determine Tention in a string

[staceyrho]

I have this problem but I'm having trouble getting started.

A 4.12 kg block is placed on top of a 8.6 kg block. A horizontal force of F=54.6 N is applied to the 8.6 kg block, and the 4.12 kg block is tied to the wall. The coefficient of kinetic friction between all moving surfaces is 0.104. There is friction both between the masses and bwtewwn the 8.6 kg block and the ground.
The acceleration of gravity is 9.8 m/s^2.
Determine the tension T in the string. Answer in units of N?

Then determine the magnitude of the acceleration of the 8.6 kg block. Answer in units of m/s^2

I tried to draw out what I have where I typed block I couldn't figure out how to make a block so that should be a square block.


T
_______[block 4.12 kg] F

u=0.104 [block 8.6kg]--------->

[u= 0.104]

 

[skeeter]

let m = mass of the small block
M = mass of the large block
g = acceleration due to gravity
u = coefficient of kinetic friction

the small mass is not moving, so it is in a state of equilibrium. therefore, net force on the small mass = 0.

forces acting on the small mass ...

T = Tension to the left
f_k1 = force of kinetic friction to the right
mg = weight down
N₁ = normal contact force up

since the small block is in equilibrium ...
T = f_k1

since f_k1 = u*N₁ ...

T = u*N₁

and, since N = mg ...

T = u*mg

forces acting on the large block ...

F = applied force to the right
f_k1 = force of kinetic friction from the upper mass to the left
f_k2 = force of kinetic friction from the ground to the left
Mg = weight down
N₁ = mg = normal contact force of the small mass down
N₂ = normal force of the ground up

the large mass is in equilibrium in the y-direction, so N₁ = Mg + mg

in the x-direction ...

F_net = F - f_k1 - f_k2

Ma = F - f_k1 - f_k2

finish up by solving for "a"

 

[staceyrho]

I am trying to follow what your doing but i'm really having trouble. Do you have an easier way to explain it?

 

[stapel]

I am trying to follow what your doing but i'm really having trouble.

You've had quite a few of this sort of exercise worked out for you. At which line in the explanation did you bog down?

Please be specific. Thank you.

Eliz.

 

[staceyrho]

I guess in physics the formulas don't make sence. It's not you do one formula then use that answer to apply to the next step and so on. It seems like a lot of work is done on the forumlas and the answer is not even applied to what your looking for. I have worked with tutors and re-read and re-read the chapter again and agian. I can't grasp the formulas and why we are using these. I just need to get through this semester. I am a mother of 3 and one on the way in a couple of months i'm just really having a hard time.

 

[stapel]

I guess in physics the formulas don't make sence. It's not you do one formula then use that answer to apply to the next step and so on....

No, physics isn't simple plug-n-chug algebra. You do have to understand the concepts behind the values, expressions, and formulas, and use this understanding to develop your own formulas, appropriate to each situation.

But then, this is a large part of the "point" of university studies: moving beyond simplistic and rote procedures, toward broad and deep understanding.

Eliz.