# Errata for Physics for Game Developers

The errata list is a list of errors and their corrections that were found after the product was released. If the error was corrected in a later version or reprint the date of the correction will be displayed in the column titled "Date Corrected".

The following errata were submitted by our customers and approved as valid errors by the author or editor.

Color Key: Serious Technical Mistake Minor Technical Mistake Language or formatting error Typo Question Note Update

Version Location Description Submitted By Date Submitted Date Corrected
Printed
Page 7
2nd display equations

all the occurances of m in the second display equations on this page should be m_i

Anonymous
Printed
Page 19
3-rd equation from the bottom

The integrated expression (vector product): (r x w x r) NOW READS: (r x (w x r))

Anonymous    Jan 01, 2004
Printed
Page 19
last paragraph

"... and (w x r) is angular momentum of each elemental mass..." NOW READS: "... and (r x (w x r))dm is angular momentum of each elemental mass..."

Anonymous    Jan 01, 2004
Printed
Page 50
figure 2.8

Body axes x and y must be normal. in Figure 2-8 they kind of look not perpendicular. It's just an illustration issue...

Anonymous
Printed
Page 52
figure 2.10

vector v must be normal to radius r

Anonymous
Printed
Page 53
figure 2.11

vectors an and at must be normal (perpendicular).

Anonymous
Printed
Page 55
figure 2.12

vestor vt must be normal to radius r

Anonymous
Printed
Page 56
figure 2.13

vectors at & an must be normal

Anonymous
Printed
Page 70
the last equation

The equation: sum(F) = sqrt{sum[(Fx)^2] + sum[(Fy)^2]} NOW READS: sum(F) = sqrt{(sum Fx)^2 + (sum Fy)^2}

Anonymous    Jan 01, 2004
Printed
Page 72
4-th equation

3 parentheses were opened but only 2 closed. An additional closing parenthesis HAS BEEN ADDED just after the Cv variable.

Anonymous    Jan 01, 2004
Printed
Page 72
7-th equation from the bottom;missing open and close brackets

The equation IS NOW in the form: [expression substituted for v2]dt = ds

Anonymous    Jan 01, 2004
Printed
Page 75

It NOW READS: Sum F = SQRT[ (sum Fx)^2 + (sum Fy)^2 + (sum Fz)^2 ]

Anonymous    Jan 01, 2004
Printed
Page 76
last three equations at bottom

NOW READS: All minus signs in these equations HAVE BEEN CHANGED to plus signs.

Anonymous    Jan 01, 2004
Printed
Page 77
2-nd equation fron the bottom and the last equation of this page

2-nd eq. from the bottom: on the right side of the equation in the expression exp[-(Cd/m)t] The "d" portion of the exponent NOW APPEARS as a subscript. The last eq. on this page: - in exp[-(Cd/m)t] the "d" portion of the exponent NOW APPEARS as a subscript. - cw portion of the equation (two instances) NOW APPEARS as Cw (C is now uppercase).

Anonymous    Jan 01, 2004
Printed
Page 77
4-th equation from the bottom

In this equation: Cd, Cw, vw, vx1, vx2 symbols: d, w, x1 and x2 NOW APPEAR as indexes (smaller letters lower than the main line of text). In x1 and x2 numbers 1 and 2 NOW APPEAR as subindexes.

Anonymous    Jan 01, 2004
Printed
Page 77
5-th equation from the bottom

The left side of the equation: (Cd/m)t=.... The "d" portion of the exponent NOW APPEARS as a subscript.

Anonymous    Jan 01, 2004
Printed
Page 78

In all occurrences of the exp[(-Cd/m)t] on this page, The "d" portion of the exponent NOW APPEARS as a subscript.

Anonymous    Jan 01, 2004
Printed
Page 79
3-rd line from the bottom

A minus "-" HAS BEEN ADDED at the end of this line.

Anonymous    Jan 01, 2004
Printed
Page 80
3rd line from the top

A minus "-" HAS BEEN ADDED at the end of this line.

Anonymous    Jan 01, 2004
Printed
Page 83
4-th equation

The equation was incorrect. It NOW READS: Sum F = SQRT[ (sum Fx)^2 + (sum Fy)^2 ]

Anonymous    Jan 01, 2004
Printed
Page 85
Figure 4-7.

The vector N=mg cos(theta) must be perpendicular to the vector Ft and tu the inclined plate. illustration issue...they should be drawn perpendicular

Anonymous
Printed
Page 86
5-th equation

Anonymous    Jan 01, 2004
Printed
Page 92
2-nd equation from the top

i and j are both vectors and NOW APPEAR in the vector style (bold letters, no italics).

Anonymous    Jan 01, 2004
Printed
Page 92
2-nd equation from the top

The value (0.864) NOW READS (0.866)

Anonymous    Jan 01, 2004
Printed
Page 93
1-st, 2-nd and 3-rd equation

v1+ and v2+ are vectors, and NOW APPEAR in vector style (bold letters, no italics).

