Diaz07
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#1
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Hey guys, we've nearly finished chapter 2, but there's still one thing I don't get. When to use negative acceleration and displacement. I know that if the object is going up acceleration is -9.8 and when going down +9.8. But for example, if the object starts 20m above the ground and is launched upwards and you need to find the time taken for it to reach the ground or the time at which it is 40m above ground level, which values would you use for S and A?
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QuantumSuicide
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#2
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You always choose a positive direction. You can choose upwards or downwards as positive; you should still get the same answer.

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TenOfThem
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(Original post by Diaz07)
Hey guys, we've nearly finished chapter 2, but there's still one thing I don't get. When to use negative acceleration and displacement. I know that if the object is going up acceleration is -9.8 and when going down +9.8. But for example, if the object starts 20m above the ground and is launched upwards and you need to find the time taken for it to reach the ground or the time at which it is 40m above ground level, which values would you use for S and A?

(Original post by QuantumSuicide)
You always choose a positive direction. You can choose upwards or downwards as positive; you should still get the same answer.

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^^^^^^^^^^^
This

you choose which direction is positive

Then draw a diagram and use arrows to decide if each vector is in the positive or negative direction
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Diaz07
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#4
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So for the example above the displacement could either be + or - 20m. But what if the object is going up and then down? Would you use positive or negative acceleration?
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TenOfThem
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(Original post by Diaz07)
So for the example above the displacement could either be + or - 20m. But what if the object is going up and then down? Would you use positive or negative acceleration?
What direction have you decided is positive?
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Diaz07
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#6
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If the object is going downwards I always use negative and if upwards I use positive
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Diaz07
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#7
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(Original post by Diaz07)
If the object is going downwards I always use negative and if upwards I use positive
****the other way around! If the object is going upwards I always use -9.8ms and vice versa
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Diaz07
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#8
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But I'm not sure what to do if the object goes both up and back down again like in the example
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TenOfThem
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(Original post by Diaz07)
But I'm not sure what to do if the object goes both up and back down again like in the example
I can only say this so many times

YOU CHOOSE A POSITIVE DIRECTION


Acceleration due to gravity acts downwards
Therefore
If you choose down as positive then acceleration due to gravity is positive
If you choose up as positive then acceleration due to gravity is negative
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Diaz07
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#10
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I choose down as positive, but the object is going both up(negative) and then back down(positive) do you see my problem now?
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TenOfThem
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(Original post by Diaz07)
I choose down as positive, but the object is going both up(negative) and then back down(positive) do you see my problem now?
Not really

If down is positive then acceleration due to gravity is always positive

If the item is moving down then velocity is positive
If the item is moving up then velocity is negative
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TLK
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(Original post by Diaz07)
I choose down as positive, but the object is going both up(negative) and then back down(positive) do you see my problem now?
Split it into two problems.

1. Going up


2. Going down
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Diaz07
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#13
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Okay, so if I throw an object upwards from 10m above ground. And I need to work out the total time it is above 15m while the object is above 15m it will be moving up as well as down, which acceleration value would I use considering I choose down as the positive value?
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Diaz07
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#14
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(Original post by TLK)
Split it into two problems.

1. Going up


2. Going down
Okay, this makes more sense thanks alot!
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Asciant
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#15
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(Original post by Diaz07)
Okay, so if I throw an object upwards from 10m above ground. And I need to work out the total time it is above 15m while the object is above 15m it will be moving up as well as down, which acceleration value would I use considering I choose down as the positive value?
9.8, it is always 9.8 as gravity is constant. It doesn't change depending on whether the particle is travelling up or down
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TenOfThem
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(Original post by Diaz07)
Okay, so if I throw an object upwards from 10m above ground. And I need to work out the total time it is above 15m while the object is above 15m it will be moving up as well as down, which acceleration value would I use considering I choose down as the positive value?
In this example

If down is positive

Then a = 9.8 and s = -15
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Diaz07
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#17
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(Original post by Asciant)
9.8, it is always 9.8 as gravity is constant. It doesn't change depending on whether the particle is travelling up or down
Oh alright, I wasn't sure whether to use negative or positive 9.8
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TenOfThem
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#18
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(Original post by Asciant)
9.8, it is always 9.8 as gravity is constant. It doesn't change depending on whether the particle is travelling up or down
It is 9.8 in a downwards direction

If the OP chose to take up as the positive direction then a would be -9.8
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Diaz07
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#19
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(Original post by TenOfThem)
In this example

If down is positive

Then a = 9.8 and s = -15
Ahh okay, so if I pick down as positive both gravity and displacement will be positive in the downwards direction, I get it now thanks! But could you explain why you used positive 9.8 as the object is going up and then down, not just down if you get me?
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TLK
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(Original post by Asciant)
9.8, it is always 9.8 as gravity is constant. It doesn't change depending on whether the particle is travelling up or down
It can since its a vector quantity. It can have a negative value depending on which direction you make positive.
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