General Physics I. Lecture 6: Conservation of Momentum. Prof. WAN, Xin 万歆.

Transcription

1 General Physics I Lecture 6: Conservation of Momentum Prof. WAN, Xin 万歆 xinwan@zju.edu.cn

2 Outline Importance of conservation laws in physics Applications of the conservation laws Conservation of linear momentum Collisions Many-particle systems Center of mass Collisions in the center-of mass frame

3 A Question From You We have learn the work-kinetic energy theorem. What happens to it under Galileo transformation? (1) Nothing happens, we get the exactly same work-kinetic energy theorem. () Since velocity and displacement are reference frame dependent, the work-kinetic energy theorem only works at an inertial frame at rest (w.r.t. fixed stars). (3) Some additional law will emerge to ensure the work-kinetic energy theorem valid in a different inertial frame.

4 Galilean Transformation The two inertial observers agree on measurements of acceleration.

5 Let's Work it out Assume an object in 1D subject to a total force F for simplicity. W '=F d '=F (d v 0 t)=w v 0 F t K '= mv ' = m(v v 0 ) =K v0 m v+ mv 0 According to the work-kinetic energy theorem, W =K f K i W ' ( K ' f K 'i )=W ( K f K i) v0 F t +v 0 m(v f v i ) W '=( K ' f K 'i ) F t=m v f mv i

6 Impulse-Momentum Theorem The impulse of the force F acting on a particle equals the change in the linear momentum of the particle caused by that force I = Dp. Linear momentum Impulse

7 Connection to Newton's Laws Assume that a single force F acts on a particle and that this force may vary with time

8 Two-Particle System Consider two particles 1 and that can interact with each other but are isolated from their surroundings. Newton's second law Newton's third law

9 Conservation of Linear Momentum Whenever two or more particles in an isolated system interact, the total momentum of the system remains constant. The total momentum of an isolated system at all times equals its initial momentum. The only requirement is that the forces must be internal to the system.

10 A Question for You In an isolated system the validity of Galileo invariance and the conservation of kinetic energy (and mass) can lead to the conservation of linear momentum. Can you derive it? Discuss whether your derivation is still valid when some kinetic energy converts into internal energy m1 v 1i + m v i = m1 v 1f + m v f + Δ εinternal m1 v 1i +m v i = m1 v 1f +m v f Inelastic scattering involves internal energy change.

11 Conservation Laws A conservation law is usually the consequence of some underlying symmetry in the universe. Why are conservation laws powerful tools? Can assure something impossible. Applicable even when the force is unknown. An intimate connection with invariance. Convenient in solving for the particle motion. First use the relevant conservation laws one by one, then differential equations, computers, etc.

12 Rutherford Scattering

13 The Structure of Atom

14 The Standard Model

15 Large Hadron Collider (LHC)

16 Search for Higgs Boson The top event in the CMS experiment shows a decay into two photons (dashed yellow lines and green towers). The lower event in the ATLAS experiment shows a decay into 4 muons (red tracks).

17 Collision The event of two particles coming together for a short time and thereby producing impulsive forces on each other. These forces are assumed to be much greater than any external forces present.

18 Elastic and Inelastic Collision Momentum is conserved in any collision in which external forces are negligible. Kinetic energy may or may not be constant. Elastic collision between two objects is one in which total kinetic energy (as well as total momentum) is the same before and after the collision. Inelastic collision is one in which total kinetic energy is not the same before and after the collision (even though momentum is constant). Kinetic energy is constant only in elastic collisions.

19 Perfectly Inelastic Collisions When the colliding objects stick together after the collision, the collision is called perfectly inelastic.

20 Ballistic Pendulum

21 Two-Body Collision with a Spring

22 Elastic Collision in 1D m1 v 1i +m v i = m1 v 1f +m v f m1 v 1i + m v i = m1 v 1f + m v f Show that

23 Stress Reliever

24 Special Cases Particle initially at rest: v i=0 (1) m1 m v 1f v 1i, v f v 1i () m1 m v 1f v 1i, v f v i =0 Equal masses: m1=m v 1f =v i, v f =v 1i

25 Elastic Collision in D We need one more equation to solve.

26 Example: Proton Collision We are going to show: Whenever two equal masses collide elastically in a glancing collision and one of them is initially at rest, their final velocities are always at right angles to each other.

27 An Equivalent Problem Conservation of energy Conservation of momentum

28 The Center of Mass

29 Many-Particle Systems Consider a system of many particles in three dimensions We can think of an extended object as a system containing a large number of particles

30 The Center of Mass of a Rod Show that the center of mass of a rod of mass M and length L lies midway between its ends, assuming the rod has a uniform mass per unit length. L x CM 1 = x λ dx M 0 L x λ = M 0 λl L = = M The center of mass of any symmetric object lies on an axis of symmetry and on any plane of symmetry.

31 Nonuniform Rod Suppose a rod is nonuniform such that its mass per unit length varies linearly with x according to λ=α x

32 The CM of a Right Triangle

33 Motion of a Many-Particles System

34 Collision in the CM Frame Perfectly inelastic scattering v CM u1i = v 1i v CM ui = v i v CM m1 v 1i +m v i = m1+m m = v 1i v i ) ( m1 +m m1 = v i v 1i ) ( m1 +m u1f = uf = 0 m1 u1i +m ui = m1 u1f +m uf = 0 In the center-of-mass frame, the total momentum is zero.

35 Collision in the CM Frame Elastic scattering u1i = v 1i v CM ui = v i v CM u1f = v 1f v CM uf = v f v CM m = v 1i v i ) ( m1 +m m1 = v i v 1i ) ( m1 +m v CM m1 v 1i +m v i = m 1 +m m = v i v 1i ) ( m1 +m m1 = v 1i v i ) ( m1 +m m1 u1i + m ui = m1 u1f + m uf Conservation of kinetic energy

36 Collision in the CM Frame