AP Physics

Unit 1 - Vectors and Kinematics

MCQ
Mathematical
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Step Reasoning
Identify the relationship for acceleration.
\[ F_{\text{net}} = ma = -kx \implies a = -\dfrac{k}{m}x \]
The question asks for the magnitude of acceleration relative to its maximum.
Calculate the ratio of acceleration at the given position to maximum acceleration.
\[ a_{\text{max}} = \dfrac{k}{m}A \]
\[ a(A/2) = \dfrac{k}{m} \left( \dfrac{A}{2} \right) = \dfrac{1}{2} a_{\text{max}} \]
To verify the claim about acceleration magnitude, we must evaluate the expression at \(x = A/2\) and compare it to the value at \(x = A\).
Identify the relationship for velocity using conservation of energy.
\[ \dfrac{1}{2}mv^2 + \dfrac{1}{2}kx^2 = \dfrac{1}{2}kA^2 \]
The question asks for the magnitude of velocity relative to its maximum. Velocity in SHM does not scale linearly with displacement.
Calculate the velocity at the given position and compare it to maximum velocity.
\[ \dfrac{1}{2}mv^2 = \dfrac{1}{2}kA^2 – \dfrac{1}{2}k\left(\dfrac{A}{2}\right)^2 = \dfrac{1}{2}k\left(A^2 – \dfrac{A^2}{4}\right) = \dfrac{3}{4} \left( \dfrac{1}{2}kA^2 \right) \]
\[ \text{Since } v_{\text{max}} = \sqrt{\dfrac{k}{m}}A, \text{ then } v = \sqrt{\dfrac{3}{4}} v_{\text{max}} = \dfrac{\sqrt{3}}{2} v_{\text{max}} \approx 0.866 v_{\text{max}} \]
Evaluating the energy equation at \(x = A/2\) allows us to find the specific factor of \(v_{\text{max}}\).
Conclude the final qualitative comparison.
\[ a = 0.5 a_{\text{max}} \text{ and } v = 0.866 v_{\text{max}} > 0.5 v_{\text{max}} \]
We must match our derived ratios to the qualitative descriptions in the choices.

Why each choice is correct or incorrect:

(A) This is the correct answer.

(B) Incorrectly assumes velocity is linear with displacement (\(v \propto x\)). Velocity magnitude actually follows a circle/ellipse relationship in phase space (\(v \propto \sqrt{A^2 – x^2}\)).

(C) Mistakenly assumes acceleration magnitude scales with the square of displacement because potential energy does (\(U_s = \frac{1}{4} U_{\text{total}}\)). Acceleration is actually proportional to the force, which is linear with displacement.

(D) Combines the errors of both B and C: using energy-based scaling for acceleration and linear-based scaling for velocity.

