Laws of Motion

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