Friction- Phonebooks (Marschal A. Fazio)

Author

Marschal A. Fazio, University High School, West Los Angeles

Principle(s) Illustrated

1. Static Friction

2. Kinetic Friction

Standards

Grade Eight

Forces

2. Unbalanced forces cause changes in velocity. As a basis for understanding this concept:

a. Students know a force has both direction and magnitude.

b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

c. Students know when the forces on an object are balanced, the motion of the object does not change.

d.Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or com­ pression in matter, and friction.

Grade 9-12

Motion and Forces

1.Newton’s laws predict the motion of most objects. As a basis for understanding this concept:

a. Students know how to solve problems that involve constant speed and average speed.

b. Students know that when forces are balanced, no acceleration occurs; thus an object continues to move at a constant speed or stays at rest (Newton’s first law).

c. Students know how to apply the law F=ma to solve one-dimensional motion problems that involve constant forces (Newton’s second law).

d. Students know that when one object exerts a force on a second object, the second object always exerts a force of equal magnitude and in the opposite direction (Newton’s third law).

Engineering Connection

• Engineers must understand how friction affects a wide range of situations as they design consumer products, such as ski bottoms in which friction is a disadvantage and hiking boots where friction provides traction.

Questioning Script

Prior knowledge & experience:

• Friction, Shoes with good traction

• Leafing through a phonebook or other magazine

Root question:

• Could you and your friend be able to pull apart two phonebooks that have been interwoven

Target response:

• Yes, Absolutely. It should not be that hard, let's try it.

Common Misconceptions:

• • Friction is not a force.

  • Passive objects (stationary rope, tabletop) cannot exert a force

  • Students add forces without considering the direction of the forces, i.e., they add absolute values of the forces

  • Two forces in opposite directions will cancel each other no matter what their strengths are

One possible explanation of the event:

Calculating the force needed to separate the phonebooks

If many pages of a book are inter-woven then the force on each page always equals the weight of all the upper pages (order of magnitude m*g). So in this experiment area is increased without decreasing pressure.

We can calculate the force needed to separate two books using the formula

for friction force:

F=mu*Fn (mu is coefficient of friction, Fn is normal force between surfaces)

Average force Fn acting on a page is:

Fn=m*g/2 (half of book's weight)

Since there are N pages, the maximum friction force is:

F=N*mu*m*g/2

If m=1 kg, N=1000 and k=1 then:

F=5000 N

Photographs and Movies