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Help urgent maths

Help!!
For this question
I took h as negative and gravity as positive but the ms takes both as positive…
I’m confused.
They’re dropping/ releasing the particles so the height would be negative… and gravity is acting along side it so it would be positive (in same direction)IMG_2779.jpegit’s part 16IMG_2780.jpeg
(edited 11 months ago)
Reply 1
If everything is consistent, then you would get the same result. You could end up having signs flipped at the end, but the physical interpretation will be the same. (For instance, traveling 1m to the right is the same as traveling -1m to the left.)

(I didn't bother to check which question you're asking, nor did I read the posted MS. In other words, maybe I'm not answering your question.)
(edited 11 months ago)
Reply 2
You need to think of overall positive direction, rather than comparing to the height. If you're treating downward height as negative, then "up" is your overall positive direction, so if gravity is acting down then it's also negative.

(The mark scheme is treating "down" as positive, but the same logic applies.)
Reply 3
Original post by Interea
You need to think of overall positive direction, rather than comparing to the height. If you're treating downward height as negative, then "up" is your overall positive direction, so if gravity is acting down then it's also negative.

(The mark scheme is treating "down" as positive, but the same logic applies.)


So is height and gravity usually the same sign (positive or negative)???
Reply 4
Original post by Alevelhelp.1
So is height and gravity usually the same sign (positive or negative)???

No, in this case they are because the particle has overall moved in the same direction as gravity is acting, but that need not be the case.

If it helps, when you get a question like this, draw a picture, and draw an arrow to the side of it to be your positive direction. Then draw arrows for each force and movement (e.g. height in this case): if an arrow is in the same direction as your original arrow, then that value is positive, and if not then it's negative.

e.g. say we designate up to be positive (^). The particle is going down (v), so that arrow points in the opposite direction, so height is negative (as you found). Gravity is acting downwards (v), so again that's the opposite direction to our positive arrow, so it's also negative.

(So for the mark scheme, they've designated down to be positive (v). Since our height and gravity arrows are both also down (v), we conclude that they're both positive. It doesn't matter which direction you choose to be positive, as long as you're consistent throughout your calculation, as tonyiptony said.)
Reply 5
Original post by Interea
No, in this case they are because the particle has overall moved in the same direction as gravity is acting, but that need not be the case.

If it helps, when you get a question like this, draw a picture, and draw an arrow to the side of it to be your positive direction. Then draw arrows for each force and movement (e.g. height in this case): if an arrow is in the same direction as your original arrow, then that value is positive, and if not then it's negative.

e.g. say we designate up to be positive (^). The particle is going down (v), so that arrow points in the opposite direction, so height is negative (as you found). Gravity is acting downwards (v), so again that's the opposite direction to our positive arrow, so it's also negative.

(So for the mark scheme, they've designated down to be positive (v). Since our height and gravity arrows are both also down (v), we conclude that they're both positive. It doesn't matter which direction you choose to be positive, as long as you're consistent throughout your calculation, as tonyiptony said.)

Ahhh I see thanks..
But how come when we take down to be negative…
Gravity is also negative… because the force gravity acts down always (so wouldn’t it be positive) do you get what I mean?? I’m so stuck
Reply 6
Original post by Alevelhelp.1
Ahhh I see thanks..
But how come when we take down to be negative…
Gravity is also negative… because the force gravity acts down always (so wouldn’t it be positive) do you get what I mean?? I’m so stuck

(Forgive the following clunky maths phrasing, I'm trying to keep things simple for the method to be clear but it's making some weird sentences :redface:)

Ah, I think you're trying to compare signs rather than direction. If down is negative, and gravity is down, then gravity is negative. You always compare to whichever direction you've taken to be positive. (So it's perfectly possible for gravity to be positive, as in the mark scheme, provided you've chosen down to be your positive direction. If down is positive, and gravity is down, then gravity is positive.)

Step 1: pick which is your positive direction. E.g. up is positive (so down is negative).

Step 2: work out which direction your forces act in/your particles move in. E.g. gravity is down, movement (i.e. height in this case) is down.

Step 3: match up these directions to your positive/negative in step 1. E.g. gravity is down, down is negative, so gravity is negative. Movement is down, down is negative, so height is negative.

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