Maths Mechanics M1 HELP!

[nehaoscar]

Can anyone solve part (ii) please with workings.

 

[SebastianM]

Okay. So for Part I, you get the answer as 2 SECONDS.

Now Part II---->
The difference between the heights of P and Q is 5m. Keep that in mind.
Use the equation s = ut + 0.5at^2
Height of P= 17t - 5t^2
Height of Q= 7(t-2) - 5(t-2)^2
For Q, it's time is t-2, since Q was vertically projected AFTER 2 seconds, that's when P was already in air. Letting the time for P=t seconds, the time for Q would then be (t-2) seconds.

Moving on. I already told you to keep the difference of heights in mind.
17t - 5t^2 - 7(t-2) + 5(t-2)^2 = 5
When you solve for t,
t = 2.9 seconds

Now find velocity of each of the particles, using the equation v = u + at
Velocity of P= -12 m/s
So it is travelling with a velocity of 12 m/s downwards.

Similiarly, velocity of Q= -2 m/s
So, it travels with 2 m/s downwards.

 

[nehaoscar]

Okay. So for Part I, you get the answer as 2 SECONDS.

Now Part II---->
The difference between the heights of P and Q is 5m. Keep that in mind.
Use the equation s = ut + 0.5at^2
Height of P= 17t - 5t^2
Height of Q= 7(t-2) - 5(t-2)^2
For Q, it's time is t-2, since Q was vertically projected AFTER 2 seconds, that's when P was already in air. Letting the time for P=t seconds, the time for Q would then be (t-2) seconds.

Moving on. I already told you to keep the difference of heights in mind.
17t - 5t^2 - 7(t-2) + 5(t-2)^2 = 5
When you solve for t,
t = 2.9 seconds

Now find velocity of each of the particles, using the equation v = u + at
Velocity of P= -12 m/s
So it is travelling with a velocity of 12 m/s downwards.

Similiarly, velocity of Q= -2 m/s
So, it travels with 2 m/s downwards.

Okay I got that but then the mark scheme says that the time t=0.9 seconds whereas you (and me too) are getting 2.9 seconds...
Either way it gives you the correct answer but if you calculate, the total journey time for P and Q (in part i to find T) then...
P = 3.4seconds to complete journey
Q = 1.4seconds to complete journey
So since we are getting 2.9, the journey for Q is already complete but the question states that P and Q are both in motion.
So although the final answers are correct, you'd possible lose one mark for the wrong time in the method.
The mark scheme is getting both values : 0.9 and 2.9 seconds but since both particles should be in motion, they have used the 0.9 seconds option.
What I'm not understanding is how they are getting t = 0.9seconds?

 

[nehaoscar]

Oh WAIT nevermind! They have used 0.9seconds to find velocity of Q only since T = t-2
LOL thanks a lot!