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FRICTION. FRICTION - the force that present whenever two surfaces are in contact and always acts opposite to the direction of motion. Depends on : Type of materials in contact Surfaces of materials. Does NOT depend on: Surface area in contact Speed. F f = μ F n. - PowerPoint PPT Presentation
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FRICTION - the force that present whenever two surfaces are in contact and always acts opposite to the direction of motion.
Depends on:• Type of materials in contact • Surfaces of materials
Does NOT depend on:•Surface area in contact•Speed
Ff = μFn
Fn = Normal Force; Force acting perpendicular to the two surfaces in contact. In level surfaces, this is equal in magnitude but acting opposite to the weight
Ff = Force of Friction in N
μ = Coefficient of Friction (depends on material 0 < μ < 1
TYPES OF FRICTIONTYPES OF FRICTIONKinetic Friction - Force needed to keep it going at a constant velocity.
Ff = μ kFn
Static Friction - Force needed to start motion.Ff < μ sFn
Keeps the object at rest.Only relevant when object is stationary.Calculated value is a maximum
Fri
ctio
nal
For
ce
Res
isti
ng
Mot
ion
Force Causing the Object to Move
Kinetic RegionStatic Region
Max
kFF s
Fs ≥ Fk
Fk
Fs
Frictional Forces Occur When Materials are in Contact
Fw
FsFa
Fn
Surfaces in Contact
M1
Spring Scale
Fa = Force Causing Motion (Pull on Scale)
Fs = Force of Static Friction (Resists Motion)Fn = Normal Force (Perpendicular to Surfaces)Fw = Weight of Object ( Mass x Gravity)
Friction is a Force That Always Resists Motion
Surfaces in Contact
The Block M will only move if the Applied Force (Fa) is greater the Force of Static Friction (Fs).
M
Spring Scale
Fn
FsFa
Fw
The coefficient of friction (μ) is the ratio of the Applied Force (Fa) over the Normal Force (Fn).
Surfaces in Contact
M
Spring Scale
Fn
FsFa
Fwμ = Fa
Fn
Determine the amount of force needed to move 12.0 kg of rubber at a constant velocity across dry concrete?
Solution:F = ma FFr = μkFN
FN = weight = mg = (12 kg)(9.8 N/kg) = 117.6 N
FFr = μkFN = (0.8)(117.6 N) = 94.08 N = 90 N90 N
What is the force needed to slide a stationary 150 kg rubber block across wet concrete?
SOLUTION:F = ma FFr < μsFN
FN = weight = mg = (150 kg)(9.8 N/kg) = 1470 N
FFr = μkFN = (.7)(1470) = 1029 N = 1000 N
1000 N