Determining Scale Readings on an Atwood Machine

[doug1]

Homework Statement

Two masses, measuring 800g and 400g respectively, are attached to spring scales and an Atwood Machine. What will the reading of each scale be?

https://www.physicsforums.com/attachments/52223

Homework Equations

Fg = mg

The Attempt at a Solution

I think that the reading on each spring scale will equal the force of gravity acting on each attached mass.

For Scale 1:

Reading = Fg = mg
= (0.8kg)(9.8N/kg) = 7.84 N

For Scale 2:

Reading = Fg = mg
= (0.4kg)(9.8N/kg) = 3.92 N

Is this correct?

 

[PhanthomJay]

Can't open the attachment, but assuming a massless frictionless ideal pulley (yes??), the tension in the rope as measured by the scale must be the same on both sides of the pulley. You will need to draw a free body diagram for each mass to identify the forces acting on each, then use Newton's 2nd law on each to get 2 equations to solve for the tension force (scale reading). Note that the system is accelerating.