Conserving Momentum

Honors Physics reviewed conservation of momentum and then turned to the classes of collisions. Even though all collisions are marked by conservation of momentum, not all collisions (actually a puny proportion) will be marked by conservation of kinetic energy. Elastic collisions – no deformation of objects, no sticking together, no loss of energy to sound or internal energy – they just don’t happen on a regular basis. Some collisions get pretty close, like a hard rubber ball bouncing on a smooth marble floor, but most collisions won’t proceed with no deformation or loss of energy to a nonmechanical form. The other side of the spectrum is also rare: the perfectly inelastic collision. Objects sticking together completely and moving as one discrete unit does not describe most collisions. Normally, we see things in between these extremes and these collisions, the inelastic variety, form the bulk of common experience. For our purposes, collisions will either be elastic or perfectly inelastic and this is the types of collisions you’ll find in the homework problems tonight. Tomorrow, we’ll go over those problems and work on some mixed review material to start preparing for Tuesday’s exam.

Physical Science B and E went over their Archimedes’ Principle problems and completed their discussion of density and buoyancy. Students had time to work on some conceptual questions concerning buoyancy and Physical Science E will conduct a lab investigation tomorrow that will allow them to examine buoyancy in more detail. They will get the opportunity to measure apparent losses of weight, weight of displaced fluid and calculate the theoretical buoyant force acting on an object. Physical Science B will begin a discussion of Bernoulli’s Principle tomorrow – the motion of fluids from high to low pressure and how that motion can affect the motion of objects in the path of the fluid.

Physics F conducted the impulse-momentum lab and got results comparable to Physics G. The impulse on an object approximated, in most cases, the change of momentum of the cart and looser elastic materials showed a marked longer time interval for the momentum change to occur than did stiffer elastic materials. The force-time graph showed a shallow “hill” for the impulse acting on the object for loose elastics and a sharp peak for more elastic materials. Consider that when thinking about forces that you experience in daily life and how you could increase or reduce the size of those forces by using time.

Physics G discussed conservation of momentum – the total momentum of a system stays the same, although the momentum of individual objects in the system can change. We explored different types of interactions within systems and how momentum was conserved in each case. Tomorrow, we will discuss the different classes of collisions and emphasize that momentum is conserved for each class. Kinetic energy, though…that’s another story.

Homework

Honors Physics A: 6E and 6G. For 6G, only complete the Part B portion for number 1. You can skip it for the rest of the questions.

Physical Science B: Complete buoyancy questions
Physical Science E: Read lab protocol sheet and complete buoyancy questions
Physics F: Complete lab write up
Physics G: Read 6.3 and complete the 6.3 Section Review #2, 4 and Chapter Review #27-30