Question

What is the difference between hardness and roughness of a surface?

Answer 1

@Mashy ^________^

Answer 2

dont say rough means rough and hard means hard..duh!!

Answer 3

roughness means how rough it is.. well microscopically how much uneven the surface is


and hardness is how easily can you pierce through the surface..

Answer 4

rolling friction depends on hardness or roughness?

Answer 5

rolling friction depends on a lot of things

it not only depends on the surface but also depends on how squashed is your wheel .. the only reason we have rolling friction is because, nothing is purely rigid, the wheel deforms due to its own weight and becomes a bit squashed and thus now there is a lil surface of the wheel in contact (instead of just a point being in contact) ..

Answer 6

and i feel all kind of friction depends only no roughness.. !! cause if you smoothen it out. you are not changing the hardness .. but its the roughness

Answer 7

so hardness/roughness :P

Answer 8

well it depends on hardness...............

Answer 9

whereas sliding friction depends on roughness..

Answer 10

well yea you are right
rolling friction should depend on hardness.. cause if the ground was soft the wheel wouldn't be deformed so much :D

Answer 11

but the deformation isnt that large?

Answer 12

harder the surface larger the deformation na?

Answer 13

hmmm..makes sense <3

Answer 14

yup :D.. it just hit me though..

Answer 15

something about pure rolling..

Answer 16

v=wr is only valid for the bottom point right?

Answer 17

surface you mean!!.. its valid for ROLLING component of the velocities of all the particles on the surface

Answer 18

wait wait..let me make a diag!

Answer 19

oki

Answer 20

For pure rolling,the relative velocity has to be the same at the lowest point,
\[\LARGE V_{cm}- \omega R =V_s\]
if Vs=0 that means if surface is at rest V=WR
otherwise at other points,like at the top
V+Wr=Vs?

Answer 21

did i confuse you? o.o

Answer 22

no i went for food :D.. sorry

for pure rolling

\[V_{cm}= wR \]

Answer 23

what is Vs? :P..

Answer 24

Velocity of surface

Answer 25

???

Answer 26

did i confused you o.o

Answer 27

what youmean velocity of surface? even the surface is moving? :O

Answer 28

it isnt,tabhi to v=rw

Answer 29

then from where Vs came if surface is not moving?

Answer 30

Vs=0 its NOT necessary though?

Answer 31

if I said Vs=v and Vcm=3
then
3V-rw=V
then
rw=2V
w=2V/r
You cannot apply v=rw everywhere blindly,i expected you to know that anyway..hmm

Answer 32

if after listening to "Pure rolling",v=rw and a=alpha r strikes you then you are definitely wrong at your concepts ^_^

Answer 33

when i say Vcm = wr.. i mean it with RESPECT to the surface :P.. so if the surface is moving.. all you gotta do is add its velocity too duH :P

Answer 34

okay..tell me how is the body moving if the lowest point doesn't move O.O

Answer 35

i mean how can the lowest point and the point at surface (point of contact have equal velocities ?

Answer 36

both are 0? how is it moving :|

Answer 37

What?!.. when you have a pure rolling motion the velocity of the bottom most point (the one in contact with surface) does not move with respect to surface

cause it has two velocities, one forward due to translation and one backward due to rotation.. so they are equal and opposite and cancel out each other!

Answer 38

if the bottom point doesnt move wtf :/ how does it rotate?i mean WHAT? :?

Answer 39

??

Answer 40

thats what rolling motion is.. imagine this

imagine rolling a slab over..

the bottommost point doesnt move right.. when you just roll it over??