Reason for motorcyclist leans

Atheist

2012-02-19 10:16 pm

A motorcyclist going round a corner on a level road leans over at an angle to the horizontal. The reason for this is

A. to allow the normal reaction to exert a moment about the centre of mass of the motorcyclist.
B. to increase the frictional force between the motorcycle and the road.
C. to lower his centre of mass.
D. to provide the centripetal force.

The answer is option A.

My question is: why is the option D not correct?
If the motorcyclist does not lean over at certain angle, NO centripetal force will exist. Also, the reason for allowing the normal reaction to exert a moment about the centre of mass of the motorcyclist is to provide a centripetal force.

更新1:

天同: I didn't mean that. I just wondered if the motorcyclist does not lean inwards, why the friction force will exist at first and how he could make use of the leaning over to balance the non-exist friction force.

更新2:

But now I know that the motor could turn slightly the front part of the whole body to turn round a corner without leaning over and therefore a frictional force could exist first.

回答 (1)

天同

2012-02-20 1:18 am

✔ 最佳答案

The centripetal force is NOT provided by the "normal reaction", it is provided by the frictional force fom the road.

You may recall that the centripetal force, in such case as a circular motion round a corner, is a horizontal force pointing towards the centre of the circle. The normal reaction is a vertical force, it cannot acts as a horizontal centripetal force.

As said above, the centripetal force is given by the frictional force, which acts horizontally towards the centre of the circular path. In order to balance the moment (about the centre of mass) caused by the frictional force, the cyclist thus needs to lean inward such that the moment given by the normal reaction could balance that given by friction.

Just imagine if the cyclist attempts to trun a corner without leaning inward, the frictional force would give rise to a moment about the centre of mass of the cyclist. Such moment would topple the cyclist over in the outward direction.

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