For a car that is accelerating to the right, I don't understand for if the tyre is rotating clockwise, the force is acting to the right. http://www.welltall.com/ymc/discovery/car/wt_xfer.html From the diagram, in the above web site, the direction of Ff and Fr are to the right. For example, if u have a door in front of you, fixed to an axis on the left hand side. http://www.cofc.edu/~agrestm/RS/Door.JPG Like the diagram shown, if you exert a force on the end , then the door will rotate anticlockwise. I could understand this phenomenon. I couldn't imagine why the force is acting on the tyre to the right, whereas the rotation is clockwise. I suppose the direction should be anticlockwise if the force acting on the tyre is to the right. I have been thinking too much, and need someone's else helps. Thanks.