AP Physics C: Mechanics : AP Physics C

Study concepts, example questions & explanations for AP Physics C: Mechanics

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Example Questions

Example Question #1 : Friction

\displaystyle \begin{align*}&\text{A block of mass }19kg\text{ rests on an incline of }69^{\circ}\\&\text{What is the frictional force acting on the block from the surface?}\\&g=9.81\frac{m}{s^2}\end{align*}

Possible Answers:

\displaystyle 186.39N

\displaystyle 66.8N

\displaystyle 6.81N

\displaystyle 174.01N

Correct answer:

\displaystyle 174.01N

Explanation:

Normal

\displaystyle \begin{align*}&\text{To solve a problem of force balance, a good idea is to draw}\\&\text{it out. Since the block is at rest, i.e. stationary, the sum}\\&\text{of forces acting on it must be equal to zero. The forces acting}\\&\text{on it are gravity (in the form of the mass times the gravitational}\\&\text{constant), a normal force, and since it’s on an incline, a frictional}\\&\text{force to balance it all out:}\\&F_{mass}=m*g\\&F_{normal}=m*g*cos(a)\\&F_{friction}=m*g*sin(a)\\&F_{friction}=19kg(9.81\frac{m}{s^2})(sin(69^{\circ}))\\&F_{friction}=174.01N\end{align*}

Example Question #4 : Friction

\displaystyle \begin{align*}&\text{A block of mass }284kg\text{ rests on an incline of }48^{\circ}\\&\text{It is also partially supported by a rope.}\\&\text{The frictional force acting on the block is }207.04N\\&\text{What is the tension felt in the rope?}\\&g=9.81\frac{m}{s^2}\end{align*}

Possible Answers:

\displaystyle 2786.04N

\displaystyle 1864.22N

\displaystyle 2070.43N

\displaystyle 2507.44N

Correct answer:

\displaystyle 2507.44N

Explanation:

Normalwrope

\displaystyle \begin{align*}&\text{To solve a problem of force balance, a good idea is to draw}\\&\text{it out. Since the block is at rest, i.e. stationary, the sum}\\&\text{of forces acting on it must be equal to zero. The forces acting}\\&\text{on it are gravity (in the form of the mass times the gravitational}\\&\text{constant), the force of the rope supporting it, a normal force,}\\&\text{and since it’s on an incline, a frictional force to balance}\\&\text{it all out. Notice that the tension in the rope mitigates some}\\&\text{of the gravitational force (as illustrated in the upper left}\\&\text{corner) to lessen what is applied to the slope:}\\&F_{mass}=m*g\\&F_{normal}=(F_{mass}-F_{rope})*cos(a)\\&F_{friction}=(F_{mass}-F_{rope})*sin(a)\\&F_{rope}=F_{mass}-\frac{F_{friction}}{sin(a)}\\&F_{rope}=2786.04N-\frac{207.04N}{sin(48^{\circ})}\\&F_{rope}=2507.44N\end{align*}

Example Question #11 : Forces

\displaystyle \begin{align*}&\text{A block of mass }101kg\text{ rests on an incline of }68^{\circ}\\&\text{It is also partially supported by a rope with a tension of }693.57N\\&\text{What is the frictional force acting on the block from the surface?}\\&g=9.81\frac{m}{s^2}\end{align*}

Possible Answers:

\displaystyle 111.35N

\displaystyle 918.66N

\displaystyle 275.6N

\displaystyle 371.16N

Correct answer:

\displaystyle 275.6N

Explanation:

Normalwrope

\displaystyle \begin{align*}&\text{To solve a problem of force balance, a good idea is to draw}\\&\text{it out. Since the block is at rest, i.e. stationary, the sum}\\&\text{of forces acting on it must be equal to zero. The forces acting}\\&\text{on it are gravity (in the form of the mass times the gravitational}\\&\text{constant), the force of the rope supporting it, a normal force,}\\&\text{and since it’s on an incline, a frictional force to balance}\\&\text{it all out. Notice that the tension in the rope mitigates some}\\&\text{of the gravitational force (as illustrated in the upper left}\\&\text{corner) to lessen what is applied to the slope:}\\&F_{mass}=m*g\\&F_{normal}=(F_{mass}-F_{rope})*cos(a)\\&F_{friction}=(F_{mass}-F_{rope})*sin(a)\\&F_{friction}=(101kg(9.81\frac{m}{s^2})-693.57N)(sin(68^{\circ}))\\&F_{friction}=275.6N\end{align*}

