Practice Test: Mr Rogers AP Physics C -- Friction

Unit Plan Practice Test Study Guide

Online Quiz - Friction                                                                  8B66-D878


AP Physics Friction Practice Test

1 ) A block of mass = 2M is stacked atop a block of mass = M. The sliding COF between the blocks = 0.8 and between the lower block and floor is 0.1. A force = F is applied to the top block parallel to the ground. Find the acceleration of the lower block.  a = 1.3g
2 ) Find the acceleration of a box of mangos sliding down a slope of 40 degrees. Assume the dynamic COF = .2 and the static COF = .3. Would doubling the number of mangos in the box have any effect on the acceleration? No, a = g ( sinβ - μd cosβ ).  Note that mass is not in the equation.
3 )
If the above box of mangos rests on a 20 degree slope, will it slide?
qcr = tan-1(0.3) or 16.7 degrees. It will slide.
4 ) Find the static COF if the box of mangos starts to slide if the angle of the slope =  30 degrees. static COF = tan 30 or 0.577
5 ) The tires on Martha's 4WD SUV  have a static COF = 0.7 and a dynamic COF = 0.5 with the road surface. What is her maximum acceleration and minimum stopping distance given a starting velocity of 40 m/s? amax = 0.7g (Note that the maximum acceleration comes from the max friction force, which is the max static friction. With anti-lock brakes the wheels do not lock up and slide. They roll slower and slower as the vehicle stops, at which time they stop rolling.), Min stopping distance = 114 m
6 ) If Martha's SUV is only 2WD, what is the effect on her maximum acceleration (speeding up) and minimum stopping distance (slowing down)? Assume the COF's the same for all wheels and that the car's mass is evenly distributed (half over the front tires and half over the back tires). Her maximum acceleration (speeding up) is cut in half  and her minimum stopping distance doubled. Note that in reality there is a separate friction force acting on each wheel. If the weight of the vehicle is evenly distributed, each driven wheel will have a normal force of 1/4 mg making its static friction force = 1/4 μsmg.

4WD total friction = 1/4 μsmg +1/4 μsmg+1/4 μsmg)+1/4 μsmg

                             = μsmg

2WD total friction = 1/4 μsmg +1/4 μsmg

                             = 1/2 μsmg

Using kinematics analysis with amax based on the maximum static friction:

( min stopping dist ) = vo 2 / (2amax)

Obviously, cutting the stopping acceleration in half doubles the stopping distance.


7 ) Juan pushes downward on a box of mass = M with a force = 2Mg that makes a 45 degree angle with the horizon. By contrast Susan pulls upward on a similar box with a similar force and angle. Both boxes sit on a surface with a sliding COF = 0.2 and a static COF = 0.3. Find the acceleration of both boxes.
Juan Susan

Fn = Mg + 2Mg sin 45

      = Mg( 1 + 2 sin 45 )

Fd = μd Fn

    = μd Mg( 1 + 2 sin 45 )  


All of the acceleration is in the horizontal dimension such that:

a = Σ F / m

   = ( Fx - Fd ) / M

   = [2Mg cos 45 - μdMg( 1 + 2 sin 45 ) ]  / M

   = 2g cos 45 - μdg( 1 + 2 sin 45 )

a   = g ( 2cos 45 - μd - 2μd sin 45

Fn = Mg - 2Mg sin 45

      = Mg( 1 - 2 sin 45 )

Fd = μd Fn

     = μd Mg( 1 - 2 sin 45 )  


All of the acceleration is in the horizontal dimension such that:

a = Σ F / m

   = ( Fx - Fd ) / M

   = [2Mg cos 45 - μdMg( 1 - 2 sin 45 ) ] / M

   = 2g cos 45 - μdg( 1 - 2 sin 45 )

a   = g ( 2cos 45 - μd + 2μd sin 45 )

SAM Team--Southside High School's STEM and Computer Science extra-curricular club (Mr. Rogers Sponsor)

Mr. Rogers' Twitter Site

Mr. Rogers Teacher's Blog

Mr. Rogers T-shirts

Mr. Rogers Information for Teachers

Mr. Rogers Science Fair Information

Check out other web sites created by Mr. R:

Check out Articles by Mr. Rogers:

Insultingly Stupid Movie Physics is one of the most humorous, entertaining, and readable physics books available, yet is filled with all kinds of useful content and clear explanations for high school, 1st semester college physics students, and film buffs.

It explains all 3 of Newton's laws, the 1st and 2nd laws of thermodynamics, momentum, energy, gravity, circular motion and a host of other topics all through the lens of Hollywood movies using Star Trek and numerous other films.

If you want to learn how to think physics and have a lot of fun in the process, this is the book for you!


First the web site,

now the book!

Mr. Rogers Home | Common Sylabus | AP Comp Sci I | AP Comp Sci II | AP Physics Mech | AP Physics E&M | AP Statistics | Honors Physics|IB Design Tech | Southside

[ Intuitor Home | Physics | Movie Physics | Chess | Forchess | Hex | Intuitor Store |

Copyright 1996-2011 T. K. Rogers, all rights reserved. Forchess is a registered trademark of T. K. Rogers.
No part of this website may be reproduced in any form, electronic or otherwise, without express written approval.