Solving Chain Homework w/ Newton's Laws: Acceleration & Force

[azila]

Homework Statement

A student tries to raise a chain consisting of three identical links. Each link has a mass of 200 g. The three-piece chian is connected to a string and then suspended vertically, witht the student holding the upper end of the string and pulling upward. Because of the student's pull, an upward force of 14 N is applied to the chain by the string. Use Newton's laws to answer the following questions.

Find the acceleration of the chain

Find the force exerted by the top link on the middle link

Homework Equations

F = ma;

The Attempt at a Solution

So for the first answer, I used the F=ma equation. Force is 14 N and mass 200 g of one link. But when I divide those two, I get .07 which is not the answer. I know I am doing something wrong. I even tried converting the grams in kilograms which is .02 kg and when I divide the force of 14N by .02 kg, I get 70 which is not the answer. If you can help, I would appreciate it. Thank you.

 

[learningphysics]

Take the freebody diagram of one of the links... what are all the forces acting on it. use $$\Sigma\vec{F} = ma$$ where m is the mass of one of the links. don't forget about gravity.

 

[highcoughdrop]

Okay, I've found the acceleration of the chain, but now how do I find the force exerted by the top link on the middle link? I thought that it was F = m*a, so I took my acceleration (which is the correct acceleration) and multiplied it by the mass of one link, and then added it to the 14 N force upward. What am I doing wrong?

 

[kamerling]

This is nearly the same problem that you solved before, except that you now know the acceleration and not the force. For the bottom two links you also have F = m * a. The mass involved here is the mass of the two bottom links. F is made up of gravity and the force you want to find.