# Direction of damping force on a surface

1. Aug 5, 2011

### kakarot1905

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

2. Aug 5, 2011

### xts

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.

3. Aug 5, 2011

4. Aug 5, 2011

### xts

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)

5. Aug 5, 2011

### kakarot1905

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?

6. Aug 5, 2011

### xts

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

7. Aug 5, 2011

### kakarot1905

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))?

8. Aug 5, 2011

### xts

Oooch?
It definitely is wrong, I wonder why do you think so?

9. Aug 5, 2011

### kakarot1905

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