If Kinetic Energy KE is constant. So in a sense its in comparison with the second law of newton that we define this quantity like angular momentum. It stands to reason therefore that if the net force equals zero the rate of change of momentum must equal zero also.
Applying conservation of momentum pix pfx.
If mass m is constant. FracdPdt fracmvdt mfracdvdt ma F_net. After the collision the total momentum is pfx m Mucosth along X-axis and pfy m Musinth. Here is another way to derive the Law of Conservation of Momentum.
