Question

Block A of mass 35 kg is resting on a frictionless floor. Another block B of mass 7 kg is resting on it as shown in the figure. The coefficient of static friction and kinetic friction are 0.5 and 0.4 respectively. If a horizontal force of 100 N is applied to block B, then the acceleration of the block A will be: $(g=10m{s}^{-2})$

• A. $0.8m{s}^2$
• B. $2.4m{s}^2$
• C. $0.4m{s}^2$
• D. $4.4m{s}^2$

Answer 1

The correct option is A $0.8m{s}^2$

Static friction friction force between two blocks

$f_s\le \mu NN=mg=7\times 10=70N{f}_s\le {\mu }_s\times 70f_s\le 0.5\times 70f_s\le 35N$

here force applied is grater than static friction

hence body is in motion, then there is kinetic friction

$f_k={\mu }_k\times 70f_k=0.4\times 70f_k=28N$

for mass 35 kg force applied is only kinetic friction

$f_1=f_k=28N$

acceleration of mass 35 kg is

$a=\frac{f_1}{35}=\frac{28}{35}=0.8m/s^2$

Option A is corrrect