| Step | Reasoning |
|---|---|
| Identify the requirement for work to be done by a force on an object. \[ W = \int \vec{F} \cdot d\vec{s} \] |
To determine if static friction does work, we must evaluate the relationship between the force and the displacement of the point where the force is applied. |
| Apply the rolling without slipping condition to the motion of the cylinder. \[ \vec{v}_{point} = \vec{v}_{cm} + \vec{\omega} \times \vec{r} = 0 \] |
Rolling without slipping implies a specific relationship between the translational velocity of the center of mass and the angular velocity of the object. |
| Evaluate the displacement of the application point of the static friction force. \[ P = \vec{F}_s \cdot \vec{v}_{point} = \vec{F}_s \cdot 0 = 0 \] |
Since the point of contact is the specific location where the static friction force is applied, its instantaneous velocity determines the rate of work done. |
| Determine the impact on the system’s mechanical energy. \[ \Delta E_{mech} = W_{ext} = 0 \] |
Because the work done by static friction is zero, no energy is removed from the mechanical system (cylinder-Earth) to be converted into thermal energy. |
Why each choice is correct or incorrect:
(A) Incorrectly treats the displacement of the center of mass as the displacement of the point of contact. This would only be true for kinetic friction during sliding.
(B) Incorrectly assumes that because friction provides a torque that increases rotational kinetic energy, it must do work. In reality, the force facilitates the transfer of energy from translational to rotational modes without doing net work itself.
(C) This is the correct answer.
(D) Static friction is not a conservative force. While the work done in this specific scenario is zero, the justification that it is ‘conservative’ like gravity or a spring force is physically incorrect.
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An engineer is testing two different designs for a rotating flywheel. Design A consists of a uniform solid disk with mass \(M\) and radius \(R\) mounted on a low-friction axle. When a motor applies a constant net torque \(\tau\) to Design A, it experiences an angular acceleration \(\alpha_0\). Design B consists of a uniform solid disk with mass \(2M\) and radius \(2R\). If the motor is adjusted to apply a constant net torque of \(4\tau\) to Design B, what is the resulting angular acceleration of Design B in terms of \(\alpha_0\)?

A spring of ideal spring constant \(k\) hangs vertically from a ceiling. When the spring is unextended, its bottom end is at position \(y = 0\). The positive \(y\)-direction is defined as downward. A block of mass \(m\) is attached to the spring and gently lowered until it hangs at rest at its equilibrium position \(y_{eq}\). The block is then pulled down an additional distance \(A\) to a maximum position \(y_{max} = y_{eq} + A\) and released from rest.
A block of mass \(M\) is attached to an ideal spring and undergoes simple harmonic motion on a frictionless horizontal surface. The equilibrium position of the block is at \(x = 0\). Which of the following graphs best represents the acceleration \(a\) of the block as a function of its displacement \(x\)?

A block of mass \(M\) is attached to the lower end of a vertical spring with spring constant \(k\), while the upper end of the spring is fixed to a ceiling. A second block of mass \(m\) is placed on top of the first block. The two-block system is set into vertical simple harmonic motion with amplitude \(A\). At the instant the blocks pass through the equilibrium position while moving downward, which of the following correctly describes the magnitude of the force exerted by the spring on the bottom block, \(F_{spring}\), and the magnitude of the normal force exerted by the bottom block on the top block, \(F_{normal}\)?

A block of mass \(m\) is attached to an ideal horizontal spring with spring constant \(k\). The system oscillates on a frictionless surface with amplitude \(A\). Which of the following expressions represents the kinetic energy of the block when its displacement from the equilibrium position is \(x = \dfrac{1}{3}A\)?

Two uniform, thin rods, Rod A and Rod B, both have the same mass \(M\). Rod A has length \(L\) and Rod B has length \(2L\). Both rods are pivoted at one end so they can rotate in a vertical plane. The rods are held in a horizontal position and released from rest at the same time. What is the ratio \(\dfrac{\alpha_B}{\alpha_A}\) of the magnitude of the initial angular acceleration of Rod B to that of Rod A?

A drone of mass \( m \) is flying in a straight horizontal line at a constant speed \( v \) and a constant altitude \( H \) above the ground. A stationary observer is located at point \( P \) on the ground. At a certain moment, the horizontal distance between the drone and the observer is \( x \). Which of the following is a correct expression for the magnitude of the angular momentum of the drone relative to the observer at this moment?

A uniform rigid rod of mass \(M\) and length \(D\) is suspended vertically from a horizontal, frictionless pivot at its top end. The rotational inertia of the rod about the pivot is \(I_{rod} = \dfrac{1}{3}MD^2\). A small lump of clay of mass \(m\) is thrown horizontally with speed \(v_0\) toward the rod. The clay strikes the rod at a distance \(L\) from the pivot and sticks to it. The collision occurs almost instantaneously, and the rod-clay system swings upward after the collision.

A solid sphere of mass \(M\) and radius \(R\) is released from rest at the top of an incline of height \(H\) and rolls without slipping to the bottom. A block of mass \(M\) is released from rest at the top of an identical incline that is frictionless and slides to the bottom. If the translational speeds of the sphere and the block at the bottom of the incline are \(v_{sphere}\) and \(v_{block}\), respectively, what is the ratio \(\dfrac{v_{sphere}}{v_{block}}\)?

A small ball of mass \(m\) travels horizontally with speed \(v\) on a frictionless surface. It strikes the bottom end of a uniform rod of mass \(M\) and length \(L\) that is suspended vertically from a frictionless pivot at its top end. The ball sticks to the rod, and the ball-rod system begins to rotate about the pivot. Which of the following correctly compares the linear momentum of the ball-rod system immediately before the collision, \(p_i\), to the linear momentum of the system immediately after the collision, \(p_f\), and provides the correct physical justification?
C
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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] |
Metric Prefixes
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] | 0.000000000001 |
Nano- | n | [katex]10^{-9}[/katex] | 0.000000001 |
Micro- | µ | [katex]10^{-6}[/katex] | 0.000001 |
Milli- | m | [katex]10^{-3}[/katex] | 0.001 |
Centi- | c | [katex]10^{-2}[/katex] | 0.01 |
Deci- | d | [katex]10^{-1}[/katex] | 0.1 |
(Base unit) | – | [katex]10^{0}[/katex] | 1 |
Deca- or Deka- | da | [katex]10^{1}[/katex] | 10 |
Hecto- | h | [katex]10^{2}[/katex] | 100 |
Kilo- | k | [katex]10^{3}[/katex] | 1,000 |
Mega- | M | [katex]10^{6}[/katex] | 1,000,000 |
Giga- | G | [katex]10^{9}[/katex] | 1,000,000,000 |
Tera- | T | [katex]10^{12}[/katex] | 1,000,000,000,000 |
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