# Homework Help: Finding Force with friction on a slope

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1. Sep 27, 2015

### PAstudent

1. The problem statement, all variables and given/known data

I was wondering if my phi angle was in the right position from the picture. Also, it is correct that my normal force FN is perpendicular to the surface?

2. Relevant equations
Newton's second law
fk=ukFN

3. The attempt at a solution
Refer to picture

2. Sep 27, 2015

### TSny

The angle Φ is not correctly indicated in your diagram. Note that Φ is the angle F makes to the vertical direction. Your normal force is correctly drawn.

3. Sep 27, 2015

### PAstudent

So the angle would just be on the other side of that Force vector

4. Sep 27, 2015

### TSny

I'm not sure exactly what you mean. Could you be more specific or attach another figure?

5. Sep 27, 2015

### PAstudent

6. Sep 27, 2015

### TSny

That's still not quite right. You now have Φ as the angle between F and your y axis. But Φ is the angle between F and the vertical direction.

7. Sep 27, 2015

### PAstudent

I get what you are saying about the vertical direction, but I just don't get how that vector wouldn't be in that quadrant

8. Sep 27, 2015

### TSny

You have F in the correct quadrant of your x-y coordinate system. But you do not have the angle Φ indicated correctly. Φ is not the angle between F and the y axis. Φ is the angle between F and the vertical direction. The vertical direction is along the same line that mg acts. Draw a dotted line along mg and extend it upward into the same quadrant that F is in.

9. Sep 27, 2015

### PAstudent

10. Sep 27, 2015

### TSny

You rotated F to the wrong side of the vertical. In the original figure, F points up and to the left. You now have F pointing up and to the right.

11. Sep 27, 2015

### PAstudent

Hopefully this is correct. Sorry to put you through all of that. I just struggle with these type of angle concepts

12. Sep 27, 2015

### TSny

Yes, your free body diagram is now correct. You just need to find the angle that F makes to the x axis (or y axis).

13. Sep 27, 2015

### PAstudent

Thank you for all your help