1. Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Sections 801, 802, 803 Lecture 7

2. Overview of Today’s Class Hw Quiz Vectors

3. A rocket, initially at rest, is fired at t=0 vertically down from a building of height H. The rocket’s acceleration, including the effects of gravity, is downwards with increasing magnitude given by a(t)=  t, where  is a known constant. When does it hit the ground and how fast is it going when it hits?

4. A ball is thrown vertically upward with a velocity of magnitude v 1 from a window at height H. In addition to gravity acting on the ball there is another force so that the acceleration in the up direction is –g+  t where  is a constant and t is the time. What is the ball’s position when the acceleration is zero?

5. Falling with air resistance

6. 16 m You want to measure the height of a building. You stand 2m away from a 3m pole and see that it’s “in line” with the top of the building. You measure 16 m from the pole to the building. What is the height of the building?

7. Adding Vectors by Components 1.Draw a diagram 2.Choose x and y axes. Choose them in a way that make your work easier. (E.g. choose one axis along the direction of one of the vectors so that the vector will have only one component). 3.Resolve each vector in x and y components 4.Calculate each component using sine and cosine. Be careful with signs: any component that points along the negative x or y axis gets a negative sign. 5.Add the x components together to get the x component of the resultant. Similar for y: V x =V 1x +V 2x +… V y =V 1y +V 2y +…

8. 6. If you want to know the magnitude and direction of the resultant vector,

9. Russian fable: Swan, Crawfish, and Pike Lake FcFc FpFp River FsFs Find F s and F c if F p, θ, and  are given Despite their huge effort the box does not move!

10. Have a great day! Hw: All Chapter 3 problems and exercises