of the car.does this mean that you do less work on the car than if you had lifted the car directly?explain.can the total work done on an object during an displacement be negative?explain.if the total work is negative,can its magnitude be larger than the initial kinetic energy of the object?explain.
No, you move the jack handle many times to raise the car inches, so the distance you exert that force is much more. With a jack with no losses, the force you exhert x the distance you move the handle equals the weight of the auto x the distance you lift it. edit: polarity of work or energy is an arbitrary thing. If you pick positive work as that done by you on an object lifting it, then the work it does falling back down would be considered negative. Me, I ignore polarity and use common sense. The work done in lifting an object to height h equals the PE it has from that height, which equals the KE it gains in falling from that height.
Know the difference between force and work. Using simple machine we can increase or decrease a force and also we can change the direction of force at our will. We don’t waste or gain any energy by changing the force. Only while moving a force through some distance, energy comes into play. Once again it is repeated that force can be changed easily with out any actual movement of the force. And it can be done by simple machines. An ant can give the same force as an elephant, if the ant uses a simple machine. But the work or energy is measured by the distance moved by the force. The same amount of work is done while moving a 2 N force through 4 m and a 4N force through 2 m . The simple relation is 2*4 = 4*2 = 8. You know 8 can be obtained by choosing two numbers in an infinite ways. To state some of them, 1*8, 0.5*16, 0.1* 80, 100 * 0.08 and so on =============== An automobile jack is a simple machine. A small force applied at the end of the lever provides a large force at the platform which supports the car. What we have gained is the gain in force and not in energy. In fact we should not use the word gain here. The reason for saying this is we inherently assume a gain of energy . The apt word is advantage. Thus we have a mechanical advantage while using a jack. The advantage is we get more force from a small force. When the lever is moved through a large distance , the car is lifted up through a small distance . Work done by the small force = work done by the force acting on the platform. Hence the same amount of work is done by both forces . No gain or loss of energy. ====================== But in practice, due to friction and other energy losses , the work out put is always less than the work input . And hence we say about the efficiency of the machine. An ideal machine will have cent per cent efficiency. All actual machines will have efficiency less than 100 %. =========================
