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Objective
Student will: Incorporate Force of Friction into
calculationStatic FrictionKinetic Friction
Cornell Notes (1/3)Friction: The force exerted by a surface as an object moves across it or tries to move across it.
Force of Friction = Coefficient of Friction x Normal Force
Force: Newton (N):no units
Normal Force: Newton (N)
Questions1) How is friction
define?
Know how to calculate Normal Force
Cornell Notes (2/3)Two Kinds Of Friction Coefficient
s
Coefficient of static
friction ()
Use if two surfaces are not moving
Coefficient of kinetic friction ()
Use if two surfaces
are moving
Cornell Notes (1/5)
Example: FrictionThe Flash is pulling a car, that weighs 1,200 kg, to a nearby gas station and makes the car accelerate at 10 m/s2. Assume friction and the coefficient of kinetic friction "µk" is 0.9. Compute the Flash’s applied force on the car.
Cornell Notes (2/5)
The Flash is pulling a car, that weighs 1,200 kg, to a nearby gas station and makes the car accelerate at 10 m/s2. Assume friction and the coefficient of kinetic friction ("µ)" is 0.9. Compute the Flash’s applied force on the car.
Given:m = 1200 kg = 10 m/s2
µk = 0.9 = 9.8 m/s2
Unknown: = ?
Steps1) Define
Cornell Notes (3/5)Choose an equation or situation:
Rearrange the equation to isolate the unknown:
2) Plan
F⃗net=m a⃗
F⃗app=m a⃗
F⃗app+(−F⃗ fr )m
= a⃗
F⃗app−μm g⃗
m= a⃗
F⃗ fr=μ F⃗ N⃗FN=m g⃗
F⃗app− μm g⃗= a⃗m
F⃗app= a⃗m+μm g⃗F⃗app+(−μ F⃗ fr )
m=a⃗
Cornell Notes (4/5)Substitute the values into the equation and solve
3) Calculate
F⃗app=22584
F⃗app=22584 N
F⃗app=10 ∙1200+0.9∙1200 ∙9.8
F⃗app=10 ∙1200+0.9∙1200 ∙9.8
F⃗app= a⃗m+μm g⃗
Cornell Notes (1/5)
Example: Coefficients of FrictionWhen Colossus pushes a 2000 kg shipping box resting on
a road, he exerts a force of 1800 N to start it sliding. Once it is sliding, he uses a force of only 1500 N to keep the shipping box moving with constant speed. What are the coefficients of static and kinetic friction between the shipping box and the road?
Cornell Notes (2/5)Given:
m = 2000 kg 1800 N1500 N = 0 N = 0 N
Unknown:
Steps1) Define
When Colossus pushes a 2000 kg shipping box resting on a road, he exerts a force of 1800 N to start it sliding. Once it is sliding, he uses a force of only 1500 N to keep the shipping box moving with constant speed. What are the coefficients of static and kinetic friction between the shipping box and the road?
Cornell Notes (3/5)Choose an equation or situation:
Rearrange the equation to isolate the unknown:
2) Plan
Fn = mg because it is
on a horizontal surface
�⃗� 𝑎𝑝𝑝 . 𝑠− �⃗� 𝑓𝑟 . 𝑠=𝑚�⃗�s�⃗� 𝑎𝑝𝑝 .𝑘−𝐹 𝑓𝑟 .𝑘=𝑚�⃗�𝑘
�⃗� 𝑎𝑝𝑝 . 𝑠− �⃗� 𝑓𝑟 . 𝑠=0�⃗� 𝑎𝑝𝑝 . 𝑠=�⃗� 𝑓𝑟 .𝑠
�⃗� 𝑎𝑝𝑝 . 𝑠=𝑢𝑠 ∙ �⃗�𝑁
�⃗�𝑎𝑝𝑝 . 𝑠
𝑚�⃗�=𝑢𝑠
�⃗� 𝑎𝑝𝑝 . 𝑠=𝑢𝑠 ∙𝑚�⃗�
�⃗� 𝑎𝑝𝑝 .𝑘−𝐹 𝑓𝑟 .𝑘=0�⃗� 𝑎𝑝𝑝 .𝑘=�⃗� 𝑓𝑟 .𝑘
�⃗� 𝑎𝑝𝑝 .𝑘=𝑢𝑘 ∙𝐹 𝑁
�⃗�𝑎𝑝𝑝 .𝑘
𝑚�⃗�=𝑢𝑘
�⃗� 𝑎𝑝𝑝 .𝑘=𝑢𝑘 ∙𝑚�⃗�
�⃗� 𝑎𝑝𝑝 . 𝑠− �⃗� 𝑓𝑟 . 𝑠=𝑚�⃗� s⃗𝐹 𝑎𝑝𝑝 .𝑘−𝐹 𝑓𝑟 .𝑘=𝑚�⃗�𝑘
Cornell Notes (4/5)Substitute the values into the equation and solve
3) Calculate
.091743119266
.092
.076452599388
.076
18002000 ∙9.8
=𝑢𝑠
Coefficient of frictions do not have units
15002000 ∙9.8
=𝑢𝑘
�⃗�𝑎𝑝𝑝 . 𝑠
𝑚�⃗�=𝑢𝑠
�⃗�𝑎𝑝𝑝 .𝑘
𝑚�⃗�=𝑢𝑘