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Suppose a car/bike is moving in a circular manner on a flat surface having some friction. If it's moving with a constant speed, am I right in the below conclusions with the reasons? (Assume the person who is driving the car/bike doesn't lean or do any such type of movements).

  • There cannot be a friction component along the tangential direction of the circular path it makes. It can have a friction component along the tangential path, if and only if the car speed was increasing/decreasing from the internal mechanism inside car.
  • In the constant speed case, the friction is only acting radially and it is static in nature.
  • Even though it might seem that as the car is moving, friction must be acting in a kinetic nature as kinetic friction is only acting while something is moving. But it's not so in this case, is it the only case where friction is static even though car is moving.
  • Only the radial static friction is responsible for this circular motion. No other horizontal force is there which can contribute in making the bike/car turn
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    $\begingroup$ Re, "...might seem as if the car is moving...friction is static..." Don't forget that the car's wheels roll along the road. At any given instant in time, the instantaneous velocity of the part of the wheel that is touching the road is zero, and the instantaneous velocity of the opposite part of the wheel is twice the velocity of the car's body. $\endgroup$
    – Solomon Slow
    Commented Jun 10, 2022 at 0:59
  • $\begingroup$ @Paracetamol: When you write "But it's not so in this case, is it the only case where friction is static even though car is moving." Do you mean: "But it's not so in this case, it is the only case where friction is static even though car is moving." ? $\endgroup$
    – Brendan Darrer
    Commented Jun 10, 2022 at 9:30
  • $\begingroup$ @SolomonSlow can you elaboarate more , i understand car wheels bottom point when taking turn has no velocity but still i dont get how you get that opposite part of wheel is twice the cars body ? What opposite part of wheel and can you show it ? The relation you got derive $\endgroup$
    – Paracetamol
    Commented Jun 13, 2022 at 14:09
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    $\begingroup$ Yeah right @BrendanDarrer i was asking wuestion whether it is the only case where it happens or not $\endgroup$
    – Paracetamol
    Commented Jun 13, 2022 at 14:10

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No, first statement is not true. The number of friction forces which are acting here are 2, i.e one along the circular path which is making the car move even if it's not being accelerated (although the net force will be zero on the vehicle in the case of constant speed if you only look tangentially. But yes, two types of friction forces are present) and the second will be radially inwards which will act as a centripetal force directed radially inwards.

Statement 2: It's correct if there is no slipping along the radius of curvature then friction will be static.

Statement 3: It's also correct the friction will be static even though the car is moving, because the friction force has direction and according to the well said statement about kinetic friction i.e "kinetic friction opposes the relative motion", so according to this, There should be a relative motion along the radius of curvature to introduce kinetic friction in the game Radially. As stated in this scenario there is no relative motion with the ground radially, so the kinetic friction will not be acting here. But in the case of tangential motion, yes it will.

Statement 4: First of all the statement can be much better if you clarify the word Horizontal force. Coming to the last statement which states Only the radial static friction is responsible for this circular motion comes out to be very true.

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  • $\begingroup$ Why is 1st not correct as such i said in that fricntion component is there along it iff the car is spedding up that is having a acceleration tangentially ? $\endgroup$
    – Paracetamol
    Commented Jun 13, 2022 at 14:20
  • $\begingroup$ "As stated in this scenario there is no relative motion with the ground radially, so the kinetic friction will not be acting here" you mean because it has wheels therefore ? Which do pire rolling thats why? $\endgroup$
    – Paracetamol
    Commented Jun 13, 2022 at 14:26
  • $\begingroup$ And shouldnt actually its only one friction only isnt ? We just have two comoponents , you said number of friction forces is two which confuses me . Also "But in the case of tangential motion, yes it will." You mean in the cosntant speed turning kinetic tangetial friction force is acting ? I think it should be zero $\endgroup$
    – Paracetamol
    Commented Jun 13, 2022 at 14:27

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