Efficiency is calculated through dividing the actual mechanical advantage by the hypothetical mechanical advantage:
- the actual mechanical advantage is 9J because that's how much work the light bulb doing
- the hypo. mechanical advantage is 100J. Ideally, in a perfect world, the light bulb can convert 100J input into 100J output, but do to resistance and other factors it is not possible.

change the decimal to a percentage:

the light bulb had 9% efficiency
The direction of the motion is constantly changing during
motion over any closed path, not only circular.
Think about it like this, the more mass there is, the faster its going to go. If you took a golf ball and a ping pong ball and you held them each separately, you would notice that the golf ball is heavier. If they move with the same kinetic energy, but the golf ball WEIGHS more, then the golf ball will have the greater speed. If you think about it, the ping pong ball may be taking its time to get to wherever its going.
How do you ask a question but don’t know the answer we will never know