A tunnel is bored through the center of a planet, as shown in the Figure (this drawing is NOT to scale and the size of the tunnel is extremely exaggerated). Assume that the planet is a homogenous sphere with a total mass M = 3.6 × 1024 kg and a radius R = 7300 km. A package of mass m = 7.8 kg is dropped into the tunnel. If the tunnel is used to deliver mail from one side of the planet to the other, how long would it take for a letter to travel through the planet? I got the magnitude of gravitation as 23.1 Newtons, but I'm not sure what to do with it and how to use it to figured out how long it would take to travel through the planet. My next question: Four spheres form the corners of a square whose sides are 10 cm long. The masses of the spheres are m1 = 950 kg, m2 = 200 kg, m3 = 600 kg, and m4 = 950 kg. What is the magnitude of the net gravitational force from them on a central sphere with mass m5 = 950 kg? img:http://i242.photobucket.com/albums/ff106/jtdla/prob10.gif [Broken] I tried using the pytagorean theorem to get a hypotenouse (5^2+5^2=radical(50)) and putting that in the denomentator of the various masses times 950. After summing them up I still didn't get the answer. Any ideas?