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KinematicsForces
\(\Delta x = v_i t + \frac{1}{2} at^2\)\(F = ma\)
\(v = v_i + at\)\(F_g = \frac{G m_1 m_2}{r^2}\)
\(v^2 = v_i^2 + 2a \Delta x\)\(f = \mu N\)
\(\Delta x = \frac{v_i + v}{2} t\)\(F_s =-kx\)
\(v^2 = v_f^2 \,-\, 2a \Delta x\) 
Circular MotionEnergy
\(F_c = \frac{mv^2}{r}\)\(KE = \frac{1}{2} mv^2\)
\(a_c = \frac{v^2}{r}\)\(PE = mgh\)
\(T = 2\pi \sqrt{\frac{r}{g}}\)\(KE_i + PE_i = KE_f + PE_f\)
 \(W = Fd \cos\theta\)
MomentumTorque and Rotations
\(p = mv\)\(\tau = r \cdot F \cdot \sin(\theta)\)
\(J = \Delta p\)\(I = \sum mr^2\)
\(p_i = p_f\)\(L = I \cdot \omega\)
Simple Harmonic MotionFluids
\(F = -kx\)\(P = \frac{F}{A}\)
\(T = 2\pi \sqrt{\frac{l}{g}}\)\(P_{\text{total}} = P_{\text{atm}} + \rho gh\)
\(T = 2\pi \sqrt{\frac{m}{k}}\)\(Q = Av\)
\(x(t) = A \cos(\omega t + \phi)\)\(F_b = \rho V g\)
\(a = -\omega^2 x\)\(A_1v_1 = A_2v_2\)
ConstantDescription
[katex]g[/katex]Acceleration due to gravity, typically [katex]9.8 , \text{m/s}^2[/katex] on Earth’s surface
[katex]G[/katex]Universal Gravitational Constant, [katex]6.674 \times 10^{-11} , \text{N} \cdot \text{m}^2/\text{kg}^2[/katex]
[katex]\mu_k[/katex] and [katex]\mu_s[/katex]Coefficients of kinetic ([katex]\mu_k[/katex]) and static ([katex]\mu_s[/katex]) friction, dimensionless. Static friction ([katex]\mu_s[/katex]) is usually greater than kinetic friction ([katex]\mu_k[/katex]) as it resists the start of motion.
[katex]k[/katex]Spring constant, in [katex]\text{N/m}[/katex]
[katex] M_E = 5.972 \times 10^{24} , \text{kg} [/katex]Mass of the Earth
[katex] M_M = 7.348 \times 10^{22} , \text{kg} [/katex]Mass of the Moon
[katex] M_M = 1.989 \times 10^{30} , \text{kg} [/katex]Mass of the Sun
VariableSI Unit
[katex]s[/katex] (Displacement)[katex]\text{meters (m)}[/katex]
[katex]v[/katex] (Velocity)[katex]\text{meters per second (m/s)}[/katex]
[katex]a[/katex] (Acceleration)[katex]\text{meters per second squared (m/s}^2\text{)}[/katex]
[katex]t[/katex] (Time)[katex]\text{seconds (s)}[/katex]
[katex]m[/katex] (Mass)[katex]\text{kilograms (kg)}[/katex]
VariableDerived SI Unit
[katex]F[/katex] (Force)[katex]\text{newtons (N)}[/katex]
[katex]E[/katex], [katex]PE[/katex], [katex]KE[/katex] (Energy, Potential Energy, Kinetic Energy)[katex]\text{joules (J)}[/katex]
[katex]P[/katex] (Power)[katex]\text{watts (W)}[/katex]
[katex]p[/katex] (Momentum)[katex]\text{kilogram meters per second (kgm/s)}[/katex]
[katex]\omega[/katex] (Angular Velocity)[katex]\text{radians per second (rad/s)}[/katex]
[katex]\tau[/katex] (Torque)[katex]\text{newton meters (Nm)}[/katex]
[katex]I[/katex] (Moment of Inertia)[katex]\text{kilogram meter squared (kgm}^2\text{)}[/katex]
[katex]f[/katex] (Frequency)[katex]\text{hertz (Hz)}[/katex]

Metric Prefixes

Example of using unit analysis: Convert 5 kilometers to millimeters. 

  1. Start with the given measurement: [katex]\text{5 km}[/katex]

  2. Use the conversion factors for kilometers to meters and meters to millimeters: [katex]\text{5 km} \times \frac{10^3 \, \text{m}}{1 \, \text{km}} \times \frac{10^3 \, \text{mm}}{1 \, \text{m}}[/katex]

  3. Perform the multiplication: [katex]\text{5 km} \times \frac{10^3 \, \text{m}}{1 \, \text{km}} \times \frac{10^3 \, \text{mm}}{1 \, \text{m}} = 5 \times 10^3 \times 10^3 \, \text{mm}[/katex]

  4. Simplify to get the final answer: [katex]\boxed{5 \times 10^6 \, \text{mm}}[/katex]

Prefix

Symbol

Power of Ten

Equivalent

Pico-

p

[katex]10^{-12}[/katex]

Nano-

n

[katex]10^{-9}[/katex]

Micro-

µ

[katex]10^{-6}[/katex]

Milli-

m

[katex]10^{-3}[/katex]

Centi-

c

[katex]10^{-2}[/katex]

Deci-

d

[katex]10^{-1}[/katex]

(Base unit)

[katex]10^{0}[/katex]

Deca- or Deka-

da

[katex]10^{1}[/katex]

Hecto-

h

[katex]10^{2}[/katex]

Kilo-

k

[katex]10^{3}[/katex]

Mega-

M

[katex]10^{6}[/katex]

Giga-

G

[katex]10^{9}[/katex]

Tera-

T

[katex]10^{12}[/katex]

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