How is it possible to blow a balloon?

Explanation:

To blow up a balloon, start by pinching the neck of the balloon with your index finger and thumb. Then, take a deep breath and put the opening of the balloon in between your lips. When you're ready, loosen your grip on the neck and blow into the balloon opening so the balloon starts to fill with air.May

a) the acceleration is -9.8 m/s^2 (gravity's acceleration), the magnitude is 9.8 m/s^2 and the direction is downwards.

The pebble is not decelerating since the magnitude of it's speed is increasing, even if it is on a negative direction.

b) after 0.5 s the pebble is 5.725 meters down. (or -5.725m from it's initial position.)

Explanation:

a) the acceleration is given only by gravity, it's the only force acting on the pebble, the slingshot only gives the pebble an initial speed.

b) To calculate the position we use the equation

y0 + v0·t + ½·g·t² = yf

replacing:

0 + (-9.0m/s) * 0.5s + 1/2 (-9.8)*0.5² = yf

-5.725 = yf