physics of fetch

This weekend i was playing fetch with my dog chewy and i realized that the two toys that i was throwing had to very different behaviors. The first toy i threw was stuffed animal. When i threw this toy it would fly through the air in a projectile like path and hit the ground and slide to a stop. The second object i threw was a tennis ball which also traveled in a projectile like path but it hit the grown and changed velocities in a bouncy collision several times before rolling to the rest. Both of these objects have about the same mass and were thrown at around the same velocity and same trajectory however, the tennis ball had a larger impulse. This is because the tennis ball had a greater change in velocity due to the bouncy than the stuffed animal did that had no vertical velocity after it hit the ground. I also noticed that the coeficeint of friction between the stuffed animal and the floor was greater than the tennis ball rolling on the floor because the ball traveled further after it stopped bouncing.

Energy of a Football Kickoff

This Friday i was at the Iolani homecoming Football game against Word of Life. While i was sitting in the hot son watching Andrew Skalman set the football on the kickoff tee i realized what he is about to do is what we are talking about in physics this week. The law of conservation of energy states that energy is conserved, so ignoring air resistance the sum of the PE and KE before the ball is kicked should be equal to the sum of PE and KE throughout the flight of the ball. However, when the ball is resting on the tee the ball has no KE because its velocity is zero and very low PE with respect to the ground. At the peak of the ball flight, the ball has very high PE with respect to the ground and a high KE because the ball is traveling foward. So how did the ball get the energy to accomplish it's flight. The answer is work is being done to the system by Andrew Skalman when he kicks the ball. The work he inputs (work=F x delta X) is equal to the sum of PE and KE (TE) at any point after the ball is kicked minus the KE and PE (TE) before the ball is kicked (ignoring air resistance). This proves that the law of conservation of energy holds true even if the sum of KE and PE are not equal at different points of the ball's path because work is being done.