Example Question #91 : Mechanics Exam

\displaystyle \begin{align*}&\text{A block of mass }187kg\text{ rests on an incline of }70^{\circ}\\&\text{What is the frictional force acting on the block from the surface?}\\&g=9.81\frac{m}{s^2}\end{align*}

Possible Answers:

\displaystyle 1834.47N

\displaystyle 627.43N

\displaystyle 63.96N

\displaystyle 1723.84N

Correct answer:

\displaystyle 1723.84N

Explanation:

Normal

\displaystyle \begin{align*}&\text{To solve a problem of force balance, a good idea is to draw}\\&\text{it out. Since the block is at rest, i.e. stationary, the sum}\\&\text{of forces acting on it must be equal to zero. The forces acting}\\&\text{on it are gravity (in the form of the mass times the gravitational}\\&\text{constant), a normal force, and since it’s on an incline, a frictional}\\&\text{force to balance it all out:}\\&F_{mass}=m*g\\&F_{normal}=m*g*cos(a)\\&F_{friction}=m*g*sin(a)\\&F_{friction}=187kg(9.81\frac{m}{s^2})(sin(70^{\circ}))\\&F_{friction}=1723.84N\end{align*}

Example Question #12 : Friction And Normal Force

\displaystyle \begin{align*}&\text{A block of mass }208kg\text{ rests on an incline of }28^{\circ}\\&\text{What is the frictional force acting on the block from the surface?}\\&g=9.81\frac{m}{s^2}\end{align*}

Possible Answers:

\displaystyle 2040.48N

\displaystyle 1801.64N

\displaystyle 957.95N

\displaystyle 183.65N

Correct answer:

\displaystyle 957.95N

Explanation:

Normal

\displaystyle \begin{align*}&\text{To solve a problem of force balance, a good idea is to draw}\\&\text{it out. Since the block is at rest, i.e. stationary, the sum}\\&\text{of forces acting on it must be equal to zero. The forces acting}\\&\text{on it are gravity (in the form of the mass times the gravitational}\\&\text{constant), a normal force, and since it’s on an incline, a frictional}\\&\text{force to balance it all out:}\\&F_{mass}=m*g\\&F_{normal}=m*g*cos(a)\\&F_{friction}=m*g*sin(a)\\&F_{friction}=208kg(9.81\frac{m}{s^2})(sin(28^{\circ}))\\&F_{friction}=957.95N\end{align*}

Example Question #1 : Understanding Gravity

A block with a mass of \displaystyle 10kg is traveling at \displaystyle 50\frac{m}{s} when it impacts the ground. From how many meters off the ground was the block dropped?

Round to the nearest whole number.

Possible Answers:

\displaystyle 130m

\displaystyle 126m

\displaystyle 128m

\displaystyle 124m

\displaystyle 132m

Correct answer:

\displaystyle 128m

Explanation:

Set the gravitational potential energy and kinetic energy equal to each other and solve for the height.

\displaystyle mgh = \frac{1}{2}mv^2

Mass cancels.

\displaystyle gh = \frac{1}{2}v^2

Isolate the height and plug in our values.

\displaystyle h = \frac{v^2}{2g} = \frac{(50\frac{m}{s})^2}{2*9.8\frac{m}{s^2}} = \frac{2500}{19.6} = 127.551m

Rounding this gives \displaystyle h = 128m.

Example Question #1 : Gravity

An object of mass \displaystyle \small m is dropped from a tower. The object's drag force is given by \displaystyle \small F_{d}=-bv^{2} where \displaystyle \small b is a positive constant. What will the objects terminal velocity be?

