Friction

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Which Experiences More Friction?

Left or right. Which experiences the most friction? Why?

School Blocks YouTube? Download the file below

Kittens_SlideTreadmill_.wmv

PBS Friction Video

D) Friction — force between 2 surfaces that opposes motion Fill In the Blank Notes

Caveman rubbing sticks together to start a fire

Friction Depends Only On:

1. Texture of Each Surface

What do plows have on their wheels to increase friction?

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Chains

2. Force Holding Objects Together

What do race cars have mounted on their rear hood to increase their friction?

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Rear-engine dragster with a rear spoiler

Spoilers increase the force between tire and road

Why do some people put barbells in their car trunk on a snowy day?

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Weights in a car truck
increase the tires force on the road

Two Kinds of Friction

(f_s) 1) Static Friction — force of friction that must be overcome to move an object at rest.

(f_k) 2) Kinetic Friction — force of friction that opposes a moving object.

Kinetic friction is weaker than static friction

f_k < f_s

Friction Equations

Static friction: f_s = u_sF_n

Kinetic friction: f_k = u_kF_n

f_s = force of static friction

f_k = force of kinetic friction

F_n = normal force (supporting force)

Box on a ramp diagram showing the normal force

u_s = coefficient of static friction — “the roughness number”

u_k = coefficient of kinetic friction — “the roughness number”

1. Relates to texture of surfaces in contact

2. Calculated in lab

Table of kinetic and static friction coefficients for waxed ski on snow, wood on wood, steel on steel, and copper on steel

ice and skate - low uk

What do skiers do to their skis to lower the coefficient of friction between their skis and the snow?

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Skiers put wax on the bottom
of their skis to lower the coefficient of friction

Try it

What is the direction of the friction at P?

Check Your Understanding

5 quick questions — pick an answer to see if you’re right.

1. What is friction?

2. According to this lesson, friction depends only on…

3. How do static friction and kinetic friction compare for the same two surfaces?

4. A box is pressed against a floor with a normal force of 50 N. The coefficient of kinetic friction is 0.30. What is the kinetic friction force?

5. Why do some drivers put heavy weights in the car trunk on a snowy day?

Friction Relationships →