Question

1. what causes an explosion to occur?

objects in motion within a system have momentum. an internal force acts on the objects to move them apart, causing the explosion.
objects at rest within a system have no momentum. an internal force acts on the objects to move them apart, causing the explosion.
objects in motion within a system have momentum. an external force acts on the objects to move them apart.
objects at rest within a system have no momentum. an external force acts on the objects to move them apart, causing the explosion.

Explanation:

An explosion within a system occurs due to internal forces. Objects at rest within a system initially have no momentum. When an internal force acts on the objects, it causes them to move apart, resulting in an explosion. External forces are not responsible for explosions within a system as they act from outside the system. Objects in motion within a system already have momentum, and an internal force acting on them to move apart is the cause of an explosion.

Answer:

Objects at rest within a system have no momentum. An internal force acts on the objects to move them apart, causing the explosion.