Question on Newton second's law

[UNknown 2010]

Hello,

This is my first post on the forum, I hope that I will get an answer for my question as soon as possible.


http://img512.imageshack.us/img512/5765/55503111px2eq6.png

Thanks in advance

 

[arildno]

Well, what have you done so far?

 

[UNknown 2010]

Hello,

Did you understand my question ?
because it is translated and my english is not good enough .

My try:
Total Mass = 48 kg ==> 480 Newtons

(1)
F=ma
480-250=48a
a = 4.79 m/s^2

(2)
F = 32(10-4.79) = 166 Newtons

I know it is wrong but after I read the sticky topic I saw that I should put my tryhelp me please

 

[Redbelly98]

There is another force acting on the boy+seat which you are not accounting for. Look at the picture carefully to see what that might be.

Also, be careful about direction (upward or downward) and +/- signs. I suggest using + for the upward direction, and - for downward direction.

 

[UNknown 2010]

I couldn't find the third foce

 

[Redbelly98]

In addition to the downward force of weight (gravity):

One end of the rope is attached to the scale, pulling upward.

The other end of the rope is attached to the boy+board.

 

[UNknown 2010]

http://img261.imageshack.us/img261/7338/23325527qn0.png
If it is not correct please give me the solution steps

 

[Doc Al]

In your FBD for the board: You have the force "b" acting upward, but the boy exerts a downward force on the board.

In your FBD for the boy: Again, the direction of force "b" is backward. The board pushes up on the boy. Also, you neglected the force on the boy due to the rope he's holding.

 

[UNknown 2010]

Thank PF members especially Doc Al

I solved it and I checked the answer from the final answer that is provided under the question and it is correct