Here's the formula for distance of fall from rest (without air resistance):
Distance = (1/2) (gravity) (time)²
For this problem:
125 m = (1/2) (9.8 m/s²) (time)²
Divide each side
by 4.9 m/s² : 125/4.9 sec² = time²
Square root
each side: time = √(125/4.9 sec²)
= 5.05 seconds (rounded)
By the way . . . . . Notice that we didn't need to use the mass
of the penny anywhere in the calculations. That's because it
doesn't matter. If there's no air resistance, then ALL objects ...
a leaf, a hammer, a dead mosquito, a rock, a feather, a car,
an ice cube, a school bus, a tree, a battle ship, a bullet shot,
a bullet dropped, a bowling ball, a tissue ... all fall at the same
rate and hit the ground at the same time.
