Homework Help: Two blocks on a tabletop

1. Feb 15, 2010

cdotter

1. The problem statement, all variables and given/known data
Block A (mass 2.25 kg) rests on a tabletop. It is connected by a horizontal cord passing over a frictionless pulley to a hanging block B (mass 1.30 kg.) The coefficient of kinetic friction between block A and the table is 0.450. After the blocks are released from rest, find a. the speed of each block after moving 3.00 cm and b. the tension of the cord.

2. Relevant equations

3. The attempt at a solution
Block A:
Fgravity: (2.25kg)(9.8m/s2) = 22.05N
Fnormal: 22.05N
Ffriction: (22.05N)(0.450) = 9.92N
Resultant force: 22.05N - 9.92N = 12.13N

Block B:
Fgravity: (1.30kg)(9.8m/s2) = 12.74N

I have no idea what to do after this. I found the exact same problem through a search but I don't understand the explanation.

2. Feb 15, 2010

tiny-tim

Hi cdotter!
Sorry, but this is completely wrong

you haven't used the tension, T, at all;

and the 22.05 and the 9.92 are perpendicular, so you can't add (or subtract) them anyway.

Try again.

3. Feb 15, 2010

cdotter

I now understand that the forces are perpendicular and that I can't subtract them, but how are my normal force and frictional force calculations incorrect?

4. Feb 16, 2010

tiny-tim

Your normal force and frictional force were correct, but your "resultant force" equation was completely wrong.

Try again, using T.

5. Feb 17, 2010

cdotter

Block B pulls down with 12.74N of force. Block A has a frictional force of 9.92N. Shouldn't the tension then be 12.74N+9.92N = 22.66N? My book says it's 11.7N. I have absolutely no idea where 11.7N comes from.

6. Feb 17, 2010

tiny-tim

No, everything is accelerating, and you've left "a" out of your F = ma.

7. Feb 17, 2010

cdotter

Oh, but I don't understand how to put what I have into the equation?

Block A: $F_{fric}-T_{rope}=ma$
Block B: $F_{gravity}-T_{rope}=ma$

Right?

edit: I guess not. I'm so lost haha.

Last edited: Feb 17, 2010
8. Feb 17, 2010

tiny-tim

there are only two forces on it, and their total has to equal its mass times its acceleration.

9. Feb 17, 2010

cdotter

Could you show me what you mean because I have absolutely no idea what to do. It has two forces acting on it: weight pulling down and the tension of the rope pulling up. I don't know how to calculate those or put them into the equation.

10. Feb 17, 2010

cdotter

I figured it out by reading another post, thank you for the hints tiny-tim.