ASTRON 2B03 Lecture Notes - Lecture 9: Electromagnetism
12 Oct 2018
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Astron 2B03 Lecture 9
Gravitational force
• Between 2 objects depends on the masses of the objects
• F=Gm1m2/r^2 (r=distance b/w them, G=gravitational constant)
• Gravitational force appears to be large only because of large masses involved
• G is very small
How big are typical masses
• Adult masses~ 45-130 kg
• Mass of Earth is 6*10^24 kg
• Mass of our Su is 2*10^30 kg
• Mass of the stars in our galaxy is 4*10^41 kg
• Even though G is small, the cumulative effect of all this mass is so large that it
is of critical importance. So arguable the “most important force” in the
universe—what dictates how structure grows- is gravity
Why is gravitational constant G so very small?
• We now have some hints at an answer: “extra dimensions
3+1 dimensions
• We talk about left and right
• Back and forth
• Up and down
• 3 space dimensions (3D)
• Those are all the directions that we see in our everyday
• We will see that we should also include time as a fourth dimension.
• However, there may be (many) more space dimensions
Everything in nature is about balance of forces
• What keeps you in your chair
• The electromagnetic force must outweigh gravity (the only 2 long range
forces)
• The electrons in your body repel the electrons in the chair
