1. Pluto moves in a fairly elliptical orbit around the sun. Pluto's speed at its closest approach of 4.43×109km is 6.12 km/s. 2. Relevant equations: L=mvr, F_g: (GMm/r^2), A_c: mv^2/r 3. The attempt at a solution: I found the answer here, but I'm more interested in why we would use angular momentum. I took L=mvr and used the given variables to set Pluto's momentum at its furthest point to its momentum at its closest. so I went from L=mvr to miviri=mfvfrf. The masses cancel, so the equation simplifies to viri = vfrf. I only came to this conclusion after a classmate hinted at me to think about angular momentum, so I'm still confused as to why angular momentum is the key to solving this, rather than Newton's universal law of gravitation set to centripetal acceleration, which gives me the wrong answer. I'm sorry if I was unclear on anything, thanks for your time.