calculating distance/acceleration?

Question

if a calculator falls off a desk and hits the floor 0.39 seconds later, how tall is the desk and how fast would the calculator be traveling when it hits the floor?

Answer 1

Use the regular equations of motion, but keep the following facts in mind (which are IMPLIED but not stated):

initial speed = 0 m/s
acceleration = 9.8 m/s^2 (sometimes rounded off to 10 m/s^2)

And yes, for those questions dealing with throwing something vertically upwards, the reverse should be assumed:

final velocity = 0 m/s
acceleration = - 10 m/s^2 (note the negative)

Do try to remember these the next time you're taking a quiz and do not seem to have enough data to work with.

Answer 2

y = 1/2gt^2

g = 9.8m/s^2

y = 1/2(9.8)(.39)^2

y = .7453m

use the height we just found to find the veloicity
Vf^2 = 2ad + Vi^2
Vf^2 = 2(9.8)(.7453) + 0^2

Vf^2 = 14.61

Vf = 3.82m/s

hope this helps