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M1 SUVAT help

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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Reply 1

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

Reply 2

TenOfThem

Original post by Diaz07

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.

Posted from TSR Mobile

^^^^^^^^^^^
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

Reply 3

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?

Reply 4

TenOfThem

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?

Reply 5

Diaz07

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

Reply 6

Diaz07

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

Reply 7

Diaz07

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

Reply 8

TenOfThem

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

Reply 9

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?

Reply 10

TenOfThem

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

Reply 11

TLK

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

Reply 12

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?

Reply 13

Diaz07

Original post by TLK

Split it into two problems.

1. Going up

2. Going down

Okay, this makes more sense thanks alot! :biggrin:

Reply 14

Asciant

Original post by Diaz07

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 :smile:

Reply 15

TenOfThem

Original post by Diaz07

In this example

If down is positive

Then a = 9.8 and s = -15

Reply 16

Diaz07

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 :smile:

Oh alright, I wasn't sure whether to use negative or positive 9.8

Reply 17

TenOfThem

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 :smile:

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

Reply 18

Diaz07

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?