1. What is Friction? How does it work?
4/18/2008
What is Friction? How does it work?
Lecture 8

2. 4/18/2008
Friction is a Force
Force on box
by person
Force on person
by box
Force of friction on floor by box
Force of friction on box
by floor
When you push on a box, it pushes back. You need to push harder than the force of friction between the box and the floor to move it!
Lecture 8

3. 4/18/2008
Friction Mechanism
Corrugations in the surfaces grind when things slide.
Lubricants fill in the gaps and let things slide more easily.
Lecture 8

4. What Causes Friction?
Friction is the force that opposes the motion between two surfaces that touch.
The surface of any object is rough.
Even an object that feels smooth is covered with tiny hills and valleys.
The contact between the hills of valleys of two surfaces causes them to stick, resulting in friction.

5. What affects friction? The amount of friction depends on:
Roughness of the surfaces
Force pushing the surfaces together

6. Types of Friction
Kinetic friction occurs when force is applied to an object and the object moves.
Examples:
Sliding Friction: pushing an object across a surface
Rolling Friction: between wheels and a surface
Fluid Friction: opposes the motion of objects traveling through a fluid (air or water)

7. Types of Friction
Static friction occurs when force applied to an object does not cause the object to move.

8. Affecting Friction
To reduce the amount of friction, apply a lubricant between two surfaces.
Motor oil, wax, and grease are examples.
Friction can also be reduced by rolling, rather than pushing, an object.

9. 4/18/2008
What Causes friction?
Even though a surface may look smooth, if you magnify the surface it isn’t smooth.
Everything is made up of molecules. The molecules come together to make bumps. Some surfaces have molecules that leave large bumps and some leave smaller bumps, but all surfaces have bumps.
Microwelds occur when two bumpy surfaces are rubbed up against each other and they stick together
Lecture 8

10. 4/18/2008
Sticking Together
The greater the force on the object, the greater the force of the microwelds and the greater force is needed to overcome the microwelds to move the object
Lecture 8

11. Static and Sliding (Dynamic) Friction
4/18/2008
Static and Sliding (Dynamic) Friction
Static frictional force: when nothing is sliding
Sliding frictional force: when surfaces are sliding
Static frictional forces always greater than sliding ones
Lubrication provides microscopic rollers between surfaces
Lecture 8

12. 4/18/2008
Big Consequences!
(USGS)
Lecture 8

13. Calculating Friction
4/18/2008
Remember: The rougher the surface, the greater the force of friction
The more force pressing the two surfaces together, the greater the force of friction
The force of friction
Ff = (a measurement of surface roughness)*(how heavy the object is)
Ff = m*Weight, where m is called the coefficient of friction
(USGS)
Lecture 8

14. Coefficient of Friction
4/18/2008
Coefficient of Friction
Material on Material
s = static friction
k = kinetic friction
steel / steel
0.6
0.4
add grease to steel
0.1
0.05
metal / ice
0.022
0.02
brake lining / iron
0.3
tire / dry pavement
0.9
0.8
tire / wet pavement
0.7
Lecture 8

15. 4/18/2008
Demo static vs. kinetic
Which force is bigger?
Lecture 8

16. Trajectories with Air Resistance
4/18/2008
Trajectories with Air Resistance
Baseball launched at 45° with v = 50 m/s:
Without air resistance, reaches about 63 m high, 254 m range
With air resistance, about 31 m high, 122 m range
Vacuum trajectory vs. air trajectory for 45° launch angle.
Lecture 8

17. 4/18/2008
Air Trajectories, cont.
Now v = 40 m/s, optimal angle (45° for vacuum, 40° for air):
Optimal angle for 40 m/s in air gets 97 m range vs. 164 m for vacuum trajectory.
Golf balls actually have optimal launch of 25-30°, helped by lift
forces associated with spin of the ball.
Lecture 8