# Incline problem

1. Oct 12, 2007

### iHate Physics

1. The problem statement, all variables and given/known data
Two packages at UPS start sliding down the 20 degree ramp shown in the figure. Package A has a mass of 7.00 kg and a coefficient of kinetic friction of 0.180. Package B has a mass of 12.0 kg and a coefficient of kinetic friction of 0.150.

How long does it take package A to reach the bottom?

http://img362.imageshack.us/img362/1479/knightfigure0825pe6.th.jpg [Broken]

2. Relevant equations

F=ma (sorry but thats all I know for this question)

3. The attempt at a solution
I hate incline problems with a passion, and I have no idea how to start one. All I have done is draw free body diagrams for this question. I know you are supposed to apply Newton's second law, but it get confused when i do this on an incline question

Last edited by a moderator: May 3, 2017
2. Oct 12, 2007

On your free body diagram, if you take the incline as your x-axis, this should help simplify things.

3. Oct 12, 2007

### bob1182006

can you show us what you got for Newton's second law when applied to each block individually?

For this problem you should first find the acceleration of package A. From there you can work out the time it takes to fall down the 2m @ 20* incline.

4. Oct 13, 2007

### iHate Physics

k well for box A i got

Fnet = F B on A - Ff

and for Box B i got

F net = -F a on B - Ff

but what would F b on a be equal to if i broke it down into f=ma format? Would it be f= mass of b times the acceleration of b?

Last edited: Oct 13, 2007
5. Oct 13, 2007

### bob1182006

and yes the force B exerts on A will be the net force that B is experiencing.

6. Oct 13, 2007

### iHate Physics

oh crud i forgot, it should be Fgx in both questions.

7. Oct 13, 2007

### iHate Physics

so i should work with diagram b right? to find the acceleration of block a.

8. Oct 13, 2007

### bob1182006

it doesn't matter which one you work from, they both experience the same acceleration.
And since you need to substitute in force of block a on b, or force of block b on a. Eventually you will get down to the same equation with the same answer.

9. Oct 14, 2007

### iHate Physics

k but like what do you mean substitute?

10. Oct 14, 2007

### iHate Physics

i got 4.3586 as my acceleration..

11. Oct 14, 2007

### bob1182006

could you show the work please? I got something like lower than your value for acceleration.

When you have the 2 formulas for the net force of A and B you need to consider the effect that B has on A because A has a higher coefficient of friction it will probably be slowing down B so B will be pushing down on A and thus giving it a bit higher accleration.