Why? Well, I suspect there are some slight frame rate problems with my screen capture. Before that, the kinetic energy SHOULD be constant – but there are some spikes in the data. I added the red arrow to indicate the location on the graph where the bird entered the "sphere of gravity". If I assume a bird mass of 1 unit (call it kg if you like) and a scale where the sling shot is 4.9 meters tall (from the Angry Birds Terrestrial game) then this would be a plot of kinetic energy vs. How? First, get some screen captures of motions in the game (using the desktop version of the game) and then use the free (and awesome) video analysis program Tracker. I can't directly measure the potential energy for this system. Please don't confuse momentum with the CHANGE in momentum. In fact, for circular motion the force and the momentum are NOT in the same direction. Although the change in momentum is in the same direction as the force, the momentum might not be. This might seem like a great way to go, but the problem is that both the force and the momentum are vectors. In the momentum principle, I can find the forces on the bird (probably just the gravitational force) and in a short time interval, I can write: When dealing with orbits, it is easier to use the Work-Energy principle than it will be to use the momentum principle. That is not a perfectly circular orbit, but it will work. What if I shot a bird (not shot THE BIRD) in such a way that it sort of went around the asteroid, like this: But how could I test if this is indeed the way gravity works in Angry Birds Space? Honestly, I think the best thing is to look at orbital motion. Here, G is the gravitational constant the m's are the masses of the two objects and r is the distance between their centers.
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