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Common Introductory Physics Mistakes
Listed are some common mistakes, categorized by their first occurrence. I know you are saying that you
would never make these silly mistakes, but alas, you may. Be forewarned. Many of these mistakes will
continue to haunt (and hurt) you as you move on in physics, not only in the chapter indicated. (Note:
Letters in boldface are used to represent vectors.) (6th ed. of Physics by Giancoli)
Chapter 1 and General
 Please read the instructions!!!
 Conversions between grams kilograms and centimeters meters
 Sometimes Arctan()  Tan1() as computed by your calculator.
 Units matter.
 Careful with conversions of powers (e.g. 20.0 cm = 0.20 m, but 20.0 cm2 = 0.0020 m2, and 20.0 cm3 =
0.000020 m3)
 Significant Figures matter (e.g. 2.00  2)
 We use  so much you might think all angles are . Sometimes they aren’t.
 When using your calculator sometimes you’ll want to be in degree mode, and sometimes in radian mode.
Chapter 2
 Average velocity and instantaneous velocity are not always equal. Therefore it is not always correct to
write v = x/t and plug in any old numbers.
 v, Δv, v0, and v are used to represent different quantities. Make sure you understand the differences.
 As used in this class, g is +9.81 m/s2. In equations remember that g is down and often a negative sign must
be included.
Chapter 3
 Vectors have a magnitude and a direction. (Often forgotten!)
 F1 + F2  F1 + F2 , when we deal with vectors!
 When using sine and cosine functions, be careful about what mode your calculator is in (radian verses
degree).
 Range, and the range equation, specifically requires that the projectile return to the same height from which
it was launched. Also, make sure you write the formula correctly.
 Relative velocity: the answer is not always the hypotenuses, and you don’t always have a right triangle.
Chapter 4
 Weight and mass are two different concepts.
 Which component of “mg” is down the plane and which is perpendicular to the plane!
 If an object is accelerating down, and you pick up as positive, a negative sign is needed in front of the “ma”
in F = ma.
 The normal force, FN, is so named because it is always perpendicular (“normal”) to the surface. But the
normal force is not always mg or mgcos.
 If you don’t know the acceleration of an object, please don’t simply assume it is “g”.
 (advanced) Friction is not always in the opposite direction as the motion of an object.
Chapter 5
 Gg
 An object moving in a circle at constant speed IS accelerating. The magnitude is ac = v2/r.
 There is no “centripetal force”, per se, and so never appears on an FBD. It is just a name we use.
Chapter 6
 Using W = Fd cos is valid only if the force is constant.
 Don’t forget to square the velocity (really, speed) when computing kinetic energy.
 The spring constant, k, does have units. In the mks system N/m are appropriate.
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Don’t confuse force (Newton) with the work (Joule) done by the force, esp. with a spring.
The net work (done by all forces) equals change in kinetic energy, not simply W = KE.
(advanced) A ball that “rolls without slipping” is most likely not on a frictionless surface (otherwise it
would just slide!) but the work done by this frictional force is zero. Cool.
Chapter 7
 P (power) and p (momentum) are not the same thing. Don’t mix them in your equations.
 By definition, during an elastic collision, KE is conserved, not mechanical energy.
Common Introductory Physics Mistakes

If momentum is conserved, then it is conserved in the x, y, and z direction independently.
Chapter 8
 Be careful with unit conversions, especially with omega. 1 revolution = 2 radians!
 Torque is a vector, and its units are Nm, not N. Also, don’t write this as a Joule.
 Centripetal/Radial acceleration and angular acceleration are not the same concept.
 When writing KEROT for a disk, there are two ½ terms! (KE = ½I2, I= ½mR2).
Chapter 9
 There is, in general, a force at a pivot point that has components in both the x and the y direction. Often
you don’t know anything about their magnitudes and directions.
Chapter 11
 When using the x, v, and a equations for SHM, be careful about what mode your calculator is in (radian
verses degree).
 When squaring T = 2(m/k), don’t forget to square the 2 and the .
 Once again, when calculating wave quantities, mind the radian/degree problem.
 Don’t confuse the wave number k with the spring constant k.
Chapter 12
 Memorizing all the resonance equations won’t help you one bit if you can’t think. In fact, I guess this
applies to just about every equation in physics.
Chapter 16
 2q  q2 (ok you math nerds, unless q = 0 or 2)
 F and E are vectors!
 Repulsive/attractive mean different directions depending on orientation.
 You must differentiate between the effects of a point charge and a uniform electric field.
Chapter 17
 V is not a vector! (…and there was much rejoicing.)
 Many people confuse electric potential and (electric) potential energy. Don’t you do it, because V  PE.
 Again, differentiate between the effects of a point charge and a uniform electric field.
 Don’t get confused between the symbol for electric potential (V) and the units for it (Volts, or V for short).
 The electric dipole moment is often measured in units of Cm. That’s Coulombs times meters, NOT
centimeters!
 Don’t get confused between the symbol for capacitance (C) and the units for charge (Coulombs, or C for
short).
 For goodness sake, V for electric potential is not the same as v for speed. There are problems with both.
Chapter 18
 I think Ohm’s Law is the most MISUSED formula in all of intro physics. Please understand it!
Chapter 19
 Two resistors may be in parallel, or in series, or neither. Same for capacitors.
 Don’t forget the generic formula for parallel resistors is one-over-RP (1/RP = …).
 Capacitors add in the opposite manner to resistors. Parallel = Straight addition, Series = Reciprocals
 EMF  E. Electromotive force is not the same concept as electric field. Heck, they don’t even have the
same units!
Chapter 20
 Right hand rule! (Don’t use your left hand).
 The force on a negative charge is opposite the direction of the force on a positive charge.
Chapter 21
 (change in flux) and (flux) may not be the same. Sometimes they are, but only if f or i is zero.
 Watch out for factors of two in  (e.g.  to ,  to ×, etc.). In these situations, if B, A, and are constant
in magnitude, then  = 2BAcos
 The area vector is perpendicular to the plane of the loop.
Chapter 23
 Angles used in Snell’s law are measured with respect to the normal line.