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# Mechanics M1 Help Watch

1. need help on Q18 all parts, is it simultaneous eq's? Thanks
2. (Original post by Sniperdon227)
need help on Q18 all parts, is it simultaneous eq's? Thanks
Not as such.

a) You should be able to do.

b) reset the clock so that t=0 when B starts, and now work out s (displacement for A and for B). When they meet, they're equal, so equate and solve for t.
Displacement for A needs a bit of thought - post your initial equation/thoughts, if in doubt.

c) Knowing t, you can find the velocities.

Edit: B_9710 has given a slightly different, more standard, method below, so be clear to whom you're replying, if you do.
3. Just use the SUVAT equations to find the distance in terms of t, and then equate the 2. remember though, for the car B, it is initially 5 seconds behind, so replace t with (t-5) when you find an expression for the distance that B has travelled after t seconds. (Because when t=5 A has moved but B hasn't).
4. Actually, looking at it there are probably better ways of doing this.
5. The answers are a)25 b)20seconds
Okay when I used; s= ut +0.5a(t)^2 where
A)
s=
u=0
v=
a=2
t=(t+5)

B)s=
u=0
v=
a=3.125
t=t

it worked however when I did what B_9710 said it never worked A(let t=t) B(let t=(t-5))
6. (Original post by Sniperdon227)
Okay when I used; s= ut +0.5a(t)^2 where
A)
s=
u=0
v=
a=2
t=(t+5)

B)s=
u=0
v=
a=3.125
t=t

it worked however when I did what B_9710 said it never worked A(let t=t) B(let t=(t-5))
You have to take great care doing it the way I suggested which is why I said that other ways are probably a bit easier.
7. (Original post by Sniperdon227)
it worked however when I did what B_9710 said it never worked A(let t=t) B(let t=(t-5))
With B_9710's method it works fine, but you get t=25. Since you've used t as the time for A, then it's 25 seconds after A started, which makes it 20 seconds after B started - the desired result.
8. (Original post by ghostwalker)
With B_9710's method it works fine, but you get t=25. Since you've used t as the time for A, then it's 25 seconds after A started, which makes it 20 seconds after B started - the desired result.
I get it the answer in each case needs interpreting, thanks for your help guys

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