Question

1. The problem statement, all variables and given/known data
two blocks of masses 8kg and 4kg respectively are connected by a string as shown in firgure. calculate their accelerations if they are initially at rest on the floor. after a force of 100N is applied on the pulley in the upward direction (g=10 m/s).
upload_2018-10-6_10-27-46.png
2. Relevant equations
Σf=ma

3. The attempt at a solution
okay, i am used to find the acceleration for the two blocks incase the pulley is fixed (same acceleration for both masses). and i know how to find acceleration for two masses, one is hanged from fixed pulley and the other from moving pulley (a fixed = 2 a moveing) . but it's my first time with this type of problem, the fixed pulley itself it moving.

now logically, the whole system will have acceleration, and i think i can find the whole acceleration for the system by considering both masses as one object being lifted (ignoring the mass of the pulley) with a force of 100N.

but How to find each block's acceleraion that's what i don't know. and i found an answer for this question which makes sense but still wasn't enough. it says:

"
force applied on the pulley will be 100 N
since pulley is ideal so we can say tension in the string must be 50 N
now we will have
for 4 kg block
plug in all values
weight of other block of mass 8kg is 80 N
its weight is more than the tension in the string so this block will remain at rest and its acceleration must be ZERO"

when i imagine it, i also end up saying block 8kg will have a=0. but i need to reach that by equations.

by the way, i think this system is similar to the atwood's machine which i didn't understand clearly :/ (see attachment )

Answer 1

first find acceleration of single blocks name them a and a2.after that find acceleration of the whole system or u can say acceleration of pulley and then apply the formula 2A=a +a2 where A is acceleration of the pulley and a of 8 kg can't be zero because 8kg is more heavy then 4 kg and it will tend to accelerate downward then too if u can't reach the answer comment me an pls rate my ans