Phys 201, Lecture 5

1. Phys 201, Lecture 5 Chapter 3: Motion in Two and Three Dimensions Phys 201, Spring 2011

2. 3-D Kinematics • The position, velocity, and acceleration of a particle in 3 dimensions can be expressed as: • r = x i + y j + z k • v = vx i + vy j + vz k(i,j,kunit vectors ) • a = ax i + ay j + az k (time-independent) • We have already seen the 1-D kinematics equations: Phys 201, Spring 2011

3. 3-D Kinematics • For 3-D, we may simply apply the 1-D equations • to each of the component equations. • These can be combined into the vector equations: Phys 201, Spring 2011

4. Displacement of a particle in two dimensions y = r sin θ θ x = x cos θ Phys 201, Spring 2011

5. Change of displacement of a moving particle average velocity is Phys 201, Spring 2011

6. Magnitude of velocity vector: Direction of velocity vector described by θ: y vy θ vx x Phys 201, Spring 2011

7. Average velocity over time interval Δt: Instantaneous velocity: in the tangential direction on the r-t graph, independent of the r direction. Phys 201, Spring 2011

8. Relative velocity • Velocity is defined relative to a frame of reference. vx = 0 vx ≠ 0 Mathematically, a vector sum: Vresultant = Vrelative + Vframe Phys 201, Spring 2011

9. Example 3-2: Flying plane in wind VAG : air (wind) relative to ground: east VpG : plane relative to ground: Go north VpA : plane relative to air Pilot of plane that flies 200 km/h wishes to fly due north. What direction should he point? Phys 201, Spring 2011

10. Example 3-2: Flying plane in wind Pilot of plane that flies 200 km/h wishes to fly due north. What direction should he point? To go due north (y), this east (x)-component of the resultant velocity must be zero: sinθ = vAG/vpA = (90 km/hr)/(200 km/hr) sinθ = 0.45  θ = 0.47 radians from N [ or, in degrees, 0.47 radians x (360 degrees)/(2π radians) = 27° W of N] The velocity magnitude: vpG = vpA cosθ = 180 km/hr Phys 201, Spring 2011

11. Question Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth (B) swims perpendicular to the flow, Ann (A) swims upstream, and Carly (C) swims downstream. Which swimmer wins the race? A) Ann B) Beth C) Carly Phys 201, Spring 2011

12. Question Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth (B) swims perpendicular to the flow, Ann (A) swims upstream, and Carly (C) swims downstream. Which swimmer wins the race? A) Ann B) Beth C) Carly correct Time to get across = width of river/perpendicular component of velocity. Beth has the largest perpendicular component of velocity. Phys 201, Spring 2011

13. Question (seagull) A seagull flies through the air with a speed of 10 m/s in the absence of wind. Assume it can only make the same effort while flying in wind. It makes a daily round-trip to an island one km from shore. Compare the time it takes for the seagull to fly on a calm day to the time it takes when the wind is blowing constantly towards the shore at 5 m/s. a. The round-trip time is the same with and without the wind b. The round-trip time is always longer with the wind c. The round-trip time can be shorter than without the wind Total round trip time in the absence of wind is 2×(1000 m)/(10 m/s) = 200 s. In the presence of wind, the seagull’s speed going towards shore is 15 m/s and away from shore is 5 m/s. The time to go out to the island & the time to return: (1000 m)/(5 m/s) = 200 s, and (1000 m)/(15 m/s) = 67 s, so the total time in the presence of wind is 267 s. In general: t = 2 d / v (1-vw2/v2) Phys 201, Spring 2011

14. Acceleration vectors Average acceleration over time interval Δt: Instantaneous acceleration: in the tangential direction on the v-t graph, Not tangential to r-t graph, independent of the r, v directions. Phys 201, Spring 2011

15. Simplify to 2-D Kinematics Planar 3-D problems can be reduced to 2-D’s: Choose y axis to be along direction of acceleration. Choose x axis to be along the “other” direction of motion.  Perpendicular to y, but co-planar with the trajectory. Phys 201, Spring 2011

16. Projectile motion y For projectile motion: Horizontal acceleration is zero ( horizontal velocity is constant) Vertical acceleration is -g (constant in magnitude g, directed downward) x The horizontal and vertical motions are independent, except that the object stops moving both horizontally and vertically at the instant it hits the ground (or some other object, which determines y-range). Phys 201, Spring 2011

17. Without air resistance, an object dropped from a plane flying at constant speed in a straight line will A. Quickly lag behind the plane. B. Remain vertically under the plane. C. Move ahead of the plane. There is no acceleration in the horizontal direction – object continues to travel with the same horizontal velocity (same as the plane). Due to gravitational acceleration, the object accelerates downward, so its speed downwards increases. Phys 201, Spring 2011

18. Vertical and horizontal motions are independent For projectile motion, the vertical positions of these two balls are the same at each time: x = vx0 T Vertical motion and horizontal motion are independent: y = y0 + vy0 T + ½ (-g)T2 Phys 201, Spring 2011 Fig 3-12

19. Horizontal range of a projectile The horizontal range is the product of the horizontal speed (x component of the velocity) and the total time that the projectile is in the air. If object starts at height y=0, then T, the time in the air, is determined by finding when it reaches height y=0 again: The two solutions of this equation are T=0 (as expected) and T=2v0y/g=2v0sinθ/g. The horizontal range is then v0xT = (v0cosθ)(2v0sinθ/g) = v02sin(2θ)/g. Phys 201, Spring 2011

20. The range of a projectile depends on initial angle. Starting at ground level (y=0), the range is maximized for θ=45°. range = v02sin(2θ)/g Phys 201, Spring 2011

21. If a projectile lands at an elevation lower than the initial elevation, the maximum horizontal displacement is achieved when the projection angle is somewhat less than 45°. Phys 201, Spring 2011

22. A battleship simultaneously fires two shells at enemy ships from identical cannons. If the shells follow the parabolic trajectories shown, which ship gets hit first? 1. Ship A 2. Ship B 3. Both at the same time The higher the shell flies, the longer the flight takes: T=2v0y /g=2v0sinθ/g. A B Phys 201, Spring 2011

23. Acceleration for a general curved path Instead of considering a = ax i + ay j + az k (time-independent) Decomposed into: a = at + ac the tangential acceleration: at = dv/dt and centripetal acceleration: ac Phys 201, Spring 2011

24. Velocity: r change rate Acceleration: v change rate • Centripetal acceleration: • v/r = dv/dr • dv/dt = v/r dr/dt • Thus, • ac = dv/dt = v2/r • Directing toward center! Phys 201, Spring 2011

25. Example 3-4 Acceleration for uniform circular motion. Initial velocity has magnitude v and points due east. Final velocity has same magnitude v and points due north. Velocity has changed  particle is accelerating! Acceleration constant in magnitude, direction changing with time. the tangential acceleration: at = dv/dt = 0 the centripetal acceleration: ac = v2/r Phys 201, Spring 2011

26. Phys 201, Spring 2011

27. Non-uniform circular motion: A pendulum In general, a = at + ac Both non-zero. Phys 201, Spring 2011