Question

An student dropped their book from the top of a canyon into a river 40 meters below.

How long does it take for the book to hit the water?


2.0 s


2.9 s


4.3 s

Answer 1

Use this formula for an object starting from rest and accelerating:

Distance = (1/2) (acceleration) (time)²

In this problem, the distance is 40 meters, and acceleration is gravity.

So . . .

40 m = (1/2) (9.8 m/s²) (time)²

Divide each side by (4.9 m/s²) :

time² = (40 m) / (4.9 m/s²)

time² = 8.16 sec²

Take the square root of each side :

√(time²) = √(8.16 sec²)

time = 2.86 seconds

Round it to 2.9 sec (choice B)