Chapter 8. Momentum, Impulse and Collisions. 10/22/14 Physics 218

Transcription

1 Chapter 8 Moentu, Ipulse and Collisions 0//4 Physics 8

2 Learning Goals n n n n n n The eaning of the oentu of a particle(syste) and how the ipulse of the net force acting on a particle causes the oentu to change. The conditions under which the total oentu of the syste of particles is constant (conserved). How to solve two-body collision probles. The distinction between elastic, inelastic and copletely inelastic collisions. The definition of the center of ass of a syste and what deterines how the center of ass oves. Lastly, a very brief introduction to rocket propulsion. 0//4 Physics 8

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5 Not so any equations if you use these as your tool kit. 0//4 Physics 8 5

6 Definition of Moentu (revisited) dv d F Net ( v) dt dt we defined the quantity v p v as the oentu of the particle dp F Net dt as an alternate forof Newton's nd Law 0//4 Physics 8 6

7 The Definition of Ipulse 0//4 Physics 8 7 assuing a constant net force ( ) Δ t F t t F J net ) ( this becoes substituting for theipulse, p p J p p t t F t t p p F Net net

8 For forces that are not constant the ipulse becoes t J t F net dt p p generaldefinition of an ipulse (ipulseis a vector and has units of newton -secondsor kg /s) 0//4 Physics 8 8

9 Figure 8.3

10 Collisions and Ipulse! n Do not do this at hoe!! 0//4 Physics 8 0

11 Q8. ball (ass 0.40 kg) is initially oving to the left at 30 /s. fter hitting the wall, the ball is oving to the right at 0 /s. What is the ipulse of the net force on the ball during its collision with the wall?. 0 kg /s to the right B. 0 kg /s to the left C. 4.0 kg /s to the right D. 4.0 kg /s to the left E. none of the above

12 8. ball (ass 0.40 kg) is initially oving to the left at 30 /s. fter hitting the wall, the ball is oving to the right at 0 /s. What is the ipulse of the net force on the ball during its collision with the wall?. 0 kg /s to the right B. 0 kg /s to the left C. 4.0 kg /s to the right D. 4.0 kg /s to the left E. none of the above

13 Q8. You are testing a new car using crash test duies. Consider two ways to slow the car fro 90 k/h (56 i/h) to a coplete stop: (i) You let the car sla into a wall, bringing it to a sudden stop. (ii) You let the car plow into a giant tub of gelatin so that it coes to a gradual halt. In which case is there a greater ipulse of the net force on the car?. in case (i) B. in case (ii) C. The ipulse is the sae in both cases. D. not enough inforation given to decide

14 8. You are testing a new car using crash test duies. Consider two ways to slow the car fro 90 k/h (56 i/h) to a coplete stop: (i) You let the car sla into a wall, bringing it to a sudden stop. (ii) You let the car plow into a giant tub of gelatin so that it coes to a gradual halt. In which case is there a greater ipulse of the net force on the car?. in case (i) B. in case (ii) C. The ipulse is the sae in both cases. D. not enough inforation given to decide

15 Q8. You are testing a new car using crash test duies. Consider two ways to slow the car fro 90 k/h (56 i/h) to a coplete stop: (i) You let the car sla into a wall, bringing it to a sudden stop. (ii) You let the car plow into a giant tub of gelatin so that it coes to a gradual halt. In which case is there a greater net stopping force on the car?. in case (i) B. in case (ii) C. The stopping force is the sae is the sae in both cases. D. not enough inforation given to decide

16 Conservation of Moentu n If the vector su of the external forces on a syste is zero, the total oentu of the syste is conserved. dp F Net 0 dt and the oentu of the syste is constant 0//4 Physics 8 6

17 Figure 8.8 For a two body syste with no external forces acting. dp dpb FBon ; FonB dt dt dp dpb FBon + FonB + 0 dt dt d( p + pb ) 0 dt sothe total oentu is constant

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20 Recoil of a rifle 0//4 Physics 8 0

21 Figure 8.

22 Elastic collisions in -D 0//4 Physics 8 before and for the velocities after the collison Wecan solvethese equations B B B B B B after B B after total before total v v v v v v KE v KE v v p v p

23 Elastic collisions n 0//4 Physics 8 3