# AS-Level Mechanics Newtons 3rd Law

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#1
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.
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1 month ago
#2
(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
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1 month ago
#3
I assume the lift is being pulled upwards?
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1 month ago
#4
I got 2.9 for the acceleration. idk the second part tho
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1 month ago
#5
which way is the lift moving
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1 month ago
#6
If the lift is moving upwards: 2800-220g=220a
if moving down: 220g-2800=220a
Find a by re-arranging equation
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#7
(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
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#8
(Original post by dude999)
I got 2.9 for the acceleration. idk the second part tho
yh same thx
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#9
(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
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1 month ago
#10
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
Last edited by EnglishNOOb101; 1 month ago
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1 month ago
#11
The thing is, normally it says whether the lift is going up or down, so i am not 100% sure if this is right
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1 month ago
#12
(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.
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1 month ago
#13
Ohhh i am so sorry i didn't know
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1 month ago
#14
(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.
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1 month ago
#15
the reaction force will differ depending on whether the acceleration is up or down.
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1 month ago
#16
(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
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1 month ago
#17
(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

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