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AS-Level Mechanics Newtons 3rd Law

A box of mass 20kg is in a lift of mass 200kg. If the tension in the cable is 2800N then work out the acceleration of the lift and the reaction force between the box and the lift.
Reply 1
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dude999
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Original post by j_kingzz
A box of mass 20kg is in a lift of mass 200kg. If the tension in the cable is 2800N then work out the acceleration of the lift and the reaction force between the box and the lift.


I don't know about newtons 3rd law but his 2nd law uses the equation force equals mass times acceleration
Reply 2
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dude999
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I assume the lift is being pulled upwards?
Reply 3
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dude999
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I got 2.9 for the acceleration. idk the second part tho
which way is the lift moving
If the lift is moving upwards: 2800-220g=220a
if moving down: 220g-2800=220a
Find a by re-arranging equation
Reply 6
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j_kingzz
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Original post by EnglishNOOb101
If the lift is moving upwards: 2800-220g=220a
if moving down: 220g-2800=220a
Find a by re-arranging equation

yh so it doesnt say which way it is moving but i got 2.9 too so i jus put both positive and negative
Reply 7
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j_kingzz
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Original post by dude999
I got 2.9 for the acceleration. idk the second part tho

yh same thx
Reply 8
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j_kingzz
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Original post by EnglishNOOb101
If the lift is moving upwards: 2800-220g=220a
if moving down: 220g-2800=220a
Find a by re-arranging equation

Did you do the second part tho, i jus wanna know if i got it right
Assuming that it goes up,2800-220g=220a
2800-220*9.8=220a
644=220a
a=644/220
a=2.9 m/s2 (1dp)

As we r still assuming that lift is going up, R-20g=20a
R=20g+20a
R=20*9.8+20*2.9
and then solve to find R
(edited 3 years ago)
The thing is, normally it says whether the lift is going up or down, so i am not 100% sure if this is right
Original post by EnglishNOOb101
The thing is, normally it says whether the lift is going up or down, so i am not 100% sure if this is right


Please be aware that posting full solutions is contrary to the maths forum rules.

That said, it doesn't matter whether the lift is going up or down.

Taking "up" as positive, 2800-220g=220a gives a=644/220, a positive quantity; so the "acceleration" is upwards. The lift is either moving upwards and speeding up or moving downwards and slowing down. Doesn't matter.
Ohhh i am so sorry i didn't know
Original post by EnglishNOOb101
Ohhh i am so sorry i didn't know


No biggeee. There's a sticky thread at the top of the forum on asking/answering questions on this particular forum.
the reaction force will differ depending on whether the acceleration is up or down.
Reply 15
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dude999
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Original post by ghostwalker
No biggeee. There's a sticky thread at the top of the forum on asking/answering questions on this particular forum.

Wait where? I didn't know that either
Reply 16
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davros
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Original post by dude999
Wait where? I didn't know that either

You can't really miss it - it's the sticky post that says Posting Guidelines :smile:

https://www.thestudentroom.co.uk/showthread.php?t=4919248

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