IIT JEE , AIEEE Forum - Physics , Mechanics - Why Kinetic friction is less than Static frinction?

cheeky_virus

Sir, according to me

Kin.friction < Stat.friction

_k x N₁ <

_s x N₂

_k =

_sas the surface is same.

Therefore the only difference lies in the normal force on the body. But I am not able to understand how can it be.

It can be that the angle which the contact force makes with surface during kinetic phase would be greater than the angle which the contact force makes with surface during static phase.

deedee

kinectic fric acts wen a body is in motion

whereas static fric acts wen a body is at rest .....

wen a body is at rest ...itz surface in contct wid other surface is more...

thus......friction value is more

wen a body is in motion only a part of it is in contact wid d other surface.thus d value of fric is less

also

d force required 2 get or start d body(move it)is more

then d force required 2 keep it in motion......

thus kinectic fric is less then static fric

hope u get it.....

do rate ma reply if u find it useful...

cheeky_virus

Mam, Kindly don't feel bad but I disagree with your given reason in the following points.

1.Frictional force , whether it is Kinetic friction or Static friction , is independent of the area of the surface of contact.

2.Mam, you have reasoned according to your experience . I want a sure proof.

Please don't feel bad for my disagreement for your answer.

singhshivendra

when 2 bodies in contact move with respect to each other,rubbing the surfaces, the fric is kinetic.(force is applied).

but when equal force is applied and body is not moving, the fric is static.

so that means co efficient of static fric is greater than that of kinetic fric.

hence kinetic fric is less rhan static fric.

pls rate me if u r satisfied

shinee

coff of kinetic friction is less than coff of static friction
suppose, a body is in the state of rest and u want to move it, so u kept on applying force on it till u are able to come over the frictional forces{i.e ur applied force is more}, then the body moves{remember u kept on applying same force but but u didnt increase the magnitude ]. According to Neutons first law, a body moves only if a difference if force is created and since the magnitude of applied force is same and it is being applied for a long period of time,first the body would be able to move thereby indicating that it overcame static friction and still if u keep on applying the same initial applied force, then the body slides with more ease,{as the applied force is now sufficiently large to create motion because it overcame static friction} hence kinetic friction is less than static friction.

rajat.khanduja

Firstly, the argument that co-efficient of friction is the same is wrong. For kinetic friction

is lesser than in case of static friction.

Now when a body is at rest, you need to apply more force to make it start moving, hence we conclude that the static frictional force is higher than kinetic friction as we need to apply comparatively lesser force to keep a body in motion.

Though I'm not sure of the reason, I think it is because :

1. When at rest, the electromagnetic forces of attraction between the two bodies in contact is pretty large and to overcome it you need to apply a lot of force.
2 But when the body is in motion, the electromagnetic forces must be lesser as the distance between the molecules change ( between two objects) and hence less force is needed to oppose the electromagnetic forces.

Hope this is correct and you find this useful. Rate me if convinced

deedee

@ cheeky virus ...

plz don call me mam.........

2 add in it........

d adhesive forces ..betwn those subtances(surfaces)

wen d body is at rest is more......

then wen d body is in motion

wen an object is in motion ......bonds....betwn the different atoms on both d surfaces in contact ..... .r made.....n broken easily ...

thus ...........friction ie kinectic fric is less.......

but wen its at rest a strong bond is formed due 2 adhesive attraction......

thus .....more force is required 2 break them

thus static fric is more.........

hope u r satisfied........

this was d xplaination wich our classes sir gav us,.........

i don remember if he specified any thing else..... :)

tarinbansal

Yaar iska bada hi simple and straightforward reason hai-
When the body is at rest, there is bond formation between the molecules of the 2 bodies in contact.
When the bodies are in relative motion, the bonds can not be formed and hence the bonds are broken.
Now the static friction is more becoz, you need to apply more force to break bonds while in kinetic friction, the bonds are only broken and you need to apply only that much force which is required to overcome friction between their rough surfaces.

Hence kinetic friction is less than static.

Rate if satisfied.

tka

friction is a kind of electromagnetic force.

it arises due to electrostatic forces acting between two surfaces. compare it to the so called van der waals forces, or dispersion forces, or london forces.

when in rest, the charges arrange themselves so that they are all at equilibrium.

something like -

++-+-+-++--

--+-+-+--++

this electrostatic attraction varies from material to material

motion tends to affect this equilibrium. so it takes some force (call it F1) to set the body in motion.

once the body is set into motion, the electrostatic attraction still acts between the surfaces, but due to constantly changing relative positions of the charges, the force acting now, is different. it is infact lesser than the maximum (that is, at the equilibrium position)

well so the force of static friction is different from (more than) the force of kinetic friction. the normal reaction remains the same, so we infer that coefficient of static friction is more than the coefficient of kinetic friction.

i hope that helps

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