Why is it easy to rotate the steering wheel while the car in motion, but difficult while it is at rest?
S. BRUDNELL SAM
Difficulty experienced in moving physical bodies, arises basically from two reasons; friction and inertia. While friction opposes motion of a body in contact with another or the medium through which the body moves, inertia of a moving object opposes any alteration of the state of motion independent of its surrounding. There are two types of friction — static friction and dynamic friction.
Static friction opposes motion when a body at rest is moved, and is larger in magnitude compared to the opposition by dynamic friction experienced. Again depending on the type of motion there are two kinds of friction — sliding friction and rolling friction. The resistance due to sliding friction is higher than that due to rolling friction.
The static sliding friction would offer maximum resistance and the dynamic rolling friction would do it the least. On the other hand, mass is the index of inertia for linear motion, implying more massive objects to be less easy to move as compared to relatively lighter bodies.
Similarly, once a heavier body is in motion, it is not so easy to stop or even slow it down as it is for a lighter body. In the case of rotational motion, the role of mass is played by the moment of the mass distribution about the rotation axis, known as the moment of inertia and it can be regarded as the “inertia” for rotational motion.
When we have the vehicle in motion and wish to turn the steering, and hence the wheels, we have to work against the dynamic friction, dominantly rolling friction.
Further, the inertia of the vehicle would keep it moving and the small perturbation by turning the steering will not involve direct linear movement but will only alter the orientation of the vehicle in motion.
If we need to turn the wheels of the static vehicle, we need to overcome the large static sliding type of friction. And, in this case, no advantage from the inertia of the moving vehicle is available; instead, it offers additional resistance. Thus turning the steering wheel of the static car necessitates large amount of energy or effort.
Prof. H. K. SAHU
Chennai Mathematical Institute, Chennai