Friction and Normal Force

I. Normal Force The Force that opposes gravity. A 2 kg book is resting on a table. FN The Net Force is zero. Fg What is the Normal Force? Fg = FN

B. According to Newton’s 3rd Law For every action, there is an equal and opposite reaction. The action is Gravity. The reaction is called the Normal Force. Lucky for us, The two are equal on a flat surface.

C. When are they not equal Normal Force F of gravity

II. Friction Force that opposes motion. Whenever two surfaces are in contact. Always opposite direction of motion Depends on surface type and Normal force ONLY. NOT SURFACE AREA!

B. Kinetic Friction Friction on a moving object. Ff = k FN k = coefficient of kinetic friction. It is a constant that depends on surface type You look it up on a chart.

C. Static Friction Friction on a stationary object. Ff = s FN s is the coefficient of static friction. It is always higher than kinetic friction.

EXAMPLE You push a 25 kg box across the floor. It is travelling at a constant speed of 1.0 m/s. If the coefficient of kinetic friction is .20, what force are you pushing with? F net = Fpush - Ff F push F f