Difference between applied tension and tension due to weight?

[Petrikovski]

im trying to do this AP problem that involves a block on a table attached to a string which hangs over a pulley. in Case I 50 N is applied to the string and in Case II a 5 kg block is attached. the tension in Case I is 50, but according to my packet in case II the tension in Case II is 40. why would 50 N applied directly be more tension than Fw = 5x10 = 50 N?

 

[Petrikovski]

btw the block on top is 10kg and there is a coefficient of friction of .2

 

[daniel_i_l]

Because in the second case there is more mass so there's less acceleration:
mg - Fs = (M+m)a
50 - 20 = 15a
a = 2
Now looking at the 5kg weight:
50 - T = 5a
T = 50 - 10 = 40