Friction A force that opposes motion

Causes Of Friction Matter is not as smooth on its surface as it appears to be; tiny irregularities get “stuck” on one another On very smooth surfaces, the atoms can become so close that there are electrical forces acting between them Sliding objects often experience “jerky” motion as bonds or “jags” are broken

Types of Friction Kinetic Friction – friction of moving objects Amount of kinetic friction depends on - nature of surfaces (type of material) - normal force pushing surfaces together Does not depend on the amount of surface area

Proportionality F f = μ k F N μ k is called the coefficient of kinetic friction μ k depends on the nature of the two surfaces in contact More friction, higher μ k

Example What frictional force will a.75kg waxed ski experience as it slides on wet snow? Draw a free body diagram

Static Friction Friction that must be overcome to start motion It is higher than kinetic friction μ s = F f / F N

Example A 10.0 kg box rests on the floor. The coefficient of static friction is.40 and kinetic friction is.30. Find the force of friction acting on the box if it is pushed by a) 0 N b) 10 N c) 20 N d) 38 N e) 40 N

Question Your little sister wants a ride on her sled. Will you use more force if you push or pull her? Assume the same angle in each case.