$A$ spring-block system is placed on a rough horizontal floor. The block is pulled towards the right to give the spring some elongation and then released. The block will have maximum velocity when:

"When a man walks on a rough surface,the frictional force acts in the direction opposite to his motion." Is this statement correct? Give reasons.

$A$ heavy box of mass $50 \,kg$ is moving on a horizontal surface. If the coefficient of kinetic friction between the box and the horizontal surface is $0.3$, then the force of kinetic friction is: (in $\,N$)

$A$ stone weighing $1$ kg and sliding on ice with a velocity of $2$ m/s is stopped by friction in $10$ sec. The force of friction (assuming it to be constant) will be ......... $N$.

$A$ movable steel plate is placed between fixed steel and brass plates and the stack of plates is subjected to a weight of $100 \ N$ as shown in the figure. The coefficient of kinetic friction for steel on steel is $0.57$ and for steel on brass is $0.44$. Assuming that the entire weight comes onto the stack and that the weight of the plates is negligible in comparison to the applied weight,the force required to move the middle plate (in $N$) is

$A$ marble block of mass $2 \, kg$ lying on ice,when given a velocity of $6 \, m/s$,is stopped by friction in $10 \, s$. The coefficient of friction is: