Laws of Motion

goals
A large truck and a car, both moving with a velocity of magnitude $v$, have a head-on collision. If the collision lasts for $1\ s$

The correct statement of third law is:

A small block of mass $m$ is fixed at upper end of a massless vertical spring o spring constant $K=\dfrac {4mg}{L}$ and natural length $'10\ L'$. The lower end of spring is free and is at a height $L$ from fixed horizontal floor as shown, the spring is initially unstressed and the spring block system is released from rest in the shown position.
A small bar starts sliding down an inclined plane forming an angle $\alpha$ with the horizontal. The friction coefficient depends on the distance $x$ covered as $k=ax$, where $a$ is a constant.
All the surfaces and pulleys are frictionless in the shown arrangement. Pulleys $P$ and $Q$ are massless. The force applied by clamp on pulley $P$ is