1. Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Sections 807, 808, 809 Lecture 8

2. Quiz (folders)

3. Reading quizzes (Chs 5,6)

4. Hockey Puck Which of these three best represents a hockey puck in the real world? a) b) c)

5. Newton’s 1 st Law A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration

6. Aristotle: a natural state of an object is at rest; a force is necessary to keep an object in motion. It follows from common sense. Galileo: was able to identify a hidden force of friction behind common- sense experiments 1564-1642 384-322 B.C.

7. Galileo: If no force is applied to a moving object, it will continue to move with constant speed in a straight line Galilean principle of relativity: Laws of physics (and everything in the Universe) look the same for all observers who move with a constant velocity with respect to each other. Inertial reference frames

8. Newton’s second law The vector acceleration of an object is in the same direction as the vector force applied to the object and the magnitudes are related by a constant called the mass of the object.

9. A Recipe for Solving Problems 1.Sketch Isolate the body (only external forces but not forces that one part of the object exert on another part) 2. Write down 2 nd Newton’s law Choose a coordinate system Write 2 nd Newton’s law in component form: 3. Solve for acceleration

10. Is it better to push or pull a sled? You can pull or push a sled with the same force magnitude, F P, and angle , as shown in the figures. Assuming the sled doesn’t leave the ground and has a constant coefficient of friction, , which is better? FPFP FPFP

11. Kinetic Friction For kinetic friction, it turns out that the larger the Normal Force the larger the friction. We can write F Friction =  Kinetic N Here  is a constant Warning: –THIS IS NOT A VECTOR EQUATION!

12. Static Friction This is more complicated For static friction, the friction force can vary F Friction   Static N Example of the refrigerator: –If I don’t push, what is the static friction force? –What if I push a little?

13. H Coefficient of friction:  What is the normal force? What is the velocity of the block when it reaches the bottom?

14. Newton’s 3 rd Law For every action there is an equal, but opposite, reaction

15. Skater Skater pushes on a wall The wall pushes back –Equal and opposite force The push from the wall is a force –Force provides an acceleration –She flies off with some non-zero speed

16. P m1m1 m2m2 No friction m1m1 Free body diagram m1m1 m1gm1g P N1N1 F 21 m2gm2g F 12 N2N2 m2m2 F 12 =F 21

17. Quiz a) A crate of mass m is on the flat bed of a pick up truck. The coefficient of friction between the crate and the truck is . The truck is traveling at the constant velocity of magnitude V 1. Draw the free body diagram for the crate. b) The truck starts to accelerate with an acceleration a c. Draw the free body diagram for the crate, if the crate does not slip.

18. A small block, mass 2kg, rests on top of a larger block, mass 20 kg. The coefficient of friction between the blocks is 0.25. If the larger block is on a frictionless table, what is the largest horizontal force that can be applied to it without the small block slipping?

20. Have a great day! Reading: Chapter 5,6 Hw: Chapter 6 problems and exercises