1. Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Sections 818, 819, 820, 821 Lecture 11

2. Quiz (folders) A rocket is fired from a tower of height H, at t=0, with an initial velocity of magnitude W at an angle Θ above horizontal. The rocket’s engine supplies an acceleration that increases with time according to a(t)=αt 2. The acceleration is always directed at an angle β with the horizontal. The rocket’s fuel runs out after 2 sec. What will its velocity and position be when the fuel runs out? Consider α, β, Θ, H, and W as known constants and neglect gravity.

3. 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.

4. 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!

5. 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

6.  Pulling Against Friction A box of mass m is on a surface with coefficient of kinetic friction . You pull with constant force F P at angle  The box does not leave the surface and moves to the right. 1.What is the magnitude of the acceleration? 2.What angle maximizes the acceleration?

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

8. First exam Tuesday, September 22 102 CHEN Sections 818 and 819 start at 7 pm Sections 820 and 821 start at 8:05 pm

9. All liquids, gels and aerosols must be in three-ounce or smaller containers. No calculators and cell phones BRING YOUR STUDENT ID!

10. What is included? Chapters 1-6 Kinematics, vectors, Newton’s 2 nd Law

13. Kinematics

14. In components:

15. If is given, you can find and

16. Vectors

17. Newton’s 2nd Law

18. Friendly advice DO NOT…

19. DO NOT use Const acceleration case formulas when acceleration is a function of time. You have to integrate or differentiate! What are the Const acceleration case formulas?

20. DO NOT write the vector sign over a projection!

21.  DO NOT forget that F p has two components: F p cos θ and F p sin θ

22. Do Not Forget to Write down what is given and express the answer in terms of what is given Box the answer Indicate the origin and positive x and y

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