Net Forces, Friction, Coefficients of Friction

1. Net Forces, Friction, Coefficients of Friction

2. Sliding Book Example • Why do things not continue to move at constant velocity? • If the sliding book slows down, what’s the force responsible? • How could I keep it moving at a constant velocity? Friction Friction Remove Friction or continue to add force equal to friction Create a net force of zero

3. The three types of friction • Surface or Sliding Friction – Created when an object slides or moves over a second object while their two surfaces are touching. • Fluid Friction- Created when an object slides or moves over a second object that is a fluid (Gas or liquid). This reduces the amount of Peaks & Valleys that will interact while their two surfaces are touching. • Rolling Friction - Created when an object rolls over a second object. The circular motion constantly refreshes the two surfaces that are coming into contact.

4. Forceon person by box Force on box by floor Force on box by person Force on floor by box Friction opposes MotionSurface Friction Acceleration What kind of motion is created by an Unbalanced Force? It’s thesum of all the forces that determines the type of motion.

5. Friction due to the Surface How does the corrugated surface change Friction? Corrugations in the surfaces grind when things slide. Lubricants fill in the gaps and let things slide more easily.

6. Force of Floor acting on Box Force of Earth acting on Box (weight) Why Doesn’t Gravity Make the Box Fall? Force from floor on box and gravity = Net Force of zero. If the floor vanished, the box would fall, Net force  zero

7. Forceon person by box Force on box by floor Force on box by person Force on floor by box What’s Forces are not shown? The Normal Force and the Force of Gravity

8. When we drew the box and floor, with the “normal” force and the force of gravity, these weren’t strictly force pairs • forces on the box that result in a Net zero acceleration of the box • The real pairs have to involve the earth: box floor • Force Pairs: • earth-box (grav) • box-floor (contact) • earth-satellite (grav) satellite earth

9. Questions NO Friction vector is opposite in direction of motion • Does friction always exert a force? • What does this say about the direction of the frictional force, relative to the velocity vector? • What do you think would happen if we loaded lead bricks into the box? Would it become harder to slide? • What are some ways to reduce frictional forces? Yes Lubrication, change surface, reduce normal force

10. Static and Kinetic Friction Static frictional force: when two surfaces are touching and nothing is sliding, No motion (ƒs) Kinetic frictional force: when two surfaces are touching and at least one of the surfaces are sliding or moving over the other. (ƒk) Static frictional forces are always greater than Kinetic

11. Friction is ƒ=µ N Friction = (Coefficient of Friction) (Normal Force)

12. Reaction Force From Ramp Normal Force Friction Force Decompose Vector Weight of block Weight of block “Normal” Forces and Frictional Forces “Normal” means perpendicular Friction Force = Normal Force  (coefficient of friction) Ffriction = Fnormal

13. Coefficient of Friction

14. Stay on the road! • What does it take to stay on the road around a curve? • using s = 0.8 as average for tires on road, Ffriction = 0.8mg • (Normal force is just mg on level surface) Fcurve = macurve = mv2/r • where r is radius of curve, say 50 m (e.g., cloverleaf exit ramp) • Got enough friction if Fcurve < Ffriction • happens if v2 < 0.8gr, or v < 20 m/s = 44 m.p.h.