Question

Find the frictional force between the two blocks. Take $g=10{\text{m/s}}^2$.

• A. $6\text{N}$
• B. $28\text{N}$
• C. $30\text{N}$
• D. $70\text{N}$

Answer 1

The correct option is A $6\text{N}$

By FBD,
In y - direction,
$N_1=3\text{g}$
$f_{{}_{\text{max}}}=\mu N_1=\left(0.2\right)\left(3\text{g}\right)$
$=6\text{N}$

Assume them to move together with common acceleration, $a_c=\frac{F}{m_1+m_2}=\frac{40}{10}=4{\text{m/s}}^2$

For $7\text{kg}$ body: $f=7a_c=28\text{N}$ is required to make them move together, which is not possible because maximum force supplied by friction is $f_{max}$ which is equal to $6N$. Hence the blocks moves seperately and the friction is kinetic in nature and equals to limiting friction in this case.