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Mechanics M1 Help

IMG_4479[1].jpgneed help on Q18 all parts, is it simultaneous eq's? Thanks
Original post by Sniperdon227
IMG_4479[1].jpgneed 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.
(edited 7 years ago)
Reply 2
Avatar for B_9710
B_9710
18
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).
Reply 3
Avatar for B_9710
B_9710
18
Actually, looking at it there are probably better ways of doing this.
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))
Reply 5
Avatar for B_9710
B_9710
18
Original post by Sniperdon227
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))


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.
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.
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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