Suppose a wall divides a playground, and twenty balls lie on the ground on the east side, while forty balls lie on the west side. if a child on the east side of the wall always tosses a ball over the wall at the same time a child on the west side tosses a ball over the wall, then:

The figure is missing but we can answer anyway.the correct answer is "releasing the string". in fact, when the person is holding the bow and the arrow, the kinetic energy of the arrow is still zero (because the arrow is not moving). the moment the arrow is released, it acquires a velocity v, so its kinetic energy will become different from zero: where m is the mass of the arrow and v its velocity.  all the other options, instead, do not increase the kinetic energy of the system: -  pulling farther back on the string : this will just increase the elastic potential energy of the system-  holding the arrow higher: this will increase the gravitational potential energy of the system (which is given by u=mgh, where h is the height above the ground)-  decreasing the mass of the arrow: this will decrease the gravitational potential energy of the system (which is given by u=mgh, where m is the mass)

There are no answer choices, so an example would be: it takes a long time and a lot of precision to sew a shirt by hand, so a machine was created to do the job instead.