LECTURE 15. Prof. Paul

Transcription

1 LECTURE 15 Prof. Paul

2 Review Clicker Questions:

3 Review Clicker Questions: (You can talk with others) Mark and David are loading identical cement blocks onto a truck. Mark lifts his block straight up from the ground to the truck, whereas David slides his block up a ramp on massless, frictionless rollers. Which statement is true? (a) Mark does more work than David. (b) Mark and David do the same amount of work. (c) David does more work than Mark. (d) None of these statements is necessarily true because the angle of the incline is unknown and/or the mass of one block is not given.

4 Mark and David are loading identical cement blocks onto a truck. Mark lifts his block straight up from the ground to the truck, whereas David slides his block up a ramp on massless, frictionless rollers. Which statement is true? (b) Mark and David do the same amount of work.

5 If the net work done on a particle is zero, which of the following statements must be true? (a) The velocity is zero. (b) The velocity is decreased. (c) The velocity is unchanged. (d) The speed is unchanged. (e) More information is needed.

6 If the net work done on a particle is zero, which of the following statements must be true? (a) The velocity is zero. (b) The velocity is decreased. (c) The velocity is unchanged. (d) The speed is unchanged. (e) More information is needed. W net = ΔKE = = ½ m(v f 2 v i2 )

7 Announcements me if you d like to work with others on the homework but are having a hard time finding a group for whatever reason. I ll see if I can pair you up with someone(s). Folks coming in 10 or 15 minutes late. Maybe first part of class is not useful? Start off with clicker question(s) as warm up

8 Today Types of collisions Two dimensional collisions

9 Conserva)on of Momentum, cont The principle of conserva)on of momentum states when no external forces act on a system consis)ng of two objects that collide with each other, the total momentum of the system remains constant in )me Specifically, the total momentum before the collision will equal the total momentum aaer the collision Sec)on 6.2

10 Types of Collisions Momentum is conserved in any collision Inelas)c collisions Kine)c energy is not conserved Some of the kine)c energy is converted into other types of energy such as heat, sound, work to permanently deform an object Perfectly inelas)c collisions occur when the objects s)ck together Not all of the KE is necessarily lost Sec)on 6.3

11 More Types of Collisions Elas)c collision Both momentum and kine)c energy are conserved (billiard balls are close) Actual collisions Most collisions fall between elas)c and perfectly inelas)c collisions Assume the collision is inelas)c, unless it says otherwise. In general energy is lost during collision. Sec)on 6.3

12 Bullet is shot, lodges itself into a block. Over the entire process A. Energy leaves the bullet-block system B. Momentum leaves the bullet-block system C. Momentum and Energy both leave the system D. Neither Momentum nor Energy leave the system E. Not enough information to tell

13 Bullet is shot, lodges itself into a block. Over the entire process A. Energy leaves the bullet-block system B. Momentum leaves the bullet-block system C. Momentum and Energy both leave the system D. Neither Momentum nor Energy leave the system E. Not enough information to tell

14 Bullet is shot, lodges itself into a block. What type of collision is this? A. Elastic B. Inelastic C. Perfectly Inelastic D. Not enough information to tell

15 Bullet is shot, lodges itself into a block. What type of collision is this? A. Elastic B. Inelastic C. Perfectly Inelastic D. Not enough information to tell

16 When should we use conservation of momentum? A. Time 1 Time 2 B. Time 1 Time 3 C. Time 2 Time 3 t1: just before bullet hits block t2: just after collision t3: just when block Reaches highest point D. Never

17 When should we use conservation of momentum? A. Time 1 Time 2 B. Time 1 Time 3 C. Time 2 Time 3 t1: just before bullet hits block t2: just after collision t3: just when block Reaches highest point D. Never

18 When should we use conservation of Energy? A. Time 1 Time 2 B. Time 1 Time 3 C. Time 2 Time 3 t1: just before bullet hits block t2: just after collision t3: just when block Reaches highest point D. Never

19 When should we use conservation of Energy? A. Time 1 Time 2 B. Time 1 Time 3 C. Time 2 Time 3 t1: just before bullet hits block t2: just after collision t3: just when block Reaches highest point D. Never

20 Book does this example: page 178 Δp = m block (vf vi) block + m bullet (vf-vi) bullet = 0 (between times 1 & 2) ΔEtot = ΔPE + ΔKE = 0 (between times 2 & 3) Is momentum conserved between 2 & 3? Discuss.

21 Summary of Types of Collisions In an elas)c collision, both momentum and kine)c energy are conserved: Δp total = 0; ΔE tot =0 In an inelas)c collision, momentum is conserved but kine)c energy is not Δp total = 0; ΔE tot =0 In a perfectly inelas)c collision, momentum is conserved, kine)c energy is not, and the two objects s)ck together aaer the collision, so their final veloci)es are the same Δp total = 0; ΔE tot =0; v 1f = v 2f No)ce that the equa)on for change in total momentum is the same. Sec)on 6.3

22 You try: High-speed stroboscopic photographs show that the head of a 200-g golf club is traveling at 55 m/s just before it strikes a 46-g golf ball at rest on a tee. After the collision, the club head travels (in the same direction) at 40 m/s. Find the speed of the golf ball just after impact.

23 You try: High-speed stroboscopic photographs show that the head of a 200-g golf club is traveling at 55 m/s just before it strikes a 46-g golf ball at rest on a tee. After the collision, the club head travels (in the same direction) at 40 m/s. Find the speed of the golf ball just after impact. Δp total = Δp 1club + Δp 2ball 0 = m club (v f - v i ) club + m ball (v f v i ) ball 0= 0.2(40-55) +.046(v f 0) ball V = 65 m/s

24 You try: High-speed stroboscopic photographs show that the head of a 200-g golf club is traveling at 55 m/s just before it strikes a 46-g golf ball at rest on a tee. After the collision, the club head travels (in the same direction) at 40 m/s. Find the impulse acting on each object.

25 Two dimensional momentum conservation

26 2D momentum conservation: Strategy Find sum of changes of momentum in x-direction Find sum of changes of momentum in y-direction Think about which terms are zero (check with picture) Decide if it s Elastic, Inelastic, or perfectly inelastic Elastic: use conservation of energy & conservation of momentum Inelastic: Use conservation of momentum Perfectly inelastic: use conservation of momentum & final velocities are equal Solve equations simultaneously for unknown

27 Homework Chapter 6 problems: 27, 29, 30, 38 Responsible for but don t have to turn in: 33, 36