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Kinematics and dynamics

  • Thread starter Enduro
  • Start date
  • #1
4
0
1. Homework Statement

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.
(d) How would your answers change if the elevator were
moving down at 4.0 m/s for 5.0 s?


2. Homework 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?
 

Answers and Replies

  • #2
6,054
390
At constant velocity v, what is displacement d over time t?
 
  • #3
PeterO
Homework Helper
2,425
46
1. Homework Statement

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.
(d) How would your answers change if the elevator were
moving down at 4.0 m/s for 5.0 s?


2. Homework 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?
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
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,157
503
1. Homework Statement

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.
(d) How would your answers change if the elevator were
moving down at 4.0 m/s for 5.0 s?


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

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