Because almost all of the force is done by the weight of the person and the mechanism of the swing itself, when you push someone you only give them an increase in velocity, the acceleration comes from the weight at first and then from gravity when the person is coming down, which is why we bend our legs when coming down
The kenitc energy of the bullet lowers as it keeps going up.
Because gravity is pushing the bullet down as the bullet goes up.
I'm pretty sure that the way to put this answer.