Which would have a greater effect on the kinetic energy of an object, doubling the mass or doubling the velocity?

(i) The kinetic energy of a body is directly proportional to its 'mass' (m). So, if we double the mass (so that it becomes 2m), then the kinetic energy will also get doubled.
(ii) On the other hand, kinetic energy of a body is directly proportional to the "square of its velocity"(v²). So, if we double the velocity (so that it becomes 2v), then the kinetic energy will become four times. This is because:(2 v)² =4v². It is clear from the above discussion that doubling the velocity has a greater effect on the kinetic energy of an object.