Pulley question- What have I done wrong here?

[Mayzu]

Homework Statement

http://imgur.com/ymNWTOV
Image isn't working, here is the question:
http://imgur.com/ymNWTOV

Homework Equations

F=ma
Weight force=mg
Frictional force=μ_kN

The Attempt at a Solution

So for part (a):[/B]
Force on 10kg block=mg=10x9.8=98N
This is equal to the tension force in the string, which is pulling the 5kg block right

Frictional force on the 5kg block (pulling it left):
F=μ_kN
=0.4*(mg)
=0.4*9.8*5
=19.6N

Therefore the net force on the block is 98-19.6=78.4N
F=ma
Therefore accelaration(5kg block)=F/m=78.4/5
This gave me 15.68m/s², while the answer was actually 5.23...

Part b
I have no clue how to do this part! Answer is 3.29J...

 

[Chestermiller]

Homework Statement

http://imgur.com/ymNWTOV
Image isn't working, here is the question:
http://imgur.com/ymNWTOV

Homework Equations

F=ma
Weight force=mg
Frictional force=μ_kN

The Attempt at a Solution

So for part (a):[/B]
Force on 10kg block=mg=10x9.8=98N
This is equal to the tension force in the string, which is pulling the 5kg block right

No. This is incorrect. Have you drawn a free body diagram on the 10 kg block? Do you feel that you have advanced beyond the point where you need to draw free body diagrams? Let T represent the tension in the string. What is the net force acting on the 10 kg block?

Chet