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Tough mechanics question help!!

Two blocks, A and B, of masses 2m kg and 3m kg, respectively, are attached to the ends of a light inextensible rope, which passes over a smooth fixed pulley. A is held on the floor, and B hangs in equilibrium, at a height of 2 metres above the floor. A is released. Find the time taken for B to hit the floor. Give your answer to an appropriate degree of accuracy.

I think I may have done it wrong but I can't see where in my method I went wrong.

My method:

A: T - 19.6m = 2m (a)
B: 29.4m - T = 3m (a)

A + B: 9.8m = 5m (a)
a = 9.8m/5m
= 1.96ms^-2

T - 19.6m = 3.92m
T = 23.52m

s = 2
u = 0
v = 0
a = 1.96
t = ?

2 = 0.98t^2
t^2 = 100/49
t = 10/7 seconds
Original post by psycholeo
Two blocks, A and B, of masses 2m kg and 3m kg, respectively, are attached to the ends of a light inextensible rope, which passes over a smooth fixed pulley. A is held on the floor, and B hangs in equilibrium, at a height of 2 metres above the floor. A is released. Find the time taken for B to hit the floor. Give your answer to an appropriate degree of accuracy.

I think I may have done it wrong but I can't see where in my method I went wrong.

My method:

A: T - 19.6m = 2m (a)
B: 29.4m - T = 3m (a)

A + B: 9.8m = 5m (a)
a = 9.8m/5m
= 1.96ms^-2

T - 19.6m = 3.92m
T = 23.52m

s = 2
u = 0
v = 0
a = 1.96
t = ?

2 = 0.98t^2
t^2 = 100/49
t = 10/7 seconds

Why do you think that it's wrong? It looks right to me.
Reply 2
Avatar for Notnek
Notnek
21
Original post by psycholeo
Two blocks, A and B, of masses 2m kg and 3m kg, respectively, are attached to the ends of a light inextensible rope, which passes over a smooth fixed pulley. A is held on the floor, and B hangs in equilibrium, at a height of 2 metres above the floor. A is released. Find the time taken for B to hit the floor. Give your answer to an appropriate degree of accuracy.

I think I may have done it wrong but I can't see where in my method I went wrong.

My method:

A: T - 19.6m = 2m (a)
B: 29.4m - T = 3m (a)

A + B: 9.8m = 5m (a)
a = 9.8m/5m
= 1.96ms^-2

T - 19.6m = 3.92m
T = 23.52m

s = 2
u = 0
v = 0
a = 1.96
t = ?

2 = 0.98t^2
t^2 = 100/49
t = 10/7 seconds

Was the answer given different to yours or are you asking whether your working is right or not? At first glance it looks fine.
Reply 3
Avatar for psycholeo
psycholeo
OP
9
Original post by RogerOxon
Why do you think that it's wrong? It looks right to me.


Idk. Probably because it says give your answer to an appropriate degree of accuracy at the end, and idk why it'd say that if the answer was obviously 10/7
Original post by psycholeo
Two blocks, A and B, of masses 2m kg and 3m kg, respectively, are attached to the ends of a light inextensible rope, which passes over a smooth fixed pulley. A is held on the floor, and B hangs in equilibrium, at a height of 2 metres above the floor. A is released. Find the time taken for B to hit the floor. Give your answer to an appropriate degree of accuracy.

I think I may have done it wrong but I can't see where in my method I went wrong.

My method:

A: T - 19.6m = 2m (a)
B: 29.4m - T = 3m (a)

A + B: 9.8m = 5m (a)
a = 9.8m/5m
= 1.96ms^-2

T - 19.6m = 3.92m
T = 23.52m

s = 2
u = 0
v = 0
a = 1.96
t = ?

2 = 0.98t^2
t^2 = 100/49
t = 10/7 seconds


Have a bit of confidence lol, you did it right.
Reply 5
Avatar for psycholeo
psycholeo
OP
9
Original post by Notnek
Was the answer given different to yours or are you asking whether your working is right or not? At first glance it looks fine.


No answers given with the paper.
Original post by psycholeo
Idk. Probably because it says give your answer to an appropriate degree of accuracy at the end, and idk why it'd say that if the answer was obviously 10/7


9.8 is not exact, it is only to 2 s.f. You therefore should not give a fraction as an answer as that is exact. For Edexcel, give 2 s.f. (or they will accept 3 s.f.). I'm not sure what accuracy other boards want.

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