help with maths problem (1 Viewer)

[karen88]

i hate this... dun get it... its under vertical motion under gravity

A stone is let fall from a tall building and 1 second later another stone is projected vertically downwards with a velocity of 20 m/s. When wil the second stone overtake the first?

help... i dun even noe where to start!!!

 

[Trev]

I'm not too sure, but I do know it would help if we are supposed to take gravity as -9.8m/s or -10m/s?

 

[nick1048]

-10m/s^2 use that trev

 

[tennille]

You'd probably need to work out the velocity of the first stone, but I'm not sure how to do that, because you only know the acceleration due to gravity. Is there any more info in the question?

Yo'd probably use -9.8m/s^2 as the accelearation due to gravity, not -10.

 

[Pace_T]

i hate this... dun get it... its under vertical motion under gravity

A stone is let fall from a tall building and 1 second later another stone is projected vertically downwards with a velocity of 20 m/s. When wil the second stone overtake the first?

help... i dun even noe where to start!!!


ok this is couchman textbook. im pretty sure this wont be in the HSC as this is physics...

here we go anyway..

let fall implies initial velocity (u) = 0
s(1) = ut + (1/2)(at^2)
S(2) = u(t-1) +(1/2)(a)(t-1)^2

s(1) is the distance the first one falling is
s(2) is the distance the 2nd one is thrown

for 2nd stone to overtake the 1st, s(1) = s(2)

.'. 4.9t^2 = 20(t-1) +4.9(t-1)^2
4.9t^2 = 20t - 20 + 4.9(t^2 -2t +1)
0 = 20t - 9.8t +4.9
0 = 10.2t - 15.1
t = 15.1/10.2 = 1.48

 

[shafqat]

not its not just hsc physics it can be asked as a standard projectile motion problem in maths

set up 2 columns of equations for both stones, with a = -g. for the first one initial velocity is 0, the second its 20
remember to use t and T for the 2, as the times are different
at the end sub in T = t - 1 i think after putting the displacements equal to each other

anyway, anything that can be done with the equations you learn in physics can definately be asked in maths, as you learn all the physics stuff in maths plus much more

edit: i meant that all the problems done in physics can be done in maths, but must be done so only assuming the accelerations, and using calculus from there

 

[FinalFantasy]

but i was taught that u can't use any physics motion equations in maths, unless u are given it or something... the physics motion is ONLY for acceleration constant
the maths equation can be constant acceleration and can be changing acceleration as well

 

[karen88]

thnx for da help... i got it now...

 

[acmilan]

but i was taught that u can't use any physics motion equations in maths, unless u are given it or something... the physics motion is ONLY for acceleration constant
the maths equation can be constant acceleration and can be changing acceleration as well

Its highly unlikely that you'll get non-constant acceleration in projectile motion, because technically acceleration would be constant without the existence of friction, resistance etc. all of which are not taken into account in the 3 unit course. Despite that, you still should write down any formula without showing where it came from unless given to you