Having trouble with a few problems

[ToddS]

Hello all,

I've tried to do a few problems from an extensive list of homework problems we were assigned, but I am stuck on them.. I will show you how far I got, if anyone can see where I went wrong please reply.. thanks.

51. Two blocks connected by a rope of negligible mass are being dragged by a horizontal force F. Suppose that F = 68N, mass1 = 12kg, and mass2 = 18kg, and the coefficient of kinetic friction between each block and the surface is 0.100. (b) Determine the tension T and the magnitude of the acceleration of the system

First, I created two separate free body diagrams.. and I calculated the weight of each mass. Then I attempted to use the equation Acc = F (force to acc system) / total mass .. however, this gives an incorrect result. The correct answers are tension = 27.2N and acc = 1.29m/s^2.

I have tried re-arranging amongst other things, and nothing seems to work. Does anyone see where I might have gone wrong? Thanks.

Todd

 

[Skomatth]

Acc = F (force to acc system) / total mass

This will work provided you use the net force . However I think it will be a lot clearer if you use write separate equations for each mass and combine those equations to solve for the acceleration. Since you aren't getting the correct answer your F must be the wrong value.

 

[ToddS]

F was given, I guess my mass equations are incorrect. Can you give me an idea of how I would determine them? Would you use g? u?

 

[Skomatth]

The force of friction which opposes the applied force is
$$\mu m g$$

 

[ToddS]

so.. how does this look?

F = ma
Fu = ma
uN = ma
umg = ma

?

 

[ToddS]

Skomatth, do you have AIM?

 

[Skomatth]

I have msn.

Anyway look at the system as a whole. The applied force is causing it to speed up, while the force of friction is causing it to slow down. Find the difference between these forces and you have the net force.