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# 6.1 - Gravitational Force and fields

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### 6.1 - Gravitational Force and fields

1. 1. 6.1 - Gravitational Force & Fields
2. 2. Newton’s Universal Law of GravitationEvery single point mass attracts every other point mass with a force directlyproportional to the product of the masses and inversely proportional to thesquare of their separation.Universal Constant of Gravitation: G= 6.6742 x 10-11 m3kg-1s-2
3. 3. Newton’s Universal Law of GravitationNewton calculated (using calculus) that Spheres also follow the same rule aslong as the separation is between their centre of mass. rThe Centre of Mass is a point that represents the total mass of a body andis where gravity can be said to act. For regular shapes it’s in the middle.
4. 4. Newton’s Universal Law of Gravitation on PhET http://phet.colorado.edu/en/simulation/gravity-force-lab or click on the pictureUse this to check Newton’s Law of Gravitation. What happens to the force if youdouble the distance? or double either of the masses?
5. 5. Acceleration due to GravityNewton’s Law of Gravitation Newton’s 2nd Law m + a On Earth: a = 9.81ms-2 Newton was right. This is correct! M
6. 6. Orbits: Centripetal Force & GravityAssuming the orbits are circular.Gravity causes the centripetal force + This is the speed of the orbiting object
7. 7. Orbits: Centripetal Force & Gravity (cont)1. The speed of the earth in orbit around the sun: m1 = mass of the sun = 1.99 x 1030 kg r = distance between Sun and Earth = 1.49 x 1011m G = 6.6742 x 10-11 m3kg-1s-2 2. The time for the earth to orbit around the sun: Newton was right again – 1 Year
8. 8. Orbits: Centripetal Force & GravityTry out these two great simulations to understand gravity even more. Gravity and Orbits My Solar System http://phet.colorado.edu/en/simulatio http://phet.colorado.edu/sims/my-solar- n/gravity-and-orbits system/my-solar-system_en.html or click on the pictures
9. 9. Gravitational Field StrengthDefinition: The force per unit mass experienced by a small testmass* placed in the field. For the mass m: * A small test mass is used because any larger mass might change the field.
10. 10. Field LinesThese show the direction that a mass would accelerate if placed in the field, andhelp us to imagine the field. Near the Earth the field lines are almost parallel.Around a spherical mass the field The field is uniform.lines are closer together nearer Wherever you are near the surface of the earththe surface, so the field strength you are pulled down with the same Force/Kilogramis larger. Field Strength is a vector, so two values of g can be added together