FRICTION
Static Friction (Fs) =
Force exerted on a stationery object by a surface that prevents the object from
starting to move. Fa < Fs
If the applied force becomes large enough, the body will start to move (Fa > Fs)
Kinetic Friction (Fk) = Force exerted on a moving object by a surface, acting opposite the of motion of object
𝐹
Coefficient of friction μ = 𝐹 𝑓
𝑭𝒇 = μ𝑭𝑵
𝑁
𝐹
Coefficient of kinetic friction 𝜇𝑘 = 𝐹 𝑘
𝑁
Coefficient of static friction 𝜇𝑠 =
𝐹𝑠𝑚𝑎𝑥
𝐹𝑁
𝑭𝒌 = 𝝁 𝒌 𝑭𝑵
𝑭𝒔𝒎𝒂𝒙 = 𝝁𝒔 𝑭𝑵
>>>>>>>All depend on type of surface
Example:
A 3.2 kg block of wood requires a largest force of 15 N before it will start moving on a horizontal wooden floor.
Once it starts moving, it only takes 9 N to keep it moving at constant velocity.
a) Calculate the coefficient of static friction
b) Calculate the force of friction acting on the block if a horizontal force of 7 N [E] acts on the block
c) Calculate maximum magnitude of static friction acting on the block if a 2.1 kg object is placed on top of it
d) Determine the coefficient of kinetic friction
Practice:
P171 # 1, 2
P172 # 2, 3, 5, 6, 7