I've been stalking this thread for a couple days now and I can no longer resist the urge! Mind if I joined?

I've been playing around with a physics concept- kinetic energy manipulation. The gist of it is that my character will be able to speed up, slow down an object/ persons that are in motion. 

Limitations:
- objects needs to have kinetic energy (ie: be in motion)
- objects need to be within an area of effect
- powers will not have an indefinite effect (haven't decided on time range)
- larger the mass, the more difficult it is for her to control (ie: easier for her to control a marble she throws rather than say a moving bus). Not to say that her powers wouldn't work on something as large as a bus, but it is difficult for her to control the outcome. 

I'm imagining in the beginning stages she'll only be able to manipulate the energies of objects forward, but eventually will learn to change their directions... which could lead to the use of projectiles. Maybe? perhaps? I've just been itching to write a character that manipulates the kinetic energy of steel balls (maybe a small amount like 3 or so) like projectiles. Any thoughts?