A spark is produced when two stones strike against each other. Why?

When two substances repel each other then

Assertion: On heating a sample of gas collision frequency increases. Reason: Heat is produced by the collision of gas molecules against each other.

An electric spark is struck between two electrodes placed near each other in a closed tank full of petrol. Will the petrol catch fire ? Explain your answer.

Two protons attract each other when

Two particles A and B are moving along x-axis. Their x-coordinate versus time graphs are as shown below (a) Find the time when the particles start their journey and the x-coordinate at that time. (b) Find velocities of the two particles. (c) When and where the particles strike with each other.