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# mechanics problem :s Watch

1. I am stuck on this question....... i will show my working below the question

A ball of mass 0.2kg is dropped from height of 2.5m above the horizontal ground. After hitting the ground it rises to a height on 1.8m above the ground. Find the magnitude of the impulse recieved by the ball from the ground.

s=2.5
u=0 v^2=u^2+2as
v=?? v^2=0+2 x g x 2.5
a=g v^2=5g
t=?? v=7

However after finding 'v' i thought you then make 'v' = 'u' etc whilst v=0 but i cannot find the right answer
2. I actually have no idea whatsoever. I think everything you did is correct thus far, hopefully Stephen or Mr.M will come to the rescue (TSR Dons).
3. Well u is found by The u term is 0. Using 2.5m

v is found by the same formula but v is 0 since it stops at 1.8m. Using 2.5m

Remember these are all vectors so get the +ve and -ve right.

Anyway isn't this more of a physics question?
4. it is a M1 question...

however this is what i did and managed to get v=7
but i do not know where to go from there
5. (Original post by Byrney11)
it is a M1 question...

however this is what i did and managed to get v=7
but i do not know where to go from there
Well you've found the velocity before it hits the ground (u) now find it when it just leaves the ground (v)
Then put it into the eqn:

6. right so
s=1.8
u=7
v=??
a=??
t?? or is the acceleration= to g ?
7. I'm taking a guess in the wild here, but isn't it

Impulse = mv - mu

Note u will be negative so it's effectively adding the two.

I'm assuming you can work out the velocities.
8. (Original post by soup)
Well you've found the velocity before it hits the ground (u) now find it when it just leaves the ground (v)
Then put it into the eqn:

what? impulse formula?

is this M1 or M2? I'm doing M1 and doing M2 next year!

I didn't know about impulse... I thought they were talking about force
9. right so
s=1.8
u=7
v=??
a=??
t?? or is the acceleration= to g ?
10. (Original post by Byrney11)
right so
s=1.8
u=7
v=??
a=??
t?? or is the acceleration= to g ?
Yes a is g (9.8) . But remember when it's falling its accelerating, when it's going up it's decellerating. Also take into account the different directions
11. (Original post by Byrney11)
I am stuck on this question....... i will show my working below the question

A ball of mass 0.2kg is dropped from height of 2.5m above the horizontal ground. After hitting the ground it rises to a height on 1.8m above the ground. Find the magnitude of the impulse recieved by the ball from the ground.

s=2.5
u=0 v^2=u^2+2as
v=?? v^2=0+2 x g x 2.5
a=g v^2=5g
t=?? v=7

However after finding 'v' i thought you then make 'v' = 'u' etc whilst v=0 but i cannot find the right answer
The impulse
In coordinate system where the positive axis is up v1=-7 m/s as you calculated
the v2 starting velocity of with the ball rises to a height of 1.8m

so
and the Impulse I=0.2*13=2.6 kgm/s (with g=10 m/s^2)
12. (Original post by ilyking)
what? impulse formula?

is this M1 or M2? I'm doing M1 and doing M2 next year!

I didn't know about impulse... I thought they were talking about force
Well I have absolutely no idea what M1 and M2 are. And impulse is very simple, it's just the change in momentum
13. so then i have done
s=1.8
u=7
v=-g

v^2=u^2+ 2as
v^2=49 + 2 x -g x 1.8
v^2=13.72
v=3.7

however this isnt right as i know the answer is 2.59
14. when i put it in mv-mu

i get (0.2 x 3.7) - (0.2 x -7)= 2.14 still not right
15. (Original post by Byrney11)
so then i have done
s=1.8
u=7
v=-g

v^2=u^2+ 2as
v^2=49 + 2 x -g x 1.8
v^2=13.72
v=3.7

however this isnt right as i know the answer is 2.59
u is 0
16. aaaaaaaaaaaaaaa i understand it now thanks everyone
17. (Original post by ilyking)
what? impulse formula?

is this M1 or M2? I'm doing M1 and doing M2 next year!

I didn't know about impulse... I thought they were talking about force
Impulse is change in momentum = mv-mu

In physics you wont see it till First unit of A2 (depends of exam board - ocr have one unit G484 newtonian world full of newtons laws and stuff - more mechanics )

Its in M1 but basic but in M2 its more about collisions and Newtons law of restitution

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