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I don’t get why gravity it taken as positive for part a bit negative for part b.
People say it doesn’t matter if it’s positive or negative but for part a I took gravity as a negative and I got a different answer not just different sign but different number 😭 plz help me I’m so stressedIMG_2022.jpegIMG_2023.jpeg
(edited 11 months ago)
Reply 1
going up is positive as hi=eight increases going down is negative as height above ground decreases
Reply 2
Original post by Alevelhelp.1
I don’t get why gravity it taken as positive for part a bit negative for part b.
People say it doesn’t matter if it’s positive or negative but for part a I took gravity as a negative and I got a different answer not just different sign but different number 😭 plz help me I’m so stressedIMG_2022.jpegIMG_2023.jpeg

It doesn't matter as long as you are consistent with your choice of directions. In a) it is natural to consider the starting point as displacement 0 and when it reaches the ground that is a positive displacement of 19.6, so downwards is +ve and gravity is +ve.

In b) you are starting from the ground (displacement 0) and rising up to point B, so if you take upwards as +ve, then gravity would be -ve in this scenario.

Post your working if you want it checked :smile:
Original post by Alevelhelp.1
I don’t get why gravity it taken as positive for part a bit negative for part b.
People say it doesn’t matter if it’s positive or negative but for part a I took gravity as a negative and I got a different answer not just different sign but different number 😭 plz help me I’m so stressed


If you're getting a different answer, post your working, it may just be a numerical slip, but we can't tell. It would be more useful to you to find out where you're going wrong, rather than look at abbreviated solutions.
Reply 4
Original post by Alevelhelp.1
I don’t get why gravity it taken as positive for part a bit negative for part b.
People say it doesn’t matter if it’s positive or negative but for part a I took gravity as a negative and I got a different answer not just different sign but different number 😭 plz help me I’m so stressedIMG_2022.jpegIMG_2023.jpeg


Taking up as positive, for part (a) I prefer to use g = -9.8 also, since negative acceleration implies ball is slowing down as its height increases.
This means that any position below the projection point will have negative displacement, so s = -19.6
Then -19.6 = 4v - (1/2)(-9.8)(4^2) gives v = -24.5 as velocity. So the speed at the ground is 24.5m/s, the minus sign indicates a change of direction.

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