# Homework Help: Block on a slope, friction related

1. Jan 16, 2013

### Revolt

A 300kg block is sitting on a slope of 20 degrees, the coeff of friction is 0.75, if the block is connected to a mass hanging vertically via a pulley, what is the mass of the hanging block such that it will overcome friction but not move the block

F = mgsin20 for the force acting down the slope
F = mgcos20 for the force acting perpendicular to the slope

Have attached a PDF of my attempt, but basically I found the force acting down the slope and then used this as the force in my next equation to calculate the mass,

Getting a result of 384kg, I now realise I havent properly taken friction into account and im unsure of how to do this.

#### Attached Files:

• ###### konica1wo120130116103807[1].pdf
File size:
232.9 KB
Views:
62
2. Jan 16, 2013

### apelling

In your attempt you've added the two vector components together as if they are scalar quantities ie 1006 + 2765 = 3771. I suggest you draw a free body force diagram for the 300kg block. Are we to assume it is on the point moving upwards?

3. Jan 16, 2013

### Revolt

Indeed, got another solution ill upload in a minute

4. Jan 16, 2013

### Revolt

For some reason the scanner isnt working, but basically i found the force required to in effect keep the block on the slope still, using

F = mgsin20 + umgcos20

and from this force i used m=F/g to get the mass of the block, 314kg.... this seem right ?

5. Jan 16, 2013

### Revolt

Could someone maybe run through this and tell me if they get a matching 314kg for the other block? thanks

6. Jan 16, 2013

### Revolt

Have uploaded a new file with my method, anyone comment please?

#### Attached Files:

• ###### vvvvvvvv.pdf
File size:
475 KB
Views:
84
7. Jan 16, 2013

### lep11

Well...you should draw all the forces and components acting on the block and also show clearly your reasoning. Otherwise your calculations are correct.

Last edited: Jan 16, 2013