Question

With what acceleration $a$ should the box of figure be moving upwards so that the block of mass $m$ exerts a force $\frac{7Mg}{4}$ on the floor of the box ?

• A. $\frac{g}{4}$
• B. $\frac{g}{2}$
• C. $\frac{3g}{4}$
• D. $4g$

Answer 1

The correct option is C $\frac{3g}{4}$

If $M$ exerts a force $F=\frac{7Mg}{4}$ on the floor, then from newton's third law floor will exert the same force on $m$ in the opposite direction.

$\therefore N=\frac{7}{4}Mg$

$FBD$ of block $M$ from inertial frame
From, Newton's second law of motion,

$N-Mg=Ma$

or, $\frac{7Mg}{4}-Mg=Ma$

$\therefore a=\frac{3g}{4}$

Hence, option $\left(c\right)$ is correct.