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A system designed to store energy uses a large load suspended from four steel ropes in an underground vertical tunnel. Energy is stored when the load is lifted.

Figure 8 shows a vernier caliper measuring the maximum width of one steel rope.The rope is made from seven identical wires.

Figure 7 shows the load at its lowest point. The vertical length of each steel rope is 520 m. Show that the total mass of the four ropes is approximately 9 x 10¹ kg. density of steel = 4.7 x 10³ kg m-³ The maximum tension in each rope is 1.4 x 106 N when the load is accelerated from its lowest point. The mass of the load is 4.5 x 105 kg. Calculate the acceleration of the load. The breaking stress of the steel used to make the four ropes is 890 MPa.For safety reasons, the maximum stress in each rope during the acceleration must be \text { less than } \frac{1}{5} \text { of the breaking stress. } S] Deduce whether these ropes are safe. It is suggested that the load and ropes will store 4 MW h of energy when the load hasbeen lifted by 520 m. Deduce whether this suggestion is correct.

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