Tension? Brilliant

A 3D model of brilliant.org's logo of mass 3 kg has been hung onto a flat surface using two ropes of negligible mass. Assuming the force of gravity = 10 10 m/s 2 \text{m/s}^2 , what is the sum of the tensions of the rope (in N \text N )?

Round to 2 significant digits


The answer is 41.

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1 solution

Tom Engelsman
Apr 28, 2021

Let the left-side string tension be T 1 T_{1} and the right-side string be T 2 . T_{2}. The Brilliant sphere will remain in static equilibrium according to Σ F x = 0 , Σ F y = 0 \Sigma F_{x} = 0, \Sigma F_{y} = 0 , or :

Σ F x T 1 cos ( π / 3 ) T 2 cos ( π / 6 ) = 0 ; \Sigma F_{x} \Rightarrow T_{1} \cos(\pi/3) - T_{2} \cos(\pi/6) = 0; (i)

Σ F y T 1 sin ( π / 3 ) + T 2 sin ( π / 6 ) m g = 0 \Sigma F_{y} \Rightarrow T_{1} \sin(\pi/3) + T_{2} \sin(\pi/6) - mg = 0 (ii)

We first find that T 1 = 3 T 2 T_{1} = \sqrt{3}T_{2} from (i). If we substitute this expression into (ii), then we obtain:

3 T 2 ( 3 / 2 ) + ( 1 / 2 ) T 2 = ( 3 ) ( 10 ) 2 T 2 = 30 T 2 = 15 , T 1 = 15 3 \sqrt{3}T_{2} (\sqrt{3}/2) + (1/2)T_{2} = (3)(10) \Rightarrow 2T_{2} = 30 \Rightarrow T_{2} = 15, T_{1} =15\sqrt{3} newtons.

Hence, T 1 + T 2 = 15 ( 3 + 1 ) = 40.98 T_{1} + T_{2} = 15(\sqrt{3}+1) = \boxed{40.98} newtons.

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