AP Physics

Unit 8 - Fluids

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Part A

Step Derivation/Formula Reasoning
1 \[P_{\text{gauge}} = \rho\,g\,\Delta x\] Gauge pressure at a depth is calculated as the product of the fluid’s density \(\rho\), gravitational acceleration \(g\), and the depth \(\Delta x\).
2 \[P_{\text{gauge}} = (1.025 \times 10^3\,\text{kg/m}^3)(9.8\,\text{m/s}^2)(35\,\text{m})\] Substitute the given density of ocean water, gravitational acceleration, and depth into the equation.
3 \[P_{\text{gauge}} \approx 3.53 \times 10^5\,\text{Pa}\] Performing the multiplication gives the gauge pressure on the ocean floor as approximately \(3.53 \times 10^5\,\text{Pa}\).

Part B

Step Derivation/Formula Reasoning
1 \[P_{\text{absolute}} = P_{\text{atm}} + P_{\text{gauge}}\] Absolute pressure is the sum of the atmospheric pressure and the gauge pressure.
2 \[P_{\text{absolute}} = (1.01 \times 10^5\,\text{Pa}) + (3.53 \times 10^5\,\text{Pa})\] Substitute the atmospheric pressure and the previously calculated gauge pressure into the formula.
3 \[P_{\text{absolute}} \approx 4.54 \times 10^5\,\text{Pa}\] The sum gives the absolute pressure on the ocean floor, approximately \(4.54 \times 10^5\,\text{Pa}\).

Part C

Step Derivation/Formula Reasoning
1 \[V = 1.0\,\text{m} \times 2.0\,\text{m} \times 0.03\,\text{m} = 0.06\,\text{m}^3\] Calculate the volume \(V\) of the rectangular aluminum plate using its given dimensions.
2 \[m_{\text{Al}} = \rho_{\text{Al}}\,V = (2.7 \times 10^3\,\text{kg/m}^3)(0.06\,\text{m}^3) \approx 162\,\text{kg}\] Determine the mass of the plate by multiplying its volume with the density of aluminum \(\rho_{\text{Al}}\).
3 \[W = m_{\text{Al}}\,g = 162\,\text{kg} \times 9.8\,\text{m/s}^2 \approx 1588\,\text{N}\] Compute the weight \(W\) of the plate using \(W = m \times g\).
4 \[F_B = \rho_{\text{water}}\,g\,V = (1.025 \times 10^3\,\text{kg/m}^3)(9.8\,\text{m/s}^2)(0.06\,\text{m}^3) \approx 603\,\text{N}\] Calculate the buoyant force \(F_B\) acting on the plate using Archimedes’ principle.
5 \[T = W – F_B = 1588\,\text{N} – 603\,\text{N} \approx 985\,\text{N}\] Since the plate is lifted at a constant velocity, the tension \(T\) in the cable equals the net downward force \(W – F_B\).
6 \[\boxed{T \approx 985\,\text{N}}\] This is the final tension in the cable required to lift the plate upward at a slow constant velocity.

Part D

Step Derivation/Formula Reasoning
1 \[T’ = (W – F_B) + m_{\text{Al}}\,a\] When accelerating upward, the cable must supply additional force \(m_{\text{Al}}a\) to overcome inertia, added to the weight minus buoyant force.
2 \[m_{\text{Al}}\,a = 162\,\text{kg} \times 0.05\,\text{m/s}^2 = 8.1\,\text{N}\] Calculate the extra force required for an upward acceleration of \(0.05\,\text{m/s}^2\).
3 \[T’ = 985\,\text{N} + 8.1\,\text{N} \approx 993\,\text{N}\] Add the extra force to the original tension to find the new tension in the cable.
4 \[\boxed{T’ \approx 993\,\text{N}}\] The tension increases to approximately \(993\,\text{N}\) when the plate accelerates upward at \(0.05\,\text{m/s}^2\).

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  1. \(\boxed{P_{\text{gauge}} \approx 3.53 \times 10^5\,\text{Pa}}\)
  2. \(\boxed{P_{\text{absolute}} \approx 4.54 \times 10^5\,\text{Pa}}\)
  3. \(\boxed{T \approx 985\,\text{N}}\)
  4. \(\boxed{T’ \approx 993\,\text{N}}\) indicating an increase in tension.

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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]

General Metric Conversion Chart

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]

  1. 1. Some answers may vary by 1% due to rounding.
  2. Gravity values may differ: \(9.81 \, \text{m/s}^2\) or \(10 \, \text{m/s}^2\).
  3. Variables can be written differently. For example, initial velocity (\(v_i\)) may be \(u\), and displacement (\(\Delta x\)) may be \(s\).
  4. Bookmark questions you can’t solve to revisit them later
  5. 5. Seek help if you’re stuck. The sooner you understand, the better your chances on tests.

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