# Kinematics and dynamics

1. Jul 16, 2013

### Enduro

1. The problem statement, all variables and given/known data

the question is:
A 62 kg person in an elevator is moving up at a constant
speed of 4.0 m/s for 5.0 s. T / I C

(b) Calculate the work done by the normal force on the person.
(c) Calculate the work done by the force of gravity on the
person.
moving down at 4.0 m/s for 5.0 s?

2. Relevant equations
W= F x Δd

and i think

Δd= (vf + vi/2)Δt
fg=m.g
3. The attempt at a solution

for b) i did Δd= (vf + vi/2)Δt and got displacement which is 10 m and then i did fn=fg since fnet is 0 which means that they have the same force. and for that i got 607.6 N

p.s the answer in the back is 12 kJ which i dont get?

2. Jul 16, 2013

### voko

At constant velocity v, what is displacement d over time t?

3. Jul 16, 2013

### PeterO

The lift was moving at a constant speed, so calculating displacement is much simpler than the way you did it - and your method gives the wrong answer because you assumed it started out at 0 m/s.
This 5 second interval is presumably a middle interval of the whole journey.
The lift gains speed - then travels at constant speed - then slows down and stops. The 5 sec interval is entirely in the middle section.

4. Jul 16, 2013

### PhanthomJay

I believe you meant Δd= [(vf + vi)/2)]Δt. If the speed is constant, vf and vi are the same. Try again.