Anonymous    Jan 01, 2004
Printed
Page 94
1-st and 2-nd equation from the bottom

I don't understand why the units ft/s are omitted in these equations. They were used earlier in all other equations of this kind. AUTHOR: Not an error, but for consistency they could be included...

Anonymous
Printed
Page 94
formula for coefficient of restitution e and the equation i next line

Two brackets are opened but only one is closed in the formula for coefficient of restitution e. The equations for coefficient of restitution should have closing parenthesis/brackets in the numerator.

Anonymous
Printed
Page 94
6-th and 7-th equation

v1- and v2- are vectors, and NOW APPEAR in vector style (bold letters, no italics).

Anonymous    Jan 01, 2004
Printed
Page 96
two last equations on the page

The mistake here is that the velocity terms should be shown as vectors (in bold). The same applies to just about all of the equations shown on page 96.

Anonymous
Printed
Page 98
1-st equation

the vectors in these equations should be shown in bold to denote them as vectors instead of scalars...

Anonymous
Printed
Page 99
1-st equation

tan(PHI) = F_f/F_n = (mu) The same angle is represented in different way: - in this equation (PHI) - upper case - on the figure 5.6 (phi) - lower case Font inconsistency

Anonymous
Printed
Page 105
3rd paragraph from the end

The text says that "the only formula that has changed is the formula for T", but in fact the formula for h has changed too. It should say: the only formulas that have changed are the formula for T and the formula for h"

Anonymous
Printed
Page 108
figure 6.6

The axe of velocity profile should be normal to the surface of the sphere, which means that velocity vectors should be paralel to this surface.

Anonymous
Printed
Page 109
First paragraph, just below Figure 6-7

The "separation point" is described as being at "approximately 80 degrees from the leading edge." I would have thought 90 degrees more likely, but weird things do happen in physics. If 80 degrees is correct, I feel that this should be emphasized, preferably with a few words of explanation. EDITOR: The 80 degrees is correct, and I would think more intuitive than 90. One might wonder why the angle isn't closer to 45. But it's really a peripheral point; the main idea is that the friction of a rough surface can bring the angle around to a counte-intuitive 115 degrees.

Anonymous
Printed
Page 111
Last sentence of 3rd paragraph

"In the SI system you'll get R_t in newtons (N) if you have velocity in m/s, area in m, and..." "area in m," NOW READS "area in m2,"

Anonymous    Jan 01, 2004
Printed
Page 115
A line related to the last display equation on the page,

in the sample code (cannon.c) has an error. It reads: double C = PI * RHO * RHO * radius * radius * radius * omega; that is, there is an extra factor of RHO (but would otherwise be correct).

Anonymous
Printed
Page 115
last display equation on the page

F_L = 2pi^2 ho r^4 v omega NOW READS: F_L = 2pi ho r^4 v omega

Anonymous    Jan 01, 2004
Printed
Page 126
Figure 7-6

"delected" should be "deflected" for all three sections of this diagram. Actually, "lowered" would be better.

Anonymous
Printed
Page 196
3rd line of 1st paragraph under the code

"Similarly, STBCThruster...." NOW READS: "Similarly, STBDThruster...."

Anonymous    Jan 01, 2004
Printed
Page 208
ApplyImpulse function

remove "(vCollisionNormal * vCollisionNormal)" from this function. It is not present in the example source code on the website. (and should evaluate to 1 anyway)

Anonymous
Printed
Page 309
last paragraph on the page

The second line of the last paragraph reads: "z-axis, then the y-axis, and then the z-axis, you can..." That last z-axis should be "x-axis". ------ Chapters 13 and 16 Type of error: Serious Technical Mistake Detailed Description of error: I need to clarify and correct the collision response algorithms illustrated in the sample code discussed in Chapters 13 and 16. Specifically, it should be noted that the equations for impulse in both 2D and 3D are written in terms of global, earth-fixed coordinates. Therefore, all of the variables in those equations should be in terms of global coordinates. For example, velocity, angular velocity and mass moment of inertia should all be in terms of global coordinates. For the 2D case this difference can easily go un-noticed since there's only one axis of rotation and the angular velocity and mass moment of inertia are the same in both local and global coordinates -- the z-axis is straight up in both cases. However, in the 3D case, the objects may rotate about any arbitrary axis and it's important to convert such parameters as angular velocity, and mass moment of inertia from local body fixed coordinates to global coordinates. I failed to make the conversion in the example presented in Chapter 16. While, I had in place a parameter in the rigid body structure for the mass moment of inertia in global coordinates, I inadvertently left out the conversion and that parameter went unused. Converting the inertia tensor from local to global coordinates involves transforming the inertia tensor using a rotation matrix derived from the orientation quaternion. The hovercraft example (with collision response) and the crash test example have been revised and corrected accordingly and will be posted on O'Reilly's website. Thanks, David Bourg

Anonymous