Measurement uncertainties experiment acceleration?

At home I have to drop six equal-sized pieces of play dough (balls) from different heights and then measure the time (with my cell phone) until the play dough reaches the ground. But unfortunately I'm having trouble finding measurement uncertainties. I have to find three. I could start with air resistance. It hinders the acceleration of the play dough. Maybe I could say that the cell phone doesn't record the time directly. (I.e. if I measure the exact time until the play dough touches the ground, but when I press the cell phone it takes a certain amount of time for the signal to be received and executed) I don't know if that counts as measurement uncertainty, which is why I'm asking you about other measurement uncertainties.