An open knife edge of mass $50 \ g$ is dropped from a height $2 \ m$ on a wooden floor. If the blade penetrates up to a depth of $10 \ cm$ into the wood,the average resistance offered by the wood to the knife edge is $....N$

State the principle of conservation of mechanical energy for a conservative force.

Write the equation of total mechanical energy of a body of mass $m$ falling freely from a height $H$.

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ Kinetic energy of a system can increase without applying external force on the system.
Reason $(R):$ If external forces are absent,then the work done by internal forces is equal to the change in kinetic energy.
In the light of the above statements,choose the most appropriate answer from the options given below:

$A$ bus moving on a level road with a velocity $v$ can be stopped at a distance of $x$ by the application of a retarding force $F$. The load on the bus is increased by $25\%$ by boarding the passengers. Now,if the bus is moving with the same speed and if the same retarding force is applied,the distance travelled by the bus before it stops is

$A$ mass of $2 \ kg$,initially at a height of $1.2 \ m$ above an uncompressed spring with spring constant $2 \times 10^4 \ N/m$,is released from rest to fall on the spring. Taking the acceleration due to gravity as $10 \ m/s^2$ and neglecting air resistance,the compression of the spring in $mm$ is: