# [SOLVED] Newtons Second Law - Simple Mechanics

• May 17th 2010, 05:20 AM
unstopabl3
[SOLVED] Newtons Second Law - Simple Mechanics
Kindly have a look at the attached image for the related question.

I've solved (i) to get the following values...

a= 2.5 ms^-2
T= 3.75 N

I want to inquire about part (ii)

What I want to know is that since acceleration of A was 2.5 then the acceleration of B should be -2.5 ... no?

I ask this because, for part (ii) I've got the correct answer of 'mass' by using

T-10m=2.5m

3.75/12.5=m

m=0.3 kg

But if I use -2.5 then the answer comes out to be m=0.5 kg.

What am I doing wrong?

Thanks!
• May 17th 2010, 06:15 AM
Quote:

Originally Posted by unstopabl3
Kindly have a look at the attached image for the related question.

I've solved (i) to get the following values...

a= 2.5 ms^-2
T= 3.75 N

I want to inquire about part (ii)

What I want to know is that since acceleration of A was 2.5 then the acceleration of B should be -2.5 ... no?

I ask this because, for part (ii) I've got the correct answer of 'mass' by using

T-10m=2.5m

3.75/12.5=m

m=0.3 kg

But if I use -2.5 then the answer comes out to be m=0.5 kg.

What am I doing wrong?

Thanks!

the acceleration is the same for the whole system which is + 2.5 . You only account the negative for the gravity part .
• May 17th 2010, 06:16 AM
Haytham
in body B, if you use acceleration -2.5\$\displaystyle m/s^{2}\$, this means you are considering the upwards direction to be negative

so you need to consider T also negative because its upwards and the weight to be positive because its downwards

so
-T+10m=-2.5m

you got
T-10m=2.5m
considering the opposite. downwards negative and upwards positive.
its also works
as long as same direction vectors have the same signal
• May 17th 2010, 07:44 AM
unstopabl3
Thanks for the quick replies guys!