An objects velocity will not change unless it is acted upon by ( balanced or unbalanced) forces?

" An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force."- Newtons first Law of Motion

Also, velocity is the speed of something in a given direction (So, 50mph North is an example of velocity)

Correct. If you think about a baseball that has just been launched from a spaceship into space with nothing around to interfere with it (e.g. a planet or moon's gravitational pull), it cannot just speed up or slow down just because it feels like it. Something has to act upon it from outside the system. In this case, the system is the baseball.

To use anything transparent

The sample with the lowest specific heat capacity will have the greatest temperature increase. in this case, it would be platinum.