Transcript Friction - Mr Simnett

Friction
A World Without Friction…
• https://www.youtube.com/
watch?v=VUfqjSeeZng
Friction
• A force that resists motion
• Occurs due to the attraction between
particles
▫ To walk you have to break the attraction
between the particles in your foot and the
particles in the floor
▫ To continue sliding you have to resist the
particles in your foot being attracted to the
particles in the floor
Two Types of Friction
• Kinetic friction
▫ Occurs when the object is in motion
▫ Acts against motion
• Static friction
▫ Occurs when the object is stationary
▫ Acts to resist being put in motion
Which is bigger?
Static or Kinetic Friction
• Think of Newton’s first law!
• Force to get something moving (static) is greater
than the force to keep something moving
(kinetic)
• The object at rest wants to stay at rest so the
static force is greater because the object in
motion wants to stay in motion
Special Value of Static Friction
Limiting Static Friction
• the amount of force that must be overcome to
start a stationary object moving
• The object will not move unless you apply more
than this amount of force
The Difference!
**** (Hilarious)- www.youtube.com/watch?v=JS4ODWG7Els
*** (Bill Nye) - www.youtube.com/watch?v=-pZbM17-zzk
** (Guy talking friction)www.youtube.com/watch?v=CTLXubXOTUQ
* - https://www.youtube.com/watch?v=L3if3_pOVl0
How do we measure friction?
• Weigh an object with a force scale
• Pull an object with a force scale
• Divide the two values
• We call the result the coefficient of friction
• Let’s give it a try! Materials: Mass, Newton Scale
▫ Measure:
 The force needed to start the motion
 The force needed to keep the motion constant
 Weight (Fg) of the mass
What surface is your desk?
• http://www.engineershandbook.com/Tables/fri
ctioncoefficients.htm
• Masses are steel!
Coefficient of Friction ()
• Ratio of the magnitude of the frictional
force to the normal force
• In other words… Divide how much friction
there is by the force between the two
surface
• Depends on the two surfaces in contact
The Math
s 
F fr s
F frs   s FN
FN
k 
F fr k
F frk   k FN
FN
- Coefficient of Friction (no
units)
Ffr – Force of Friction (N)
FN – Normal Force (N)
Example # 1
The coefficient of static friction between glass and
glass is 0.9. Find the minimum force required to
start an 8.0 kg mass moving.
Example # 2
• An 8.0kg teflon pot is on a steel table. If it takes
9.4N of force to keep the pot moving at a
constant speed. What is the coefficient of
friction between the pot and table?
Example # 3
A textbook has a mass of 1810 g. A force of 12 N is needed to
just start the book moving across the table.
a.) Calculate the coefficient of static friction
b.) If a second identical book is placed on top of the first
book when it is at rest, what horizontal force would be
needed to just start the book sliding?
FN = Fg = ma
= (1.810 kg)(9.81 m/s2)
= N
µS = F S
FN
= 12 N
17.8 N
= 0.67
FN = (1.810 X 2 kg)(9.81 m/s2)
= 35.5 N
Fs = µ S FN
= (0.67)(35.5 N)
= 24N
Need help?
Khan Academy
https://www.youtube.com/watch?v=ZA_D4O6l1lo
Controlling Friction
• How can we reduce friction?
• Why do we need to reduce friction?
Keep in Mind!
• Friction wastes Energy
• Turns useful energy into heat
Controlling Friction
What is used to increase friction?
Why do we need to increase friction?
Industrial Applications
• Inertial Friction Welding
▫ https://www.youtube.com/watch?v=5JbnDXw-0pM
• Creating a Friction Fire
▫ http://www.youtube.com/watch?v=VOmwLxgI06c