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

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)

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it