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Rotational Motion question

2022 · 28 Jun · Shift 2 · Q53
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  5. /2022 · 28 Jun · Shift 2 · Q53

Rotational Motion question

2022 · 28 Jun · Shift 2 · Q53

JEE MainPhysicsRotational MotionMCQ+4 / −1
A 34\sqrt {34}34​ m long ladder weighing 10 kg leans on a frictionless wall. Its feet rest on the floor 3 m away from the wall as shown in the figure. If Ef and Fw are the reaction forces of the floor and the wall, then ratio of Fw/Ff{F_w}/{F_f}Fw​/Ff​ will be : (Use g = 10 m/s2.) JEE Main 2022 (Online) 28th June Evening Shift Physics - Rotational Motion Question 92 English
  1. A
    6110{6 \over {\sqrt {110} }}110​6​
  2. B
    3113{3 \over {\sqrt {113} }}113​3​
  3. C
    3109{3 \over {\sqrt {109} }}109​3​
  4. D
    2109{2 \over {\sqrt {109} }}109​2​
View written solutionFree

Correct answer: C

  1. Geometry of the ladder

The ladder has length 34 m\sqrt{34}\,\text{m}34​m and its foot is 3 m3\,\text{m}3m away from the wall.

So the height reached on the wall is h=34−9=25=5 m.h = \sqrt{34-9} = \sqrt{25} = 5\,\text{m}.h=34−9​=25​=5m.

Thus, the ladder forms a 333-555-34\sqrt{34}34​ triangle.


  1. Forces acting on the ladder

Since the wall is frictionless:

  • At the wall, only a horizontal reaction acts: FwF_wFw​.
  • At the floor, the reaction has two components:
    • horizontal friction force fff,
    • vertical normal reaction NNN.

The resultant reaction of the floor is Ff=N2+f2.F_f = \sqrt{N^2 + f^2}.Ff​=N2+f2​.

The weight of the ladder is W=mg=10×10=100 N,W = mg = 10 \times 10 = 100\,\text{N},W=mg=10×10=100N, acting at the midpoint of the ladder.


  1. Force balance

Horizontal equilibrium

The only horizontal forces are:

  • wall reaction FwF_wFw​,
  • floor friction fff.

Hence, f=Fw.f = F_w.f=Fw​.

Vertical equilibrium

The only vertical forces are:

  • floor normal NNN upward,
  • weight 100 N100\,\text{N}100N downward.

Hence, N=100 N.N = 100\,\text{N}.N=100N.


  1. Torque balance about the foot of the ladder

Take moments about the foot, so the floor reactions produce no torque.

  • The wall reaction FwF_wFw​ acts horizontally at the top of the ladder, at height 5 m5\,\text{m}5m. So its moment is Fw⋅5.F_w \cdot 5.Fw​⋅5.

  • The weight acts at the midpoint of the ladder. Since the foot is at x=3x=3x=3 and the top at x=0x=0x=0, the midpoint is at horizontal distance 32 m\frac{3}{2}\,\text{m}23​m from the foot. So the moment of weight is 100⋅32=150.100 \cdot \frac{3}{2} = 150.100⋅23​=150.

For rotational equilibrium, Fw⋅5=150.F_w \cdot 5 = 150.Fw​⋅5=150. So, Fw=30 N.F_w = 30\,\text{N}.Fw​=30N.

Thus, f=30 N,N=100 N.f = 30\,\text{N}, \qquad N=100\,\text{N}.f=30N,N=100N.


  1. Resultant reaction at floor
= \sqrt{10000 + 900} = \sqrt{10900} = 10\sqrt{109}.$$ --- 6. **Required ratio** $$\frac{F_w}{F_f} = \frac{30}{10\sqrt{109}} = \frac{3}{\sqrt{109}}.$$ --- 7. **Option matching** $$\boxed{\frac{F_w}{F_f} = \frac{3}{\sqrt{109}}}$$ So the correct option is **C**.
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