1. Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Sections 807, 808, 809 Lecture 4

2. A “police car” problem x=0 x1x1 x2x2 V 3 =20m/s a=0 a p =kt x 2 – x 1 = 3.5 km V 1 =30m/sV 2 =40m/s 1. What was your acceleration before you meet the second police car? 2. How much time did you travel from x 1 to x 2 ? 3. Find x 1 4. At which distance the police car catches you? 5.Convert the velocity from m/s to mph a=const V(t=0)=0

3. Imagine you drop a light feather and a heavy coin from Albritton Bell Tower (138 ft.) Will they reach the ground at the same time? 384-322 B.C. Aristotle says : “No! The coin will land first because heavier objects fall faster than the lighter ones, in direct proportion to weight”. 1800 years later Galileo says : “Yes! A coin and a feather will land together if there is no air resistance!” 1564-1642

4. Free fall g-positive! On planet Earth, if you neglect air resistance, any body which is dropped will experience a constant acceleration, called g, independent of its size or weight. g=9.8 m/s 2 =32 ft/s 2

5. Galileo’s notes

6. A ball is dropped vertically down (no air resistance) from height H. Find the position x(t) and velocity v(t) of the ball as a function of time. How long is the ball in the air?

7. A person throws a ball upward into the air with an initial velocity of 15 m/s. Calculate a)How much time it takes for the ball to reach the maximum height? b)How high it goes? c)How long the ball is in the air before it comes back to hands d)The velocity of the ball when it returns to the thrower’s hand e)At what time t the ball passes a point 8.00 m above the person’s hand

8. An object’s velocity is measured to be where  =4.00 m/s and  =2.00 m/s 3. At t=0 the object is at x=0. Calculate the object’s position and acceleration as functions of time.

9. 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?

10. Have a great day! Reading: Chapter 3 Vectors Hw quiz on Tuesday: All Chapter 2 problems and exercises