Basic research is what I am doing when I don't know what I am doing. - Wernher von Braun

Chapter 8 Skill Set Solve multi-part problems by identifying appropriate physical principles for each action Solve N- body inelastic collisions in 1D and 2D (there is NO 3D for 2 bodies!) 3. Solve 2-body 1D elastic collision problems when B is stationary, given eqns. 4. When B is not stationary, change reference frame! 5. Use center-of-mass concept to find position, velocity, and acceleration of 2nd body after an event that only applies internal forces to the system (i.e. a collision). 6. Use F=dP/dt to solve constant force, changing mass probs.

Answer: when mA=mB, vA = -vB (in the c.o.m. frame) True: Two equal masses exchange velocities in a 1D collision, regardless of reference frame. So why is vA2 ≠ vB1 in the center of mass frame??? Answer: mA is not necessarily = mB. But how can the correct answer for the c.o.m. frame: vA2 = -vA1 give a “velocity exchange” when mA does equal mB ???? Answer: when mA=mB, vA = -vB (in the c.o.m. frame)

A] I,II, or III only B] II or III only C] all of them D] none of them Larger circle = larger mass. Which of the sketches could be the velocities of two masses, in their “center of mass reference frame” ? In other words, which sketches could have Ptot=0? A] I,II, or III only B] II or III only C] all of them D] none of them

For a 1D collision, seen in the c. o. m For a 1D collision, seen in the c.o.m. frame, the rebound could be II or III and still conserve momentum. (Pi=Pf=0) II is completely elastic III is partly inelastic

Center of Mass Problems