The initial force that sets the arrow in motion is muscular force:

i. This force is caused by the action of muscles in our body.

ii. The archer stretches his muscles to stretch the string of the bow.

iii. When he releases strings, the shape of the bow changed.

iv. As soon as the string released, the string regains its original position.

v. It provides the initial force to set the arrow in motion.

The arrow ultimately falls down due to force of gravity:

i. The force of gravity always acts in the downward motion.

ii. The arrow ultimately falls towards the earth because earth pulls it.

iii. Force of gravity is an attractive force.