Non-touched collisionsNiCMos-InfraredThis busy image wasFour galaxies collidingrecorded at CERN(欧洲taken by using Hubble粒子物理研究所)，inSpace Telescope.Geneva,Switzerland
This busy image was recorded at CERN(欧洲 粒子物理研究所), in Geneva, Switzerland Four galaxies colliding taken by using Hubble Space Telescope. Non-touched collisions
6-1Howto analyze a collision?In a collision, two objects exert forceson each other for an identifiable (可确认的)time interval, so we can separate themotion into three parts.Before, during, and after the collision.During the collision, the objects exertforces on each other, these forces areequal in magnitude and opposite indirection
6-1 How to analyze a collision? In a collision, two objects exert forces on each other for an identifiable (可确认的) time interval, so we can separate the motion into three parts. Before, during, and after the collision. During the collision, the objects exert forces on each other, these forces are equal in magnitude and opposite in direction
Characteristics of acollision1) We usually can assume that theseforces are much larger than any forcesexerted on the two objects by other bodys inthe environment. The forces vary with time inacomplexway2) The time interval during the collision isguite short compared with the time duringwhich we are watching.These forces are called "impulsive forces（冲力）
1) We usually can assume that these forces are much larger than any forces exerted on the two objects by other bodys in the environment. The forces vary with time in a complex way. 2) The time interval during the collision is quite short compared with the time during which we are watching. These forces are called “impulsive forces (冲力)”. Characteristics of a collision
6-2 Linear MomentumTo analyze collisions, we define a newdynamic variable, the "linear momentumas:(6-1)P=mvThe directidthe same as thepdirection ofThe momentum(like the velocity)depends on the refrence frame of theobserver, and we must always specify thisframe
6-2 Linear Momentum To analyze collisions, we define a new dynamic variable, the “linear momentum” as: (6-1) The direction of is the same as the direction of . The momentum (like the velocity) depends on the reference frame of the observer, and we must always specify this frame. P mv = P v P
A?Can p be related todpdvdmvZF=ma=mdtdtdtdpZF(6-2)dtAny conditions for existence of above Eq.?The equivalence of ZF = ma and ZF-fdtdepends on the mass being a constant
F ma = d P F dt = The equivalence of and depends on the mass being a constant. Any conditions for existence of above Eq.? Can be related to P ? dv dmv d P F ma m dt dt dt = = = = d P F dt = (6-2) 𝐹 →