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# M1 question watch

1. A particle is projected verically upwards with a speed of 30m/s from a point A. The point B is h metres above A. The particle moves freely under gravity and is above B for a time 2.4s. Calculate the value of h.

Thanks!
2. (Original post by snoopyx)
A particle is projected verically upwards with a speed of 30m/s from a point A. The point B is h metres above A. The particle moves freely under gravity and is above B for a time 2.4s. Calculate the value of h.

Thanks!
At what's the maximum hight above A that the particle reaches? And at what time does it reach that hight?
3. I think this is simplest:

First, consider the particle as it passes through B. How fast must it be going if it is to take 2.4 seconds before it comes back to B?
Then use v^2 = u^2 + 2as to work out the velocity at A.
4. (Original post by DFranklin)
I think this is simplest:

First, consider the particle as it passes through B. How fast must it be going if it is to take 2.4 seconds before it comes back to B?
Then use v^2 = u^2 + 2as to work out the velocity at A.
Which equation would you use for the first part?
5. v=u+at, but i think you would use t as 1.2, not 2.4, because after 1.2 seconds of passing B, v=0
6. What would you use as a?
7. well the acceleration is gravity (pulling down and so it would be negative) so -9.81
8. 9.8 is used
9. meh, i dunno, i'm used to doing these type of questions from AQA physics, and we always used 9.81, unless told otherwise. I think it depends on the exam board and stuff.
10. Yeah in physics we use 9.81 but as far as I know in maths 9.8 is used, for OCR anyway.
11. (Original post by snoopyx)
A particle is projected verically upwards with a speed of 30m/s from a point A. The point B is h metres above A. The particle moves freely under gravity and is above B for a time 2.4s. Calculate the value of h.

Thanks!

Let time to reach maximum height be then

The first occasion that the particle is at height is

Sub this value into
12. (Original post by steve2005)

Let time to reach maximum height be then

The first occasion that the particle is at height is

Sub this value into
Hmm. a different approach - did you get 'h' as 38.81m?
13. (Original post by matty92)
Hmm. a different approach - did you get 'h' as 38.81m?
I got 38.8623 =38.86(2*d.p)
14. Ah very good! Yeh i quoted 'g' as 9.81, not 9.8, so yeh its the same

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