I was reading through my driver's manual, when I came across this picture: enter image description here I don't think the inertia arrow is accurate: inertia/momentum would be pointing directly forwards, while centripetal force would be pointing into the turn by 90 degrees.

Is there something this arrow accurately represents or am I mistaken?

  • $\begingroup$ It is a driving manual, not a physics manual. You appear to know what you are talking about. So what is your reason for asking? $\endgroup$ – sammy gerbil Jan 8 '17 at 23:27
  • $\begingroup$ @sammygerbil I asked the question to some friends of mine, and they seemed to think the manual was accurate. I came here to confirm my suspicion. $\endgroup$ – Chris Smith Jan 8 '17 at 23:29
  • $\begingroup$ Are your friends physicists? $\endgroup$ – sammy gerbil Jan 8 '17 at 23:30
  • $\begingroup$ @sammygerbil No, and neither am I. $\endgroup$ – Chris Smith Jan 8 '17 at 23:31
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    $\begingroup$ While the drawing is incorrect, what it represents is not. It represents a car driving along a curve (making a turn). The inertia is in the direction of the car before the turn, where the inertia is temporarily straight ahead. As the car turns, the inertia moves with the direction of the car, but if they drew it that way, it wouldn't convey the message. What the drawing represents is at different times, the inertia before the turn and the car/driver having already started turn. It's the correct concept even if it's not drawn correctly for physics class. $\endgroup$ – userLTK Jan 9 '17 at 1:14

What they draw is not what we would typically draw in a physics class. Your instincts are correct. The velocity vector is straight ahead (no matter how the wheel is turned), and the force of the wheels is pulling it in the directly of the center of the curve.

Unfortunately, this is not natural for many people. Many people's intuition does not jive with this. Instead of trying to give a physics lesson, they're trying to give a safety lesson. What they draw as "the direction of inertia" is the direction of the velocity of the car if the car was not turning. They then draw the "direction of steering" as the current velocity vector.

There is no useful physics interpretation of this. Nobody would use this to, say, calculate the forces on the struts to make sure they can handle this particular radius turn at a given speed. They would do that in the way you are thinking. However, when you're driving, the "direction of steering" really does feel like the direction you are steering in, and the idea of this "direction of inertia" is close enough to the feeling of the "fictitious" centrifugal force (fictitious because it isn't a real force, but rather a re-interpretation of other forces in a rotating frame). The idea is to get people to have an intuitive grasp of what can cause them to leave the road if they speed.

When it comes to safety and other place, physical accuracy gives way to intuitive readability. The key here is that, if people have to think about it (i.e. physics lesson), they will not be able to apply it in the half second where it matters on the road.


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