Question

A steel block of mass $10\text{kg}$ rests on a horizontal floor as shown in the figure $\left(a\right)$. When three iron cylinders are placed on it as shown in the figure $\left(b\right)$, the block and cylinders go down with an acceleration $0.2{\text{m/s}}^2$. The normal reaction $R$ exerted by the floor, if masses of the iron cylinders are $20\text{kg}$ each, is -

• A. $716\text{N}$
• B. $686\text{N}$
• C. $714\text{N}$
• D. $684\text{N}$

Answer 1

The correct option is B $686\text{N}$


Assuming all four blocks are as a system.

Writing the force equation on the system in vertical direction,

$Mg-R=Ma$

$\Rightarrow (10+(3\times 20\left)\right)10-R=(10+(3\times 20\left)\right)\times 0.2$

$\Rightarrow R=70(10-0.2)=686\text{N}$

Hence, option $\left(B\right)$ is the correct answer.