Apple pencil force non-perpendular force calculation

Question

Created Jul ’17

Replies 3

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Participants 3

Apple pencil force calculation doc:

https://developer.apple.com/documentation/uikit/touches_presses_and_gestures/handling_stylus_based_input/computing_the_perpendicular_force_of_a_stylus,

indicates a division of a measured force by the sine of the attitude angle. Shouldn't this be multiplication?

Thanks in advance

Answered by PBK in 245937022

If you are pushing the pencil along its axis then the force perpendicular to a surface will be a fraction of the force along the axis - you want to divide the exerted total force along the pencil axis into two components - one perpendicular to the surface and one parallel to the suface. The component perpendicular to the surface will be smaller than the whole by a trigonometric factor - you want to multiply by the sin. BUT..........

But you don't do that because the pencil will slip along the surface! Instead you torque the pencil into the surface. (Actually you learned this technique in Kindergarden and that's why you broke all those pencil point tips because you were trying too hard.) The result is that the pencil tip only exerts a force perpendicular to the surface and there is no slipping force along the surface. The surface pushes up against the pencil point and you divide that total force into two components - one perpendicular to the axis of the pencil and one parallel to the axis of the pencil. (The component perpendicular to the axis of the pencil does not contribute to the force readout.) The component of the force parallel to the axis of the pencil is a fraction of the total force exerted perpendicular to the surface - it will be equal to the force perpendicular to the surface multiplied by a triginometric factor. So to get the force perpendicular to the surface you divide the force parallel to the axis by the trigonometric factor.

Consider the limiting case, a pencil held almost horizontally with the altitude angle close to 0. There will be little to no force along the pencil axis. So you need to convert that small force to a large force - divide by the sin of the small angle.

Consider the limiting case, a pencil help perpendicular to the surface with the altitude angle close to 90 degrees. There will be little difference between the force along the pencil axis and the force into the surface. Fortunately, the sin(90)=1.

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Answer 1

KMT OP

Jul ’17

Docs look correct to me.

A little light reading via this tutorial, for reference:

h ttps://www.raywenderlich.com/121834/apple-pencil-tutorial

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Answer 2

PBK OP

Jul ’17

Accepted Answer

If you are pushing the pencil along its axis then the force perpendicular to a surface will be a fraction of the force along the axis - you want to divide the exerted total force along the pencil axis into two components - one perpendicular to the surface and one parallel to the suface. The component perpendicular to the surface will be smaller than the whole by a trigonometric factor - you want to multiply by the sin. BUT..........

But you don't do that because the pencil will slip along the surface! Instead you torque the pencil into the surface. (Actually you learned this technique in Kindergarden and that's why you broke all those pencil point tips because you were trying too hard.) The result is that the pencil tip only exerts a force perpendicular to the surface and there is no slipping force along the surface. The surface pushes up against the pencil point and you divide that total force into two components - one perpendicular to the axis of the pencil and one parallel to the axis of the pencil. (The component perpendicular to the axis of the pencil does not contribute to the force readout.) The component of the force parallel to the axis of the pencil is a fraction of the total force exerted perpendicular to the surface - it will be equal to the force perpendicular to the surface multiplied by a triginometric factor. So to get the force perpendicular to the surface you divide the force parallel to the axis by the trigonometric factor.

Consider the limiting case, a pencil held almost horizontally with the altitude angle close to 0. There will be little to no force along the pencil axis. So you need to convert that small force to a large force - divide by the sin of the small angle.

Consider the limiting case, a pencil help perpendicular to the surface with the altitude angle close to 90 degrees. There will be little difference between the force along the pencil axis and the force into the surface. Fortunately, the sin(90)=1.

0

Answer 3

monad OP

Jul ’17

The physical explanation was quite useful. Thanks! 🙂

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