 Hi, I'm Sarah. I studied engineering and physics at the Australian National University and I now use this knowledge in my job as a management consultant. In this course, we'll cover gravity and motion. We'll learn about Newton's Law of Universal Gravitation, gravity on Earth, projectile motion, uniform circular motion, and gravitational fields. Now I'll discuss gravity as one of the fundamental forces. Currently, we believe that there are four fundamental forces in the universe, the strong nuclear force, the weak nuclear force, the electromagnetic force, and the gravitational force. The strong nuclear force stitches nucleons together, which means we get nuclei. The weak nuclear force affects nuclear decay, which gives us nuclear radiation. The electromagnetic force governs charge and magnetic fields, which means we get atoms. And finally, the gravitational force keeps large chunks of matter clumped together, giving us planets and stars, and gravity. Gravity is a bit of an odd duck compared to the other fundamental forces. Physicists have managed to unite the other three fundamental forces and explain them with one theory, the standard model, which you'll know about next semester. But gravity has stubbornly resisted integration, and we need a separate theory called Einstein's Theory of General Relativity to truly explain it. Another thing that's odd about gravity is just how weak it turns out to be. You might not believe this if you've ever tripped and fallen, but gravity is much weaker than the other fundamental forces. If we zoom down to the scale of protons and neutrons, the strong nuclear force is 10 to the 37 times stronger than gravity. The weak nuclear force is 10 to the 29 times stronger than gravity, and the electromagnetic force is 10 to the 36 times stronger than gravity. The question of why gravity is so much weaker is an open question, which will likely require uniting gravity with the other fundamental forces. This also leads us to another question. If gravity is so much weaker than the other forces, why does it seem to dominate at large scales? After all, we don't see one planet flying towards another because of the strong nuclear force, and we don't see the charge of Jupiter affecting the orbit of the moons. As it turns out, the strong nuclear force and the weak nuclear force are both short-ranged forces. After distance is much smaller than atoms, the strength of both forces is essentially zero, so they don't end up having an effect at large scales. As for electromagnetism, celestial bodies usually have as much positive charge as negative charge, so they're electrically neutral and don't generate or interact with electric fields. On the other hand, gravity is purely attractive. There's no negative mass to cancel out, so every extra bit of matter strengthens the gravitational field. This means gravity, despite being the run to the forces, is the only one left standing to influence interactions at large scales.