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AP Physics C: Mechanics : Friction and Normal Force

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

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2 Diagnostic Tests 92 Practice Tests Question of the Day Flashcards Learn by Concept

Example Questions

Example Question #1 : Friction

A truck driver is making a delivery. He takes a 287kg box out of his semi-truck by having it slide down a 33^o ramp that is 5.5m long. To ensure that the box doesn't accelerate down the ramp, the driver pushes back on it so that the box's constant speed is $1.7\frac{m}{s}$ . If the coefficient of friction is 0.4, calculate the work done by the frictional force.

Possible Answers:

9675 J

5189 J

6548 J

1049 J

3987 J

Correct answer:

5189 J

Explanation:

We can calculate the work done by friction by using $W=F_{friction}\cdot d$ .

It is just the product of the friction force and the length of the ramp. Rewrite work as $W=\mu_kF_Nd$ .

Since this problem involves a box sliding down an incline, the normal force is $F_N=mg\cos\theta$ .

Work is now written as the equation below.

$W=\mu_kmg(\cos\theta) d$

We know the following information from the question.

$\mu_k=0.4$

m=287kg

$g=9.8\frac{m}{s^2}$

$\theta=33^{o}$

d=5.5m

using these values, we can calculate work from our equation.

$W=(0.4)(287kg)(9.8\frac{m}{s^2})(\cos(33))(5.5)$

W=5189 J

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Example Question #2 : Friction

A string connected to a box sitting on the floor is used to apply a
force on the box as shown. The string is inclined at an angle of 30°
above the horizontal, the box has a mass of 15 kg, and the tension
in the string is 40 N. The box is at rest, but on the verge of slipping.
What is the coefficient of static friction between the box and the
floor?

Possible Answers:

0.338

0.154

0.212

0.273

0.479

Correct answer:

0.273

Explanation:

First, we know that the tension on the string is $40\:N$ at an angle of degrees from the horizontal. We must find the x and y components of the tension force.

$T_y = T\sin(30) = 40\cdot\sin(30)= 20\:N$

$T_x = T\cos(30) = 40\cos(30)= 34.64\:N$

Next, we must find the normal force acting on the block. The normal force of the block itself is $15\:kg \cdot 9.8\:m/s^2 = 147\:N$ , but there is a string pulling on it. We must subtract the y component of tension to get the final normal force

F_n = 147-20 = 127

Next, we know that the block is being pulled to the right, and the frictional force is acting in the opposite direction. So we get the equation:

$T_x - F_f = 0, F_f = \mu F_n$

$34.64 - \mu 127 = 0$

$\mu = 0.273$

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Example Question #1 : Friction

$\begin{align*}&\text{A block of mass }51kg\text{ rests on an incline of }11^{\circ}\\&\text{What is the frictional force acting on the block from the surface?}\\&g=9.81\frac{m}{s^2}\end{align*}$

Possible Answers:

491.12N

95.46N

50.06N

500.31N

Correct answer:

95.46N

Explanation:

Normal

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Example Question #12 : Forces

$\begin{align*}&\text{A block of mass }100kg\text{ rests on an incline of }73^{\circ}\\&\text{What is the frictional force acting on the block from the surface?}\\&g=9.81\frac{m}{s^2}\end{align*}$

Possible Answers:

938.13N

286.82N

981N

29.24N

Correct answer:

938.13N

Explanation:

Normal $\begin{align*}&\text{To solve a problem of force balance, a good idea is to draw}\\&\text{it out. Since the block is at rest, i.e. stationary, the sum}\\&\text{of forces acting on it must be equal to zero. The forces acting}\\&\text{on it are gravity (in the form of the mass times the gravitational}\\&\text{constant), a normal force, and since it’s on an incline, a frictional}\\&\text{force to balance it all out:}\\&F_{mass}=m*g\\&F_{normal}=m*g*cos(a)\\&F_{friction}=m*g*sin(a)\\&F_{friction}=100kg(9.81\frac{m}{s^2})(sin(73^{\circ}))\\&F_{friction}=938.13N\end{align*}$

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Example Question #2 : Friction

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

Possible Answers:

186.39N

174.01N

66.8N

6.81N

Correct answer:

174.01N

Explanation:

Normal

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Example Question #91 : Mechanics Exam

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

Possible Answers:

2507.44N

2070.43N

1864.22N

2786.04N

Correct answer:

2507.44N

Explanation:

Normalwrope

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

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Example Question #4 : Friction

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

Possible Answers:

918.66N

111.35N

275.6N

371.16N

Correct answer:

275.6N

Explanation:

Normalwrope

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

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Example Question #11 : Forces

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

Possible Answers:

63.96N

627.43N

1723.84N

1834.47N

Correct answer:

1723.84N

Explanation:

Normal

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Example Question #93 : Mechanics Exam

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

Possible Answers:

2040.48N

957.95N

1801.64N

183.65N

Correct answer:

957.95N

Explanation:

Normal

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AP Physics C: Mechanics : Friction and Normal Force

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

All AP Physics C: Mechanics Resources

Example Questions

Example Question #1 : Friction

Example Question #2 : Friction

Example Question #1 : Friction

Example Question #12 : Forces

Example Question #2 : Friction

Example Question #91 : Mechanics Exam

Example Question #4 : Friction

Example Question #11 : Forces

Example Question #93 : Mechanics Exam

All AP Physics C: Mechanics Resources

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