Racquetball: Long on Exercise, Short on Time

Force and momentum are intimately related. Force acting over time can change momentum, and Newton's second law of motion can be stated in its most broadly applicable form in terms of momentum. Momentum can be applied to systems where the mass is changing, such as rockets, as well as to systems of constant mass. Also, momentum continues to be a key concept in the study of atomic and subatomic particles in quantum mechanics. One can consider systems with varying mass in some detail; however, the...

Most altruistic behavior—in which one animal helps another at a cost to themselves—occurs between relatives. Scientists think these altruistic behaviors evolved because they increase the inclusive fitness of the animal providing help.