Question

1. what causes an explosion to occur?

objects at rest within a system have no momentum. an external 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 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.

Explanation:

Explosions start with a system (e.g., a bomb) at rest, so total initial momentum is 0. The force driving the explosion (like chemical combustion) is internal to the system. This internal force propels the parts outward, conserving the total momentum of the system (now equal and opposite momenta of the fragments sum to 0). External forces are not the cause of the explosion itself.

Answer:

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