The Student Room Group

2015 june m1 help

here in part a) why cant we do 255g - 150 = 255a ??? why do we only consider the crate? each method gives a different answer likee arent they supposed to be the same

and then for part b) isnt it supposed to be 150!! :frown:
(edited 7 years ago)
Original post by pondsteps
here in part a) why cant we do 255g - 150 = 255a ??? why do we only consider the crate? each method gives a different answer likee arent they supposed to be the same

and then for part b) isnt it supposed to be 150!! :frown:


You're forgetting to consider the tension.

Edit: If you consider the whole system only, you'll have two unknowns (acceleration and tension). That should be enough of a hint.
(edited 7 years ago)
Original post by pondsteps
here in part a) why cant we do 255g - 150 = 255a ??? why do we only consider the crate? each method gives a different answer likee arent they supposed to be the same

and then for part b) isnt it supposed to be 150!! :frown:


The 150N resistive force is nothing to do with the tension in the cable. If I had to guess, I'd say it's due to the runners keeping the lift in position horizontally, or air in a narrow shaft.

We don't know what the tension in the cable is, initially.

If you treat the whole thing as one at the start, we have:


225g - 150 - T = 255a

where T is the tension in the cable.

One equation, two unknowns, means insufficient information.
Original post by WinterApproaches
You're forgetting to consider the tension.

Edit: If you consider the whole system only, you'll have two unknowns (acceleration and tension). That should be enough of a hint.


oh, but if we consider the whole system there will be no S ( reaction of reaction of the crate) whereas if we consider only the lift by itself there will be S??? am i right?
Original post by ghostwalker
The 150N resistive force is nothing to do with the tension in the cable. If I had to guess, I'd say it's due to the runners keeping the lift in position horizontally, or air in a narrow shaft.

We don't know what the tension in the cable is, initially.

If you treat the whole thing as one at the start, we have:


225g - 150 - T = 255a

where T is the tension in the cable.

One equation, two unknowns, means insufficient information.


thanks a lot!!

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