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# 2015 june m1 help Watch

1. 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!!
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2. (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!!
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.
3. (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!!
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.
4. (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?
5. (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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