Static friction and motion.

This is the first so-called technical post i am posting in this blog. I started this blog to capture my thoughts regularly and register it in cyberspace so that the future-parth might look at it at some other or same part of the world. Hence this is basically like a bunch of letters to my future which is also open to others. Physics is bound to occupy a significant portion in my letters. Feel free to question or criticize any part. Or you might also like to click that red hot button at the top right corner. A wise decision, i'd say.
How do we walk? How does an automobile move?
Though one might be tempted to say its the force and 'power' of the engine, the actual force that propels a vehicle or a cheetah on the trail of a deer is friction. This is because for a system comprising the entire accelerating vehicle, the only external unbalanced force on flat surface is friction. And Newton dictates that only an external force can accelerate a rigid body. Friction propelling a vehicle.!! I got over this stunning fact an year ago-just in my third year BE, after two years of getting awarded an A in Engineering Mechanics- i already mentioned this. Another interesting thing i realized was that you can apply The Three divine laws for any part of a body or object. You are the designer of the system by choosing its boundaries. Just consider the crank shaft as the system and you would see the piston's force is an external one. Consider the engine as a whole and you may safely dispose off the complex interactions as "Ah. They are all silly internal forces. The engine's COM wouldn't budge". Nature sees to that it follows the Laws faithfully in all the systems you might think of defining. There is no preferred system. If you go deep into physics, you would repeatedly encounter this "There is no preferred x". That's one of her main attractions. An inherent impartiality. Of course there are limitations to Newtonian mechanics-quantum and relativity say hi then- yet a large part of the sense experiences are beautifully explained by Newton.


Now i was confronted with another doubt. A force has to do work to increase the Kinetic energy of the body right? If static friction is what is propelling the vehicle, it has to do work. But work cannot be done if the point of application doesn't move. Since it is static friction, there is no movement of the contact point. Hell.! I raised this question as a related one in a Physicsforums thread. Doc Al's explanation and subsequent contemplation led me to an interesting scenario which i am describing here; which is also there in that thread.
Imagine two hemispheres joined together loosely; the upper and lower hemispheres are connected by long strings. So initially the strings are flaccid. Now the conjoined hemispheres- are placed on a surface with a hemispherical cup to accommodate the lower hemisphere. Now an explosive placed inside the sphere is triggered. The lower part cannot move initially and the upper part alone shoots up wholly due to the internal force of the explosive. As for the application of newton's laws goes, the system's COM accelerates due to the normal force from the surface. Since work done requires the movement of the point of application, it is done only by the internal force increasing the KE of the upper hemisphere. Then the moving upper hemisphere pulls the lower one through the string which is also an internal force. This is what happens while walking too, i think.
So work needn't be done by the external force to increase the KE of a body. I was wrong in thinking so. It just needs to exist. When you crouch and jump, the normal force-the only unbalanced external force- merely prevents your legs from going into the surface while allowing-or shall we say remain a spectator to- the upward motion of the part of the body above the knees that results from the work done by the muscles-internal force-. Then after a few moments, your feet also rises up because the part of the body already hoisted up will now pull the feet too which didn't initially agree to join it-Again. internal force-.
Friction acts like a bridge to convert the wheels' rotational energy to the vehicle's total kinetic energy; i.e., allowing internal forces to do work on one part of the system while preventing it from doing the same on the other. This is the critical point and the fruit of my contemplation. The reason for today's happiness. A good night kiss from her.!