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Wednesday, 20 March 2013

Starting motion

Very good discussion about starting motion

What makes the car start moving forward? Let us, for the moment, forget about the details of the engine and the transmission.  The car contains all the hardware necessary to make the wheels turn.  If a forklift lifts the car so that the wheels do not touch the ground and you get in the car, start the engine, and step on the accelerator, the wheels start turning.  The car, however, does not start moving forward.

What is missing? Without frictional forces your car will not accelerate.  If you are parked on an icy surface the wheels will turn, but your car does not accelerate.  The center of mass of a system acted on only by internal forces cannot accelerate.  This is a consequence of Newton's third law.  We need an external force to accelerate the car, and that force is friction.

How does friction accelerate your car?

pwheel.gif (3472 bytes)

Assume you want the car to accelerate towards the right.  When a wheel is rolling the contact point is not sliding at all.  When a rolling wheel is accelerating, internal forces try to accelerate the contact point backward.  The force of static friction now is directed towards the right and it cancels those forces.  Fhe force of static friction is the only external force acting on the car in the horizontal direction, and without it there would be no net force to accelerate the car.

This is the same happening when we walk. Between our feet and the pavement there is a static friction.