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Physics Grav Fields
Physics Spring Y13
| Question | Answer |
|---|---|
| Type of force gravity is | Attractive so always negative. |
| What is G | Universal gravitational constant. |
| What is g | Force per unit mass (G for a specific value of R) |
| When can treat planet as point mass | When not below surface. Exponential curve downwards for g from surface. Straight line from origin to surface on the g vs r graph |
| Gravitational field shape | Radial but we treat it as uniform at the surface |
| Microgravity | g varies slightly around surface. Variation in radius and density can be measured with these g changes. The geoid is the shape the ocean surface would take due to gravity + earths rotation if winds + tides absent. |
| Ellipses | Have a minor axis and a major axis. The shortest/longest lines through centre. They have 2 foci. |
| Kepler's First Law | Planets orbit the sun in elliptical orbits with the sun at one focus. For A level *calculations* we assume circular but not theory. For planets, orbits are very close to circular. |
| Kepler's Second Law | A line joining a planet and the sun sweeps out equal areas during equal intervals of time Higher r but moves slower so same area/time. e.g. comets move very slowly because so far away then move very fast past sun occasionally. |
| Kepler's Third Law | The square of the *orbital period* of a planet is directly proportional to the cube of the *mean* radius of its orbit |
| e.g. Assuming circular orbit show Kepler's Third is true and determine constant | w = 2pi/T F = mrw^2 F = -GMm/r^2 = mr(2pi/T)^2 Cancel and rearrange for T^2 |
| What the Kepler's Third Law constant tells us | Orbital constant depends on the mass of the body being orbited but every orbiting mass has same constant around the same object. For any r, there is only one T. Also for any radius, v is constant. r depends on v. Closer = faster. |
| What the Kepler's Third Law constant tells us (condensed form) | Satellites at same altitude have same speed and orbital period |
| (manmade) Satellites | Purpose varies r. Spy satellites in polar orbit so if r is correct will eventually see whole planet as it rotates. Old SkyTV satellites geostationary with radio dishes but Starlink just a bunch of non-geostationary so less signal delay |
| Signal delay to geostationary | Geostationary satellites are very very far away |
| Speeding up a satellite | Increases its radius and time period as it leaves its orbit (but aren't things slower at higher orbits??) |
| Geostationary orbit | 24hr orbit above equator. 'Hovers' above point on the equator T must be 24hrs therefore all geostationary are same altitude |
| Geosynchronous orbit | 24 hour orbit |
| For altitude | Remove planet radius |
| Reference Kepler in Qs | |
| e.g. question gives you r[b]/r[a] and you have to find T[b]/T[a] | rb/ra = 4 (rb/ra)^3 = 4^3 = 64 By Kepler's 3rd Law, (Tb/Ta)^2 = (rb/ra)^3 = 64 Tb/Ta = 8 |
| Technical point about orbiting bodies | Conservation of momentum, so orbits never circular as both bodies orbiting their joint CoM. |
| Joint CoM calculations | But for questions about calculating the gravitational force on one body, you use their separation and the mass of each other not the total, as the orbit is of each other But for the centripetal you still use the radius about the central point I think |
Created by:
Pyrogearos2