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In "Physics for Scientists and Engineers" 9th Edition pg 311 says...

"The force required to start a car rolling is less if the force is applied to the tires than if it is applied to the car body. This is because the tires are closer to the center of mass of the car.

In "The Physics of Everyday Things" by Isaac Asimov, page 151 says

"The tires of a car are closer to the axis of rotation than the body of the car. This means that it takes less force to push the tires of a car than it does to push the body of the car. This is why it is easier to push a car if you push on the tires instead of the body."

In some random you tube vid (I know I know ... great citation) a guy purports to show how much easier it is to move his car when e pushed on the top of the tire vs the back of the car - claiming a mechanical advantage due to the radius of the tire vs the radius of the axel. A bigger wheel would make it even easier to push (which sounds to me the opposite of what Azimov said)

Finally in the 1936 auto industry educational short film 'Spinning Levers" they say "It takes a lot of force to start a freight car moving, yet, the railroad man, can start the heaviest freight cars easily, with a pinch bar. A powerful lever, which turns the wheels" They actually show a single (out of shape guy) moving a freight car all by himself by turning the wheel. This implies it is more effective to push the wheels than the body of a fright car.

So what is going on? are there 3 advantages (1: tire is closer to center of mass, 2: tire is closer to axis of rotation and 3: the tire provides leverage due to the difference between the tire's radius to the axle's radius? Or is this all bunk and/or irrelevant when talking about cars on pavement being rolled on wheels? Or something in-between. In short do we need 0,1,2, or 3 formulas to answer this, and what are they?

Secondarily what is the physics of the "pinch bar" and why don't we use em to get out cars unstuck? (Maybe that sold be a difft question)

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  • $\begingroup$ A lever of arbitrary length can give an arbitrary size mechanical advantage. To me it seems that a wheel is, at most, a 2:1 lever, so it's not going to do all that much. It is also my understanding that a modern car may not be moved without engine power unless the driven wheels are off the ground, unless damage to the transmission is of no concern. I might be wrong about that. I am not a mechanic. $\endgroup$ Jun 3, 2023 at 6:03
  • $\begingroup$ " push on the tires" you mean applied torque on the tires ? $\endgroup$
    – Eli
    Jun 4, 2023 at 15:57

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The answer to "how much easier is it to push a car..." is $2$.

Where easier is defined to be "force required".

The tires are levers, where if you push on the top of the tires you have a $2:1$ mechanical advantage, so you push 1/2 as hard for twice as far.

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