1. Tuesday Quiz What’s the difference between mass and weight? Find weight of a) 23-kg dog; b) 35-g coin c) 18-lb chicken Find mass of a) 15-lb turkeyb) 120-N fat cat How would your mass on Jupiter, where the grav. field is much stronger than on Earth, compare to your mass on Earth? How would your weight compare to your weight on Earth? What can you say about the motion of an object that is in equilibrium? What can you say about the forces on it? Draw FBD’s!

2. Vocabulary friction fA force between 2 objects that acts parallel to the surface of contact, in a direction that opposes the motion. Static friction f S (starting friction); friction betw. 2 objects that are stationary relative to each other Kinetic friction f K (sliding friction); friction between 2 objects that are sliding against each other

3. Just Consider—don’t copy The slope of the graph of kinetic friction vs. normal force between 2 objects sliding against each other is called the Coefficient of Kinetic Friction, defined as: Therefore, f k =  k  N Similarly, the slope of the graph of maximum frictional force vs. normal force between 2 objects that are stationary relative to each other is called the Coefficient of Static Friction: Therefore, f s,max =  S  N k k  s s 

4. Vocabulary friction fA force between 2 objects that acts parallel to the surface of contact, in a direction that opposes the motion. Static friction f S (starting friction); friction betw. 2 objects that are stationary relative to each other Kinetic friction f K (sliding friction); friction between 2 objects that are sliding against each other Coeff. of friction (  ) Ratio of friction force to N.  k   s 