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| Step | Derivation/Formula | Reasoning |
|---|---|---|
| 1 | \( \omega = 100,000 \, \text{rpm} \) | Given the angular velocity of the flywheel. |
| 2 | \( \omega = 100,000 \times \frac{2\pi \, \text{rad} }{1 \, \text{rev}} \times \frac{1 \, \text{min} }{60 \, \text{s} } \) | Convert from revolutions per minute (rpm) to radians per second (rad/s). |
| 3 | \( \omega = \frac{100,000 \times 2\pi}{60} \, \text{rad/s} \) | Combine the conversion factors. |
| 4 | \( \omega \approx 10472 \, \text{rad/s} \) | Simplify the expression to get the angular velocity in rad/s. |
| 5 | \( r = \frac{20 \, \text{cm}}{2} = 10 \, \text{cm} = 0.1 \, \text{m} \) | Calculate the radius of the flywheel and convert to meters. |
| 6 | \( v = \omega r \) | Use the formula for linear speed on the rim of a rotating object: \( v = \omega r \). |
| 7 | \( v = 10472 \, \text{rad/s} \times 0.1 \, \text{m} \) | Substitute the values for \( \omega \) and \( r \) into the formula. |
| 8 | \( v \approx 1047.2 \, \text{m/s} \) | Calculate the linear speed: the speed of a point on the rim of the flywheel is \( \boxed{1047.2 \, \text{m/s}} \). |
| Step | Derivation/Formula | Reasoning |
|---|---|---|
| 1 | \( \omega_i = 10472 \, \text{rad/s} \) | Initial angular velocity from part (a). |
| 2 | \( \omega_f = 0.6 \times 10472 \, \text{rad/s} \) | Angular velocity decreases by 40%, so the final angular velocity is 60% of the initial value. |
| 3 | \( \omega_f = 0.6 \times 10472 \, \text{rad/s} = 6283.2 \, \text{rad/s} \) | Calculate the final angular velocity. |
| 4 | \( \alpha = \frac{\Delta \omega}{\Delta t} \) | The formula for angular acceleration where \( \Delta \omega = \omega_f – \omega_i \) and \( \Delta t \) is the time interval. |
| 5 | \( \alpha = \frac{6283.2 \, \text{rad/s} – 10472 \, \text{rad/s}}{30 \, \text{s}} \) | Substitute the known values into the formula. |
| 6 | \( \alpha = \frac{-4188.8 \, \text{rad/s}}{30 \, \text{s}} \) | Simplify the numerator. |
| 7 | \( \alpha \approx -139.6 \, \text{rad/s}^2 \) | Calculate the angular acceleration, which is . The magnitude is \( \boxed{139.6 \, \text{rad/s}^2} \). |
| Step | Derivation/Formula | Reasoning |
|---|---|---|
| 1 | \( \theta = \omega_i t + \frac{1}{2} \alpha t^2 \) | Use the kinematic equation for angular displacement under constant angular acceleration. |
| 2 | \( \theta = 10472 \, \text{rad/s} \times 30 \, \text{s} + \frac{1}{2} \times (-139.6 \, \text{rad/s}^2) \times (30 \, \text{s})^2 \) | Substitute the known values into the formula. |
| 3 | \( \theta = 10472 \times 30 + \frac{1}{2} \times (-139.6) \times 900 \) | Simplify the expression. |
| 4 | \( \theta = 314160 – 62820 \) | Calculate the individual terms. |
| 5 | \( \theta = 251340 \, \text{rad} \) | Combine the results to get the total angular displacement in radians. |
| 6 | \( \text{Revolutions} = \frac{\theta}{2\pi} \) | Convert angular displacement from radians to revolutions. |
| 7 | \( \text{Revolutions} = \frac{251340}{2\pi} \) | Substitute the value of \( \theta \). |
| 8 | \( \text{Revolutions} \approx 40000 \) | Calculate the total number of revolutions. The rotor makes approximately \( \boxed{40000 \, \text{revolutions}} \) during these 30 seconds. |
Just ask: "Help me solve this problem."
A student is asked to design an experiment to determine the change in angular momentum of a disk that rotates about its center and the product of the average torque applied to the disk and the time interval in which the torque is exerted. A net force is applied tangentially to the surface of the disk. The rotational inertia of the disk about its center is [katex]I = MR^2[/katex]. Which two of the following quantities should the student measure to determine the change in angular momentum of the disk after 10 s? Select two answers.
A disk of radius R = 0.5 cm rests on a flat, horizontal surface such that frictional forces are considered to be negligible. Three forces of unknown magnitude are exerted on the edge of the disk, as shown in the figure. Which of the following lists the essential measuring devices that, when used together, are needed to determine the change in angular momentum of the disk after a known time of 5.0 s?
A rotating, rigid body makes 10 complete revolutions in 10 seconds. What is its average angular velocity?
A massless rigid rod of length [katex]3d[/katex] is pivoted at a fixed point [katex]W[/katex], and two forces each of magnitude [katex]F[/katex] are applied vertically upward as shown above. A third vertical force of magnitude [katex]F[/katex] may be applied, either upward or downward, at one of the labeled points. With the proper choice of direction at each point, the rod can be in equilibrium if the third force of magnitude [katex]F[/katex] is applied at point?
A string is wound tightly around a fixed pulley having a radius of 5.0 cm. As the string is pulled, the pulley rotates without any slipping of the string. What is the angular speed of the pulley when the string is moving at 5.0 m/s?
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| Kinematics | Forces |
|---|---|
| \(\Delta x = v_i t + \frac{1}{2} at^2\) | \(F = ma\) |
| \(v = v_i + at\) | \(F_g = \frac{G m_1 m_2}{r^2}\) |
| \(v^2 = v_i^2 + 2a \Delta x\) | \(f = \mu N\) |
| \(\Delta x = \frac{v_i + v}{2} t\) | \(F_s =-kx\) |
| \(v^2 = v_f^2 \,-\, 2a \Delta x\) |
| Circular Motion | Energy |
|---|---|
| \(F_c = \frac{mv^2}{r}\) | \(KE = \frac{1}{2} mv^2\) |
| \(a_c = \frac{v^2}{r}\) | \(PE = mgh\) |
| \(T = 2\pi \sqrt{\frac{r}{g}}\) | \(KE_i + PE_i = KE_f + PE_f\) |
| \(W = Fd \cos\theta\) |
| Momentum | Torque 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 Motion | Fluids |
|---|---|
| \(F = -kx\) | \(P = \frac{F}{A}\) |
| \(T = 2\pi \sqrt{\frac{l}{g}}\) | \(P_{\text{total}} = P_{\text{atm}} + \rho gh\) |
| \(T = 2\pi \sqrt{\frac{m}{k}}\) | \(Q = Av\) |
| \(x(t) = A \cos(\omega t + \phi)\) | \(F_b = \rho V g\) |
| \(a = -\omega^2 x\) | \(A_1v_1 = A_2v_2\) |
| Constant | Description |
|---|---|
| [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 |
| Variable | SI 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] |
| Variable | Derived 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.
Start with the given measurement: [katex]\text{5 km}[/katex]
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]
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]
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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