Why does the kinetic energy change? is it the
mass or the speed that causes the change?

Both speed and mass causes the kinetic energy to change.

Explanation:

The energy an object  because of its motion is called as Kinetic energy. A object that moves be it horizontal or vertical direction possess kinetic energy. Some forms of kinetic energy are rotational, vibrational, translational. The kinetic energy(K) of the object depends on two components i.e., the mass (m) of the object involved and its speed(v)

The relation between the mass, speed of the object can be represented as

 Kinetic energy$=\frac{1}{2} m \times v^{2}$

Thus when there is a increase in the mass and speed, the kinetic energy will also increase

the correct answer would be 6 feet.

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jk but i am serious about the part that most people cant answer it beacuse you did not mention the section you are talking about. but it would if you added some detail and were a bit more specific.