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Direction of damping force on a surface

by kakarot1905
Tags: damping
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kakarot1905
#1
Aug5-11, 06:31 AM
P: 18
Hi

Suppose a particle is bouncing on a surface with a viscous damping coefficient...

Question 1:
The frictional force = -c(viscous damping coefficient)*v(velocity of the particle)

But what is the direction of this force?
Perpendicular [down] to surface?

So if the surface is tilted at an angle what would the frictional force be?

Question 2:
Is it possible to express frictional force [due to damping] using force due to gravity (on the point of impact) and not velocity?

Thanks
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xts
#2
Aug5-11, 06:55 AM
P: 882
Quote Quote by kakarot1905 View Post
But what is the direction of this force?
Perpendicular [down] to surface?
Yes.

So if the surface is tilted at an angle what would the frictional force be?
if your ball hits the surface perpendicularly - the force is as you wrote.
But if you just drop a ball on tilted surface, or throw it at any other angle, you will have combined effect of damping friction (depending on velocity component perpendicular to the surface), friction in motion parallel to the surface, and energy transfer to rotation of your ball.

[quote]Is it possible to express frictional force [due to damping] using force due to gravity (on the point of impact) and not velocity?[quote]
Nope. Just take extreme case: if you lay your ball on a table (velocity is 0) there is no dumping friction, but gravity is always the same.
kakarot1905
#3
Aug5-11, 08:40 AM
P: 18
[QUOTE=xts;3437384]Yes.


if your ball hits the surface perpendicularly - the force is as you wrote.
But if you just drop a ball on tilted surface, or throw it at any other angle, you will have combined effect of damping friction (depending on velocity component perpendicular to the surface), friction in motion parallel to the surface, and energy transfer to rotation of your ball.

[quote]Is it possible to express frictional force [due to damping] using force due to gravity (on the point of impact) and not velocity?
Nope. Just take extreme case: if you lay your ball on a table (velocity is 0) there is no dumping friction, but gravity is always the same.

About the angled surface... Ignoring the surface friction, would the damping force decrease by an angle of cos(theta) or something like that?

xts
#4
Aug5-11, 08:59 AM
P: 882
Direction of damping force on a surface

About the angled surface... Ignoring the surface friction, would the damping force decrease by an angle of cos(theta) or something like that?
Yes, if you are not interested in rotation nor forces parallel to the surface, then damping friction depends on perpendicular component of the velocity - or, if you prefer, on speed*cos(theta)
kakarot1905
#5
Aug5-11, 09:54 AM
P: 18
Quote Quote by xts View Post
Yes, if you are not interested in rotation nor forces parallel to the surface, then damping friction depends on perpendicular component of the velocity - or, if you prefer, on speed*cos(theta)
Thanks for all the replies xts..

Do you know any other means of measuring the damping force [by the surface on the particle] other than using velocity?
xts
#6
Aug5-11, 10:22 AM
P: 882
You may use tensometer, or some kind of scale: eg a light plate glued on piezoelement, but velocity seems to be definitely the simplest and most feasible for no-budget home experiment
kakarot1905
#7
Aug5-11, 10:34 AM
P: 18
Quote Quote by xts View Post
You may use tensometer, or some kind of scale: eg a light plate glued on piezoelement, but velocity seems to be definitely the simplest and most feasible for no-budget home experiment
Thanks :)

One last question:
The force exerted by the plate at point of collision....
Is is it ok to say this force is damped by a factor (1(c))?
xts
#8
Aug5-11, 10:37 AM
P: 882
Quote Quote by kakarot1905 View Post
The force exerted by the plate at point of collision....
Is is it ok to say this force is damped by a factor (1(c))?
Oooch?
It definitely is wrong, I wonder why do you think so?
kakarot1905
#9
Aug5-11, 11:06 AM
P: 18
Quote Quote by xts View Post
Oooch?
It definitely is wrong, I wonder why do you think so?
Ok, is there any other possible way, the viscous damping coefficient [or the damping nature of the surface] affect the force exerted on the particle during its collision


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