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

Ff = μFn Ff = Force of Friction in N μ = Coefficient of Friction (depends on material 0 < μ < 1 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

TYPES OF FRICTION Kinetic 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

Frictional Force Resisting Motion Fs ≥ Fk Fs Max Fk Frictional Force Resisting Motion Static Region Kinetic Region The Force of Static Friction is greater than the Force of Kinetic friction. The implication is clear: It is harder to start an object moving, than it is to keep it moving. Put another way, it takes more force to start an object moving than to keep that same object moving given that all other variables (materials in contact) remain the same. Force Causing the Object to Move

CHARACTERISTICS OF FRICTION

Frictional Forces Occur When Materials are in Contact Fn Fa Fs M1 Surfaces in Contact Spring Scale Fw Contact between substances produce frictional forces that work in the opposite direction of the forces causing an object to move. In this example f s (Force of static friction) acts in the opposite direction of the pulling force F applied to the scale and string attached to the pink block. The force required to start the block moving is dependent on the smoothness of the surface, the weight or normal force acting on the block and the coefficient of friction between the surface of the block and the table it is resting on. 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 Fn Fa Fs M Surfaces in Contact Spring Scale Which Way is the pink block being pulled? What direction is the force of Static Friction acting in? Fw The Block M will only move if the Applied Force (Fa) is greater the Force of Static Friction (Fs).

FRICTION converts kinetic energy to heat energy.

Banana peel reduces friction.

INCREASING FRICTION

REDUCING FRICTION

The coefficient of friction (μ) is the ratio of the Applied Force (Fa) over the Normal Force (Fn). Fs M Surfaces in Contact Spring Scale Which Way is the pink block being pulled? What direction is the force of Static Friction acting in? Fw μ = Fa Fn

Determine the amount of force needed to move 12 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 N 90 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

Given the following : μs = .62, μk = .48 Determine the acceleration of a sliding 8.50 kg block if a force of 72 N is applied to it? 8.5 kg v FFr 72 N FFr = μkFN FN = mg FFr = μkmg FFr = (.48)(8.5 kg)(9.8 N/kg) = 39.984 N F = ma <72 N - 39.984 N> = (8.5 kg)a, a = 3.77 = 3.8 m/s2 3.8 m/s/s