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Pendulum bob with given tension

Short answer

From radial equilibrium T - W cosθ = (W/g) v²/L, with T = 2.5W, find v = 5.83 m/s. Normal acceleration is 17.01 m/s², tangential is 6.31 m/s², total acceleration 18.14 m/s².

The bob of a 2 m pendulum describes an arc of circle in a vertical plane. If the tension in the cord is 2.5 times the weight of the bob for the position shown, find the velocity and acceleration of the bob in the given position. [Figure: pendulum of length 2 m, cord at 40∘40^\circ to the vertical.]

Assumption (figure not shown): the cord makes θ=40∘\theta=40^\circ with the vertical, and the tension is T=2.5WT=2.5W. The bob moves on a circle of radius L=2L=2 m, so use normal and tangential components.

          O
          |\
          | \  L = 2 m
          |40\
          |   \
                O  bob (W)

Normal direction (towards O)

T−Wcos⁡θ=Wgv2LT-W\cos\theta=\frac{W}{g}\frac{v^{2}}{L} v2=gL (2.5−cos⁡40∘)=9.81(2)(2.5−0.7660)=34.02 m2/s2v^{2}=gL\,(2.5-\cos40^\circ)=9.81(2)(2.5-0.7660)=34.02\ \text{m}^2/\text{s}^2 v=5.833 m/sv=5.833\ \text{m/s}

Accelerations

an=v2L=34.022=17.01 m/s2a_n=\frac{v^{2}}{L}=\frac{34.02}{2}=17.01\ \text{m/s}^2

Tangential direction (only the weight component acts):

Wsin⁡θ=Wgat ⇒ at=gsin⁡40∘=6.306 m/s2W\sin\theta=\frac{W}{g}a_t\ \Rightarrow\ a_t=g\sin40^\circ=6.306\ \text{m/s}^2 a=an2+at2=17.012+6.3062=18.14 m/s2a=\sqrt{a_n^{2}+a_t^{2}}=\sqrt{17.01^{2}+6.306^{2}}=18.14\ \text{m/s}^2

The acceleration makes tan⁡−1(6.306/17.01)=20.3∘\tan^{-1}(6.306/17.01)=20.3^\circ with the cord.

Answer: v=5.83v=5.83 m/s; a=18.14a=18.14 m/s2^2 (an=17.01a_n=17.01, at=6.31a_t=6.31 m/s2^2).

✓ Answer checked by Claude Opus 5.5 (high effort)

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