ WB=FBdcos⁡60∘W_B = F_B d \cos 60^\circWB​=FB​dcos60∘

Given WA=WBW_A = W_BWA​=WB​,

FAdcos⁡45∘=FBdcos⁡60∘F_A d \cos 45^\circ = F_B d \cos 60^\circFA​dcos45∘=FB​dcos60∘

Cancel ddd:

FAcos⁡45∘=FBcos⁡60∘F_A \cos 45^\circ = F_B \cos 60^\circFA​cos45∘=FB​cos60∘

  1. Substitute trigonometric values

cos⁡45∘=12,cos⁡60∘=12\cos 45^\circ = \frac{1}{\sqrt{2}}, \qquad \cos 60^\circ = \frac{1}{2}cos45∘=2​1​,cos60∘=21​

So,

FA⋅12=FB⋅12F_A \cdot \frac{1}{\sqrt{2}} = F_B \cdot \frac{1}{2}FA​⋅2​1​=FB​⋅21​

Hence,

FAFB=1212=22=12\frac{F_A}{F_B} = \frac{\frac{1}{2}}{\frac{1}{\sqrt{2}}} = \frac{\sqrt{2}}{2} = \frac{1}{\sqrt{2}}FB​FA​​=2​1​21​​=22​​=2​1​

  1. Compare with given ratio

Given,

FAFB=1x\frac{F_A}{F_B} = \frac{1}{\sqrt{x}}FB​FA​​=x​1​

Thus,

1x=12\frac{1}{\sqrt{x}} = \frac{1}{\sqrt{2}}x​1​=2​1​

Therefore,

x=2x=2x=2

  1. Comparison with stored answer

Stored correct answer is 222, which matches the derived answer.

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