# Kinematics and dynamics

rl.bhat
Homework Helper
I'm sorry,rl.bhat.I still don't get it.
for B
final length= b+d
initial lenght = b
so substracting gives me -b

and for C,
final length = d-c
initial length = c
and subtacting to find the net change in segment gives me = d-2c ?

In the problem it is said that B moves 2.44 m in 2 seconds.
In 2.44 m(b) includes fall of the mass B, which changes the length of the rope segment, plus fall of the pulley D(d), which does not change the length of the rope segment
To calculate the net change in the length of the rope, we must know the change in the rope segment of the mass B alone.And that is b - d

To calculate the net change in the length of the rope, we must know the change in the rope segment of the mass B alone.And that is b - d

OK.Thanks.
I think I understand now,but I would like to know if this is how we generally solve for the accelerations in all pulley systems,that is by taking the displacements and then the change in rope segments?

Forget my last post.I still don't get it Since the movement of pulley D has no effect on the rope segment of B,shouldn't the change in rope segment of B,be b only?

tiny-tim
Homework Helper
Forget my last post.I still don't get it Since the movement of pulley D has no effect on the rope segment of B,shouldn't the change in rope segment of B,be b only?

Hi leena19! I haven't really been following this thread, but the depth of B (below the ceiling) is Lb + Ld, the depth of C is Lc + Ld, the depth of D is Ld, the depth of A is La, and La + Ld is constant, and Lb + Lc is constant …

you should be able to work it all out from that. (Lb is the length of rope between B and the nearest pulley, and so on )

rl.bhat
Homework Helper
Forget my last post.I still don't get it Since the movement of pulley D has no effect on the rope segment of B,shouldn't the change in rope segment of B,be b only?

Even though the movement of pulley D has no effect on the rope segment of B, it affects the distance moved by the body. In the problem b is not the change in length of the rope segment. It is the distance moved by B. If D is not moving, then b is the change in length of the rope segment B.
In all such problems we have to follow five steps.
Step 1: Find out the number of inextensible ropes. For every inextensible rope there will be one equation.
Step 2: For every inextensible rope, find the bodies that are connected to the rope.
Step 3: To relate the acceleration between the bodies, assume that the various bodies move by a distance x1, x2, ....and so on.Calculate the number of segments in the rope.
Step 4: Relate the distance moved. (Total change in the length of the rope must be zero.) To do these calculate the change in length of each segment of the rope. Then add these changes to get the total change in the rope.
Step 5: Once we get the relation between the distances moved, the acceleration relation will be the same.

Thank you SO much,rl.bhat.
I finally understand.It feels so good to finally understand,
so once again ithank you rl.bhat,tiny-tim,andrevdh and anant for all the help!

rl.bhat
Homework Helper
Thank you SO much,rl.bhat.
I finally understand.It feels so good to finally understand,
so once again ithank you rl.bhat,tiny-tim,andrevdh and anant for all the help!
You are welcome. I am happy that finally I am able to convince you.