# Help mechanics

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
(edited 11 months ago)
going up is positive as hi=eight increases going down is negative as height above ground decreases
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

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
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.
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

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.