Possible Answers:

\displaystyle \small \small v_{term}=\frac{g}{b}

\displaystyle \small v_{term}=\sqrt{\frac{g}{b}}

\displaystyle \small \small v_{term}=\sqrt{\frac{b}{mg}}

\displaystyle \small \small v_{term}=\frac{mg}{b}

\displaystyle v_{term}=\sqrt{\frac{mg}{b}}

Correct answer:

\displaystyle v_{term}=\sqrt{\frac{mg}{b}}

Explanation:

To find terminal velocity, set the magnitude of the drag force equal to the magnitude of the force of gravity since when these forces are equal and opposite, the object will stop accelerating:

\displaystyle \small F_{d}=mg

\displaystyle \small bv^{2}=mg

Solve for \displaystyle v

\displaystyle v=\sqrt{\frac{mg}{b}}

Example Question #1 : Gravity

A ship of mass \displaystyle \small m and an initial velocity of \displaystyle \small v_0 is coasting to a stop. The water exerts a drag force on the ship. The drag force is proportional to the velocity:

\displaystyle F_D =-kv where the negative sign indicates that the drag force acts in a direction opposite the motion. After the ship has coasted for a time equal to \displaystyle \frac{2}{k}, how fast (in terms of \displaystyle \small v_0) will the ship be moving?

Possible Answers:

\displaystyle \small \frac{v_0}{4}

\displaystyle \small 0.135 v_0

\displaystyle \small 0.63v_0

\displaystyle \small 0.37v_0

\displaystyle \small \frac{v_{0}}2{}

Correct answer:

\displaystyle \small 0.135 v_0

Explanation:

The ship's equation of velocity (found by solving the first-order differential equation) is \displaystyle \small v=v_0e^{-kt}

Substitute \displaystyle t=\frac{2}{k} and solve.

Example Question #21 : Forces

A spherical asteroid has a hole drilled through the center as diagrammed below:

Hole through planet

Refer to the diagram above. An object that is much smaller than the asteroid is released from rest at the surface of the asteroid, at point a. How do the velocity and acceleration of the object compare at point b at the surface, and point a, located at the center of the asteroid?

Possible Answers:

\displaystyle v_{a}< v_{b} and \displaystyle a_{a}=a_{b}

\displaystyle v_{a}< v_{b} and \displaystyle a_{a}>_{b}

\displaystyle v_{a}=v_{b} and \displaystyle a_{a}=a_{b}

\displaystyle v_{a}< v_{b} and \displaystyle a_{a}< a_{b}

\displaystyle v_{a}=v_{b} and \displaystyle a_{a}< a_{b}

Correct answer:

\displaystyle v_{a}< v_{b} and \displaystyle a_{a}>_{b}

Explanation:

Because the gravitational force depends only on the mass beneath the object (which is the gravitational version of Gauss's Law for charge), the acceleration steadily decreases as the object falls, and drops to zero at the center. Nevertheless, the velocity keeps increasing as the object falls, it just does so more slowly.

Example Question #1 : Gravity

With what minimum velocity must a rocket be launched from the surface of the moon in order to not fall back down due to the moon's gravity?

The mass of the moon is \displaystyle 7.2*10^{22}kg and its radius is \displaystyle 1.8*10^6m.

\displaystyle \small G=6.67*10^{-11}\frac{m^2N}{kg^2}

Possible Answers:

\displaystyle 2.3\frac{km}{s}

\displaystyle 520\frac{m}{s}

\displaystyle 470\frac{m}{s}

\displaystyle 730\frac{m}{s}

\displaystyle 1.6\frac{km}{s}

Correct answer:

\displaystyle 2.3\frac{km}{s}

Explanation:

Relevant equations:

\displaystyle E_{total}=K+U_g

\displaystyle K = \frac{1}{2}mv^2

\displaystyle U_g=\frac{-GMm}{r}

For the rocket to escape the moon's gravity, its minimum total energy is zero. If the total energy is zero, the rocket will have zero final velocity when it is infinitely far from the moon. If total energy is less than zero, the rocket will fall back to the moon's surface. If total energy is greater than zero, the rocket will have some final velocity when it is infinitely far away. 

For the minimum energy case as the rocket leaves the surface:

\displaystyle 0 = K + U_g = \frac{1}{2}mv_{esc}^2 - \frac{GMm}{r}

Rearrange energy equation to isolate the velocity term.

\displaystyle v_{esc}^2 = \frac{GMm}{\frac{1}{2}mr}

\displaystyle v_{esc}=\sqrt{\frac{2GM}{r}}

Substitute in the given values to solve for the velocity.

\displaystyle v_{esc}=\sqrt{\frac{2(6.67*10^{-11}\frac{m^2N}{kg^2})(7.2*10^{22}kg)}{(1.8*10^6m)}}=2310\frac{m}{s}\approx 2.3\frac{km}{s}

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