Basic mechanics question

this question is answer more than 10 times here !! hehehe
see this

Reply 2

yusufu

16

redpanda

I know this is really basic but my school only offers S1, and i didn't want to do that so i'm teaching myself M1. but anyway, there's this question in the textbook that i cannot seem to solve:

A girl throws a ball vertically upwards with speed 8m/s from a window which is 6m above horizontal groud. 1.5s later she drops a second ball from rest out of the same window. find the distance below the window of the point where the balls meet by modelling the balls as particles.

Can anyone help me?

what you want to work out is the point where the displacement of both balls is the same:

ball 1: s=s u=-8 a=9.8 t=t+1.5

ball 2: s=s u=0 a=9.8 t=t

s = ut+½at²
s = -8(t+1.5) + 4.9(t+1.5)² = 4.9t²

-8t - 12 + 4.9(t² +3t +2.25) = 4.9t²
-8t - 12 +4.9t² + 14.7t + 11.025 = 4.9t²
6.7t = 0.975
t = 39/268

sub this into the original equation to work out s

Reply 3

redpanda

OP

Thanks very much :redface:

i guess it's a good sign that i wasn't the only one who had trouble with that question

Reply 4

well it's not a good sign,,, but it shows that's not a big deal that u had some trouble with that question.. :biggrin:

Reply 5

Question - why is the acceleration of the first ball 9.8ms^-1? It's being thrown upwards, so isn't it -9.8 overall?

Reply 6

well if u took up to be -ve u have to take all the values up to be -ve and down to be +ve... (that's what Undry did) u can take down to be -ve, but then u have to change the sign of each vector...
Edit: besides: the sign of acceleration has NOTHING to do with the direction of the ball!
acceleration of gravity is always downward!

Reply 7

yusufu

16

Craver

Question - why is the acceleration of the first ball 9.8ms^-1? It's being thrown upwards, so isn't it -9.8 overall?

i'll assume youre referring to my working.
i chose to have downwards as positive. hence g acts in a positive direction.

Reply 8

Undry1

i'll assume youre referring to my working.
i chose to have downwards as positive. hence g acts in a positive direction.

(sorry that i answered, but i thought that u were offline, so i felt free to answer :rolleyes:

)
hehe

Reply 9

Taking a as -9.8 on the first, though doesn't give

s = -8(t+1.5) + 4.9(t+1.5)²

but gives

s = -8(t+1.5) - 4.9(t+15)(t+15)

what have I done wrong?

--------------

yes I agree g is downwards/positive but the first ball is thrown up so wouldn't a be negative in this case?

Reply 10

u is upward, a is downward
so one of them has to be +ve and the other has to be -ve...
u took both -ve
(u took u=-8 and a=-9.8)

Reply 11

yazan I've been taught that when being thrown upwards acceleration is -9.8

Reply 12

g doesnt change its dirction.. .it's always DOWNWARD! what ever the direction of u was acceleration due to gravity is ALWAYS downward! the first thing u should do before answering a question is putting the signs for direction and not change them...
if u took up to be +ve then in this case u is +ve and a is -ve. if the wall was thrown down, a will be -ve and u will be -ve

Reply 13

what u have been taught is wrong. what im telling u is what u shoud know!
just as i said: if u chose up to be +ve then any vector upwards is +ve, and vector downward is -ve.
if u chose up to be -ve then any vector upwards is -ve, and vector downward is +ve.
and acceleration due to gravity is always downward...

Reply 14

actually : Undry chose up to be -ve and down to be +ve. so the sign of acceleration is not fixed... it's always up to u...

Reply 15

yusufu

16

yazan_l

what u have been taught is wrong. what im telling u is what u shoud know!
just as i said: if u chose up to be +ve then any vector upwards is +ve, and vector downward is -ve.
if u chose up to be -ve then any vector upwards is -ve, and vector downward is +ve.
and acceleration due to gravity is always downward...

lol. its not entirely wrong. as long he takes anything acting upwards as +ve and anything acting downwards as -ve it will still work out.

Reply 16

Craver

Taking a as -9.8 on the first, though doesn't give

s = -8(t+1.5) + 4.9(t+1.5)²

but gives

s = -8(t+1.5) - 4.9(t+15)(t+15)

what have I done wrong?

--------------

yes I agree g is downwards/positive but the first ball is thrown up so wouldn't a be negative in this case?

that is ur mistake..
u used u = -8 , a = -9.8
although they r in opposite directions... if u used u = 8 and a = -9.8 u will get the same answer ...

Reply 17

Undry1

lol. its not entirely wrong. as long he takes anything acting upwards as +ve and anything acting downwards as -ve it will still work out.

hehe ya i know,,, but the wrong thing is that the sign of a is not always -ve... (actually i said what i said coz he made me nervous! :mad:

) hehe

Reply 18