# What's the direction

1. Aug 29, 2007

### pardesi

this is in relation to this post of mine.
my question is what is the direction of tension inside a rod (thin ) of uniform mass. the only direction i think can be along it's length but that seeems wrong because it's giving absurd answers. to the question in the link above?
so what do u all think

2. Aug 29, 2007

### Staff: Mentor

Tension in a string or rod is along the axis.

Otherwise there is bending, shear, and torsion - but none of these should be a significant factor in a conical pendulum problem.

In a conical pendulum, as another has mentioned, there are three forces: tension (along the axis), gravity (acting on the mass downward), and the centripetal force acting perpendicular to the axis of rotation, which if vertical means the centripetal force is horizontal).

3. Aug 29, 2007

### pardesi

but centripetal is no force there is just centripetal accelaration which i s a result of the circular motion and should be got by equating gravitya nd tension but why is that i am getting the wrong answer ?

4. Aug 29, 2007

### Nesk

Have you tried drawing a free body diagram, as recommended by Bel in your previous thread?

Remember that tension does not equal the force of gravity; rather, the sum of the components of tension and gravity perpendicular to the plane of movement should be zero.

5. Aug 29, 2007

### pardesi

that's what i have done in my work as my first post shows
there the only problem is $$dx$$ should be replaced by $$dT(x)$$
the differential tension but tsill the equations are the same and the problem unsolved

Last edited: Aug 29, 2007
6. Aug 29, 2007

### Staff: Mentor

The sum of components perpendicular to the plane of movement is not equal to the force due to gravity, it is the force due to gravity plus the force due to centripetal acceleration.

7. Aug 29, 2007

### pardesi

why is that

8. Aug 29, 2007

### Staff: Mentor

Because the inertial acceleration due to mv2/r is independent of the effect of gravity, mg. In addition, g acts uniformly along the rod (assuming constant linear mass density, i.e. uniform density and cross-sectional area of rod), whereas the centripetal force is not uniform on the rod because the distance from the axis of rotation varies as a function of r, and the tangential velocity is $\omega$r.

If $\omega$ = 0, i.e. v=0, then there is no centripetal force, but the force due to gravity still exists.

9. Aug 29, 2007

### pardesi

but why should i ever need a fictious force my frame of refrence is the earth in which the body is rotating uniformly

10. Aug 29, 2007

### Staff: Mentor

What fictitious force?

11. Aug 29, 2007

### pardesi

accelaration due to $$\frac{mv^{2}}{r}$$
well if u go through my proof replacing $$dx$$ by $$dT(x)$$ then u will see that i have considered a small mass $$dm$$ at a distance $$x$$ from pivot.so i can apply newton's law do what i did.
i think u r thinking that i ma applying newton's law for the entire system as a whole which i am not

12. Aug 29, 2007

### Staff: Mentor

Who says the force is ficticious? Ever make a hard turn in a car? Do you feel pressed against the window (or the window pressed against you...)? Is that a real feeling or a ficticious feeling?

People call the centrifugal force a ficticious force only because the direction is backwards (which is why I don't like it when they say that): When you make a right turn in your car, you aren't being pressed against your window, your window is being pressed against you.

13. Aug 29, 2007

### pardesi

well it's fictious because u feel the pain when u turn and i who watch u doing so don't .
well mt frame of refrence is earth so why do i ever think of it i just have a centripetal accelaration

14. Aug 29, 2007

### Staff: Mentor

Huh? If I watch a skydiver smack into the ground without his parachute, I don't feel his pain either. Does that mean it isn't real?

Regardless of what frame of reference you use, there is an acceleration and if there is an acceleration, there must be a force.

15. Aug 30, 2007

### pardesi

yes if that's what u beleive upon fine.i have no problem with that

16. Aug 30, 2007

### kaisxuans

of course it is real! yeah! listen to him. dont think other-wise!

17. Aug 30, 2007

### Staff: Mentor

You need to accept this reality in order to do your problem!

18. Aug 30, 2007

### pardesi

well why do i need to add something that is frame dependent and i say it again my frame of refrence is earth
here i can simply aplly newton's law iin the form they ae.
to me the only forces acting are gravitational and tension/internal forces between the particles on a small mass element