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# M1 question help!

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1. This is stupid. Somehow I get the answer right using some common sense and logic but I need clarification.
So Imagine the scenario where a ball is thrown into the air from a height of h. Find the speed it strikes the ground at.
Now obviously theres two ways you can do this. Long way and short way. Long way is pretty straight forward...With the short way... I know this is a daft question but is acceleration positive or negative.
I always take up as the positive direction in these situations. So the ball is launched at say 14 ms-1 from 2m off the ground/ So I write out SUVAT along with their values. S = -2, u = 14, V = ? , t = unknown. What is acceleration? Initially the acceleration is -9.8 since it is decelerating on the way up.. then it begins to accelerate on the way down. Will it be -9.8 or 9.8? I'm gonna take a guess and say 9.8 but if anyone can explain it please help!
Thanks
This is stupid. Somehow I get the answer right using some common sense and logic but I need clarification.
So Imagine the scenario where a ball is thrown into the air from a height of h. Find the speed it strikes the ground at.
Now obviously theres two ways you can do this. Long way and short way. Long way is pretty straight forward...With the short way... I know this is a daft question but is acceleration positive or negative.
I always take up as the positive direction in these situations. So the ball is launched at say 14 ms-1 from 2m off the ground/ So I write out SUVAT along with their values. S = -2, u = 14, V = ? , t = unknown. What is acceleration? Initially the acceleration is -9.8 since it is decelerating on the way up.. then it begins to accelerate on the way down. Will it be -9.8 or 9.8? I'm gonna take a guess and say 9.8 but if anyone can explain it please help!
Thanks
What direction is the acceleration due to gravity? - this is what determines the sign.
3. (Original post by samb1234)
What direction is the acceleration due to gravity? - this is what determines the sign.
Acceleration due to gravity is always downwards?
I've taken up as positive and initially the acceleration due to gravity is negative because the ball is decelerating. After it reaches maximum height, the acceleration due to gravity becomes positive as it accelerates downwards? Soooooooo.. its -9.8? since I've taken up as positive?
Acceleration due to gravity is always downwards?
I've taken up as positive and initially the acceleration due to gravity is negative because the ball is decelerating. After it reaches maximum height, the acceleration due to gravity becomes positive as it accelerates downwards? Soooooooo.. its -9.8? since I've taken up as positive?
The acceleration due to gravity doesn't become positive as it accelerates downwards - remember you took up as positive so the velocity in the second half of the motion is negative as well. You can just use -9.8 as a and the initial upwards velocity and solve it
5. (Original post by samb1234)
The acceleration due to gravity doesn't become positive as it accelerates downwards - remember you took up as positive so the velocity in the second half of the motion is negative as well. You can just use -9.8 as a and the initial upwards velocity and solve it
Ah okay, got it Thanks!
Ah okay, got it Thanks!
The key thing is to pick a direction as positive (normally up, but there is no reason you couldn't do down and might be easier for some questions) and be consistent with the directions in relation to your defined directions

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