Chapter 4 Motion in Two and Three Dimensions

Chapter 4 Motion in Two and Three Dimensions n Position vector in three dimensions 1

Instantaneous velocity The direction of the instantaneous velocity of a particle is tangent to the path at the particle’s position. The components of are : 2

Instantaneous acceleration n The components of are : 3

Projectile Motion n In projectile motion, the horizontal motion and the vertical motion are independent of each other; that is, neither motion affects the other. A projectile with an initial velocity can be written as (see figure 4 -10) : - n The horizontal motion has zero acceleration, and the vertical motion has a constant downward acceleration of - g. 4

n The range R is the horizontal distance the projectile has traveled when it returns to its launch height. 5

Examples of projectile motion 6

Horizontal motion n No acceleration 7

Vertical motion (Equations of Motion ): 1) 2) 3) 8

The equation of the path n In this equation, x 0 = 0 and y 0 = 0. The path, or trajectory, is a parabola. The angle is between and the + direction. 9

The horizontal range To find t = time of flight, y - y 0 = 0 means that : n n This equation for R is only good if the final height equals the launch height. We have used the relation sin 2 = 2 sin cos. The range is a maximum when = 45 o 10

Sample Problem n In the figure shown, a rescue plane flies at 198 km/h (= 55. 0 m/s) and a constant elevation of 500 m toward a point directly over a boating accident victim struggling in the water. The pilot wants to release a rescue capsule so that it hits the water very close to the victim. 11

(a) What should be the angle of the pilot's line of sight to the victim when the release is made? Solution Solving for t, we find t = ± 10. 1 s (take the positive root). 12

(b) As the capsule reaches the water, what is its velocity in unit-vector notation and as a magnitude and an angle? When the capsule reaches the water, 13

Sample Problem The figure below shows a pirate ship 560 m from a fort defending the harbor entrance of an island. A defense cannon, located at sea level, fires balls at initial speed v 0 = 82 m/s. (a) At what angle from the horizontal must a ball be fired to hit the ship? 14

SOLUTION: Which gives There are two solutions 15

(b) How far should the pirate ship be from the cannon if it is to be beyond the maximum range of the cannonballs? n SOLUTION: Maximum range is : - The maximum range is 690 m. Beyond that distance, the ship is safe from the cannon. 16

Sample Problem The figure below illustrates the flight of Emanuel Zacchini over three Ferris wheels, located as shown and each 18 m high. Zacchini is launched with speed v 0 = 26. 5 m/s, at an angle = 53° up from the horizontal and with an initial height of 3. 0 m above the ground. The net in which he is to land is at the same height. n (a) Does he clear the first Ferris wheel? 17

SOLUTION n The equation of trajectory when x 0 = 0 and y 0 = 0 is given by : Solving for y when x = 23 m gives Since he begins 3 m off the ground, he clears the Ferris wheel by (23. 3 – 18) = 5. 3 m 18

(b) If he reaches his maximum height when he is over the middle Ferris wheel, what is his clearance above it? n SOLUTION: At maximum height, vy is 0. Therefore, and he clears the middle Ferris wheel by (22. 9 + 3. 0 -18) m =7. 9 m 19

(c) How far from the cannon should the center of the net be positioned? n SOLUTION: 20

Avoid rounding errors n One way to avoid rounding errors and other numerical errors is to solve problems algebraically, substituting numbers only in the final step. 21

Uniform Circular Motion n A particle is in uniform circular motion if it travels around a circle at uniform speed. Although the speed is uniform, the particle is accelerating. n n The acceleration is called a centripetal (center seeking) acceleration. T is called the period of revolution. 22

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n Thus, , which means that is directed along the radius r, pointing towards the circle’s center. 24

Sample Problem n n “Top gun” pilots have long worried about taking a turn too tightly. As a pilot's body undergoes centripetal acceleration, with the head toward the center of curvature, the blood pressure in the brain decreases, leading to loss of brain function. There are several warning signs to signal a pilot to ease up: when the centripetal acceleration is 2 g or 3 g, the pilot feels heavy. At about 4 g, the pilot's vision switches to black and white and narrows to “tunnel vision. ” If that acceleration is sustained or increased, vision ceases and, soon after, the pilot is unconscious—a condition known as g-LOC for “g-induced loss of consciousness. ” 25

What is the centripetal acceleration, in g units, of a pilot flying an F-22 at speed v = 2500 km/h (694 m/s) through a circular arc with radius of curvature r = 5. 80 km? n SOLUTION: n If a pilot caught in a dogfight puts the aircraft into such a tight turn, the pilot goes into g. LOC almost immediately, with no warning signs to signal the danger. 26

Relative Motion in One Dimension n The coordinate x. PA of P as measured by A is equal to the coordinate x. PB of P as measured by B plus the coordinate x. BA of B as measured by A. Note that x is a vector in one dimension. 27

n The velocity v. PA of P as measured by A is equal to the velocity v. PB of P as measured by B plus the velocity v. BA of B as measured by A. Note that v is a one dimensional vector. We have deleted the arrow on top. 28

Because VBA is constant, the last term is zero. n Observers on different frames of reference (that move at constant velocity relative to each other) will measure the same acceleration for a moving particle. Note that the acceleration is a one dimensional vector. 29

Sample Problem Barbara's velocity relative to Alex is a constant v. BA = 52 km/h and car P is moving in the negative direction of the x axis. n (a) If Alex measures a constant velocity v. PA = -78 km/h for car P, what velocity v. PB will Barbara measure? n SOLUTION: 30

(b) If car P brakes to a stop relative to Alex (and thus the ground) in time t = 10 s at constant acceleration, what is its acceleration a. PA relative to Alex? 31

(c) What is the acceleration a. PB of car P relative to Barbara during the braking? n n n SOLUTION: To calculate the acceleration of car P relative to Barbara, we must use the car's velocities relative to Barbara. The initial velocity of P relative to Barbara is v. PB = -130 km/h. The final velocity of P relative to Barbara is -52 km/h (this is the velocity of the stopped car relative to the moving Barbara). This result is reasonable because Alex and Barbara have a constant relative velocity, they must measure the same 32 acceleration.

Relative Motion in Two Dimension 33

Homework (due Feb 1) Review Questions 72 -91 Pages 126 -126 34