Question

Block B rests on a smooth surface. The coefficient of static friction between A and B is $\mu =0.4$. When F= 30 N, then the acceleration of the upper block is:

• A. $3/2m{s}^{-2}$
• B. $6/7m{s}^{-2}$
• C. $4/3m{s}^{-2}$
• D. $3/7m{s}^{-2}$

Answer 1

The correct option is B $6/7m{s}^{-2}$

$f_{max}=\mu N$

$f_m=\left(0.4\right)100$

$f_m=40N$

In this case, $F=f$

($\because$ friction is self balancing )

$\Rightarrow$ For $B+A$ block,

$F=m_{B\times A}a=\frac{W_{A+B}}{g}a$

$\Rightarrow 30=\frac{(100+250)}{10}a$

$\Rightarrow a=\frac{30}{35}m/s^2$

$a=\frac{6}{7}m/s^2$