In the figure shown, what would be the mass of the platform so that normal reaction between the block and the platform is 20 N. (Give mass in kg)

Question

TestHub · Accepted Answer

Let the tension in the (single, ideal) rope be $T$ . The 5 kg block is supported by one movable pulley, i.e. by an upward force $2 T$

Equilibrium of the block: $2 T + N = 50$

With $N = 20$ N: $2 T = 30 \Rightarrow T = 15$ N

The platform is pulled up by four rope segments, total upward force $4 T = 60$ N

Equilibrium of the platform: $4 T = M g + N \Rightarrow 60 = 10 M + 20$

$M = 4$ kg

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