toppling in Physics

When an object is in equilibrium, there are no net force and net moment acting on the object.

In the case of a car turning round a corner, the car is definitely NOT in equilibrium, it is in a state of motion. In fact, it is under an acceleration, which is the centripetal acceleration. As such, there must be a net force, and hence net moment, acting on the car.

A zero net force and zero net moment condition is only applicable to objects that are at rest or in uniform velocity.

2010-06-01 09:12:46 補充：
There is another explanation, which I think is more acceptable. There are two normal reactions, one acts on the tyres at the inward side of the car and the other on the tyres at the outward side....(cont'd)

2010-06-01 09:13:34 補充：
.... By taking moment at the centre of mass of the car, moment due to friction + normal reaction at the inward side balances the moment due to normal reaction at the outward side. Hence, you would see that the normal reaction at the outward side is larger than that at the inward side....(cont'd)

2010-06-01 09:14:10 補充：
.... This is a common experience that the car tends to incline towards the outward side when turning a bend.

圖片參考：http://imgcld.yimg.com/8/n/HA00028060/o/701005310078813873416680.jpg

In face, the car should topple at some centripital force.As show in the picture, when the car is bending on a horizontal ground, the normal reaction is shipping to the right due since the car tend to topping outward. As a result, when the reaction become arrive the coner, it is no more place for it to ship more. Hence, this is the topping friction.In the above case, the car is a square, when fl=Rl,f=mg,the car will start to topple. Therefore, if the speed of the car is large enough to bend. The friction>mg,then the car will topple.

2010-06-01 22:04:35 補充：
What can cause an object to topple?
Net moment can cause it.As like as force, there must be a net force so that the object will accelerate.In the case of topping, a Net moment is required so that the object will topple.