- #1

- 7

- 0

## Homework Statement

A box whose mass is 4kg is at rest on the floor. It is then lifted to a height of 2m by a constant a constant force F and then released. At this point the box has upward velocity of 5m/s. Do not use Newton's laws of motion to solve this problem.

A. Find magnitude of the force F

F=65N

B. Find the speed with which the box hits the ground.

???

## Homework Equations

g = 10m/s^2

## The Attempt at a Solution

A. Wother = ΔKE + ΔPE = 65N magnitude of force

B. 0 = 1/2mvf^2-1/2m(0)^2 + mg(0) - mghi

= 1/2mvf^2 - mghi

vf = √[(2*mghi)/m] = √2*4*10*2/4 = 6.32 m/s (WRONG)

The speed is supposed to be = 8.09 m/s

I just killed the constant force in the work and substituted in the values as if the box fell to ground. I don't know what i'm missing.

I will return soon.