# Confusing quesitionWatch

#1
A person weighing 604.4 N is moving from the 1st to the 6th floor in a lift. As the lift approaches the the 6th floor, it decreases its upward speed from 8.8 ms-1 to 2.6 ms-1. in 2.5 s. What is the average force exerted by the lift floor on the person during this time?

Do i have to use F=ma? I tried and got the wrong answer :/ so not sure what to do. Thank you for the help in advance
0
4 years ago
#2
(Original post by Nirm)
A person weighing 604.4 N is moving from the 1st to the 6th floor in a lift. As the lift approaches the the 6th floor, it decreases its upward speed from 8.8 ms-1 to 2.6 ms-1. in 2.5 s. What is the average force exerted by the lift floor on the person during this time?

Do i have to use F=ma? I tried and got the wrong answer :/ so not sure what to do. Thank you for the help in advance
Have you drawn a force diagram? What was the actual answer and what did you do?
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#3
(Original post by Chlorophile)
Have you drawn a force diagram? What was the actual answer and what did you do?
Nope i cant really draw ( trying not to sound too dumb ), i dont know the answer, its an online quiz so i wont know unless i get it right, the answer i keep on getting is 149N but its not right
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4 years ago
#4
(Original post by Nirm)
Nope i cant really draw ( trying not to sound too dumb ), i dont know the answer, its an online quiz so i wont know unless i get it right, the answer i keep on getting is 149N but its not right
You don't need to be able to draw to draw a force diagram, you just need to be able to make arrows with a pen. You've got a force going down - the person's weight - and a force going up, the reaction force from the floor of the lift. Since the person is accelerating (or rather, decelerating), we know that these forces aren't balanced. Since F=ma, we also know that the resulting force is equal to the person's mass times their acceleration.
0
4 years ago
#5
Force is the rate of change of momentum. You know the person's weight, and therefore mass. You can work out the initial and final momenta of the person and from that, the rate of change of momentum, which is the force.
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4 years ago
#6
The simplest way to do this is is use suvat to find acceleration.

Then looking at the person in the lift use F=ma: R-604.4=(604.4/9.81)a
R is the normal reaction force of the person on the lift
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#7
(Original post by Chlorophile)
You don't need to be able to draw to draw a force diagram, you just need to be able to make arrows with a pen. You've got a force going down - the person's weight - and a force going up, the reaction force from the floor of the lift. Since the person is accelerating (or rather, decelerating), we know that these forces aren't balanced. Since F=ma, we also know that the resulting force is equal to the person's mass times their acceleration.
using what you just said, i keep on getting 149 as the force but it says wrong
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4 years ago
#8
(Original post by Nirm)
i keep on getting 149 as the force but it says wron
0
#9
(Original post by Chlorophile)
Nevermind i got it! Thanks for your help
0
#10
(Original post by Jordan97)
The simplest way to do this is is use suvat to find acceleration.

Then looking at the person in the lift use F=ma: R-604.4=(604.4/9.81)a
R is the normal reaction force of the person on the lift
This helped me get the right answer! Thank you so much!
0
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