1. BELLWORK 12/02/15 If you were to travel to the moon, –A. Would your mass or weight change? Why? –B. Would it be less or more on the moon? –F g is also defined as ___________ and the formula for calculating weight (F g ) is _____________?

2. Force of Friction

3. Friction Any force that resists motion It involves objects that are in contact with each other. Some scientists believe that friction is caused by uneven surfaces of the touching objects – when rubbed together resistance is offered. Experiments have shown that tiny particles are actually torn from one surface and imbedded in the other.

4. There are two types of friction:  Static Friction (F s ) – the frictional force present just before motion begins. Starting motion is when Static Friction is at is maximum.  Kinetic Friction (F k ) – the frictional force present with motion  Note: Static Friction is usually higher than Kinetic Friction

5. Friction depends on the nature of the material in contact and the smoothness of their surfaces. Static Friction is usually higher than Kinetic Friction because it is harder to get something started than it is to keep it going.

6. Free Body Diagram Normal Force F N Friction Force f f Applied Force F Gravity Force F g F g (weight) = mg F N = F g F f = uF N

7. Static Friction FNFN fsfs F app FgFg The Force of Static Friction keeps a stationary object at rest!

8. Kinetic Friction FNFN fkfk F app FgFg Once the Force of Static Friction is overcome, the Force of Kinetic Friction is what slows down a moving object! Motion

9. Types of Friction I better be safe Ump!! To initiate motion of the box the man must overcome the Force of Static Friction Upon sliding, the baseball player will come to a complete stop due to the Force of Kinetic Friction

10. CoefCoefficients of Frictiontrs Coefficient of Friction- quantity that expresses the dependence of frictional forces on the particular surfaces in contact. Represented by the symbol u (mu) Ratio of forces. Ex: u k is the ratio of F k to F n

11. Ta MaterialCoefficient of Static Friction  S Coefficient of Kinetic Friction  k Rubber on Glass2.0 Rubber on Concrete1.00.8 Steel on Steel0.740.57 Wood on Wood0.25 – 0.50.2 Metal on Metal0.150.06 Ice on Ice0.10.03 Synovial Joints in Humans 0.010.003

12. Copy all of the Formulas from Chapter 4 from Page 855 on your index card. Sub-title these formulas: “Laws of Motion”