Purpose:Demonstrate the effect of locked wheels on vehicle stability.
A plastic toy car is lightweight enough that its wheels can, when needed, be locked with a simple piece of masking tape. When the wheels are locked, it skids rather than rolling freely; but its behaviour when skidding depends on which wheels are locked.
• Challenge students to make a prediction about how the car will behave with front or back wheels locked before performing each phase of the experiment.
• With both sets of wheels free, push the car across the floor; it moves in a straight line with the orientation in which it was pushed.
• With the front wheels locked and the rear wheels free, it will also continue in the normal forward orientation, slowly skidding to a halt.
• With the rear wheels locked and the front wheels free, however, when pushed in the forward direction it will rotate so that it moves backward .
• Students may ask what happens if all four wheels are locked. Encourage them to make predictions based on what they've seen here, and on what other systems the resulting wheelless mass would resemble.
Because rolling (static) friction is generally greater than sliding friction, whichever set of wheels is sliding will end up in the forward direction. In real vehicles, this can be the cause of serious accidents!