# A few questions

1. Oct 23, 2006

### ndogg

I have a few questions:

1) Does the weight of the rope (if it is significant) affect its tension?

2) Look at this picture: http://i14.tinypic.com/3ynt3cg.jpg

3) If an object sits at rest on a record player's record that is moving around, where is its frictional force?

2. Oct 23, 2006

1) Yes, it does, but it is never significant in introductory physics problems.

2) What do the FBD-s look like? Well, the first question you need to answer is: what forces act on the box?

3) The frictional force always points in the opposite direction of the direction of motion. (Hint: which force acts on an object in circular motion?)

3. Oct 23, 2006

### ndogg

1) What if the rope weighed 10kg and the weight of the object was 15kg?

2) Tension and weight, but I'm not sure what the FBDs look like.

3) Centripetal force -- so wouldn't frictional force point opposite of the force going into the circle, meaning it points outward.

4. Oct 23, 2006

1) You should find the center of mass of the rope and add the vector of its weight placed at the center of mass.

2) In what directions do tension and weight point? Now you have all the data do draw FDB-s.

3) Correct.

5. Oct 23, 2006

### ndogg

1) Picture this: A rope is hanging from a hook on the ceiling. At the bottom of the rope is a mass of weight 15 kg. The rope itself weighs 10 kg. What is the tension in the rope?

2) I should have clarified my drawing in the first mass of 7 kg to be a person holding the string with his hand. I'm assuming the forces that act on the person are his weight (70 N), and the tension from the 6 kg mass on the other side of the pully (60 N). But I'm not sure what forces act on the 6 kg mass.

6. Oct 23, 2006

1) Again, draw a FBD with the weight of the rope acting in its center. Maybe this might help - what is the magnitude of the force which the hook exerts on the rope?

2) Hint: does the 6kg mass have a weight?

7. Oct 23, 2006

### ndogg

1) The hook exerts a force of 150 N on the rope. So would the tension in the rope just be 150 N? Or do you have to take into account the mass of the 10 kg rope, in which case the tension would be 250 N or 200 N?

2) The 6 kg mass does not have weight because it is suspended in the air, right?

8. Oct 23, 2006

### Staff: Mentor

Hint: in introductory physics textbooks, "weight" is usually a synonym for "gravitational force".

9. Oct 23, 2006

### ndogg

Oh yeah, I don't know what I was thinking. There is still weight, so the force pushing down is 60 N.

I still would like to have someone clarify #3.