# M1 SUVAT help

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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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#2

You always choose a positive direction. You can choose upwards or downwards as positive; you should still get the same answer.

Posted from TSR Mobile

Posted from TSR Mobile

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#3

(Original post by

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?

**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

You always choose a positive direction. You can choose upwards or downwards as positive; you should still get the same answer.

Posted from TSR Mobile

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

Posted from TSR Mobile

^^^^^^^^^^^

This

**choose which direction is positive**

__you__Then draw a diagram and use arrows to decide if each vector is in the positive or negative direction

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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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#5

(Original post by

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?

**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?

**decided is positive?**

__you__
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If the object is going downwards I always use negative and if upwards I use positive

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(Original post by

If the object is going downwards I always use negative and if upwards I use positive

**Diaz07**)If the object is going downwards I always use negative and if upwards I use positive

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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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#9

(Original post by

But I'm not sure what to do if the object goes both up and back down again like in the example

**Diaz07**)But I'm not sure what to do if the object goes both up and back down again like in the example

YOU CHOOSE A POSITIVE DIRECTION

Acceleration due to gravity acts downwards

Therefore

If

__down as positive then acceleration due to gravity is positive__

**you choose**If

__up as positive then acceleration due to gravity is negative__

**you choose**
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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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#11

(Original post by

I choose down as positive, but the object is going both up(negative) and then back down(positive) do you see my problem now?

**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?

__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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#12

**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?

1. Going up

2. Going down

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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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#15

(Original post by

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?

**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?

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#16

**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?

If down is positive

Then a = 9.8 and s = -15

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(Original post by

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

**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

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#18

**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

If the OP chose to take up as the positive direction then a would be -9.8

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#20

**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

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