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Why we use first gear to start our car and also to climb uphill?

Hey guys,
We know that to run our car from state of rest we give 1st gear.But do you know why?It is because you need greater force to start a motion than to maintain it.In mechanics for rotatory motion we use the term torque instead of force.Torque is the force which is required to start rotatory motion.Actualy torque equals force multiplied by the perpendicular distance from the point along which turning occurs.For layman consider force as torque only.Now to give your wheels the rotatory motion you need a greater torque which is possible by first gear whose size is biggest as compared to other gears.Having high gear ratio only it is possible to achieve more torque.Once you achieve this then you shift your gears to higher state which impart higher velocity.Since you cannot get high torque and high speed at the same time so we use different set of gears.The higher the gear more will be the velocity and less will be the torque.Now to climb uphill again you need  greater force(I.e torque) so you put it on first gear.In automatic transmission cars this change takes automatically without shifting the gears manually.This concept is taught in Mechanical engineering under the subject theory of machines in 3rd year.For layman the above concept is enough for understanding,I guess.
Thanks...

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