Snow Day Blog 2.0

The Conservation of Momentum

This blog was a production, to say the least. My first choice to demonstrate momentum was to slide down a hill into a cardboard box, sending it into motion and therefore demonstrating how although there is a transfer of energy in a collision, momentum is always conserved no matter what. There was one problem though, I don’t own a sled. I figured that a plastic container cover would suffice, but then, when I got to the hill, this happened.

Lucky for me, a little kid was nice enough to let me borrow his sled (which was much too small) and, after many, many failed attempts, one dead phone, and one failed filming job by my mom, I was able to get this collision on video.

But, of course, when I got home, I realized that I had no way of measuring the distance that I had traveled, and this meant that I had no frame of reference for Logger Pro to use to measure things such as change of position and velocity, making my efforts fruitless. After some thought, I settled on using the collision between two balls of varying weight to demonstrate the conservation of momentum. In the video below, my mom and I are 2.5 meters apart, and the medicine ball weighs 2.63 kg while the lacrosse ball weighs 170 grams.

As expected, when we threw the balls at each other, some of the medicine ball’s energy was transferred to the much lighter lacrosse ball, causing it to change directions and accelerate. Next, using Logger Pro, I created a calculated column that was equal to the square root of the x position squared plus the y position squared for each ball, which, when graphed over time, was equal to its overall velocity. Here are the respective graphs.

As you can see, the medicine ball’s velocity undergoes little change over the course of the video, while the lacrosse ball’s velocity undergoes a drastic change following the collision. Furthermore, as my work below shows, momentum was conserved, but energy was not, and therefore this was an inelastic collision. All in all, 1.792 Joules of energy were lost due to this collision. I attribute this loss of energy to the sound that this collision produced and the thermal energy produced as a result of the friction between the two balls, as the medicine ball had a rough texture. Lastly, the initial and final energies could have been different just because of inaccuracies throughout the process of plotting the path of the balls and then determining their respective velocities.

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