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M1 AQA SUVAT Question, Help Please

http://filestore.aqa.org.uk/subjects/AQA-MM1B-W-QP-JUN10.PDF
http://filestore.aqa.org.uk/subjects/AQA-MM1B-W-MS-JUN10.PDF

Q6ciii) I cant do it for some reason

My values
S=4
U=3.92 They make this negative im not sure why?
V=9.68
A=9.8
T=?

Surely if they make u negative then they'd make v negative because those values correspond to Particle A not B

Reply 1

Notnek

Original post by Sniperdon227

Before the string breaks, B is moving up towards the pulley (since A is the heavier particle).

Once the string breaks, B will continue to move up but it's acceleration will only be due to gravity which acts downwards.

So if you take downwards as positive then u will be negative when the string breaks. Eventually the particle will change direction and move down towards the surface. But the initial velocity is upwards.

(edited 7 years ago)

Reply 2

Sniperdon227

For A
S=4
U=3.92
V=9.68
A=9.8
T=?

For B
S=4
U=-3.92
V=9.68
A=9.8
T=?

Can you please explain it more, I still dont understand

Reply 3

alesha98

Original post by Sniperdon227

For A
S=4
U=3.92
V=9.68
A=9.8
T=?

For B
S=4
U=-3.92
V=9.68
A=9.8
T=?

Can you please explain it more, I still dont understand

Before the string break, particle A is moving downwards and particle B is moving upwards. Since the direction of A and B are different, so the sign is different. If the mark scheme has set a positive direction as downwards, then B will be moving in a negative direction. Therefore the signs of the particle A and B are different

Reply 4

Notnek

Original post by Sniperdon227

Can you please explain it more, I still dont understand

If you could tell us why you think

u

should be positive then that will help us explain why it shouldn't be.

Reply 5

Sniperdon227

OMG Ive been reading the question wrong all this time it goes up 4m then goes down 8m so
S= -4
U=3.92
V=9.68
A=9.8
T=?

this is taking up to be positive
t=1.39