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# Stuck on this M2 question watch

1. I feel like it's easy but I just can't do it for some reason. What am I missing?

I've tried:

If s = 15, but you can't sub t in so how do you find out A?
If s = 0, t = 0, and you can't find A here.
2. (Original post by 2014_GCSE)

I feel like it's easy but I just can't do it for some reason. What am I missing?

I've tried:

If s = 15, but you can't sub t in so how do you find out A?
If s = 0, t = 0, and you can't find A here.
What else do you know when t=0?
3. (Original post by morgan8002)
What else do you know when t=0?
initial velocity = 20

Do I differentiate the equation and put it equal to 20?
4. (Original post by 2014_GCSE)
initial velocity = 20

Do I differentiate the equation and put it equal to 20?
yeah
5. (Original post by morgan8002)
yeah
Thanks for helping me here but urhh I'm still having trouble haha

so

S = A - Ae^-kt
V = kAe^kt

20 = kAe^k(0)

20 = kA

What have I done wrong...?
6. (Original post by 2014_GCSE)
Thanks for helping me here but urhh I'm still having trouble haha

so

S = A - Ae^-kt
V = kAe^kt

20 = kAe^k(0)

20 = kA

What have I done wrong...?
Sorry. I looked at the question quickly and didn't take into account the extra k when differentiating.
Sketch a graph of s against t. What happens as ? This is another common way of getting around the problem and removing the ks.
7. (Original post by morgan8002)
Sorry. I looked at the question quickly and didn't take into account the extra k when differentiating.
Sketch a graph of s against t. What happens as ? This is another common way of getting around the problem and removing the ks.
S gets bigger and moves towards the asymptote of A?
8. (Original post by 2014_GCSE)
S gets bigger and moves towards the asymptote of A?
Yes. The question says that the particle stops at s=15m, so this is the value of A.

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