2.EGO Flashcards
U12PH4 (a) State Kepler’s three laws of planetary motion. [3]
- Planets move in elliptical orbits with the Sun at one focus (1)
- Line joining a planet to the Sun sweeps out equal areas in equal time[intervals]. (1)
- r 3 T2 r- semi major axis (or accept radius), T- period of orbit (1)
U16PH4 In many applications an approximate value for the height reached by an object is obtained by _neglecting the variation in the g_ravitational field strength with height.(e) Discuss whether the use of the approximation in part (c) is appropriate in this case. [1]
Yes, as it is likely that the % difference is less than the uncertainty because of the measurement of the velocity or suitable alternative. ecf for consistency Answer to (e) must be consistent with answer to (d)
U16PH4 (a) Describe how the spectrum was used to determine the orbital speed and orbital period. [4]
Star (and companion planet) will both orbit the common centre of mass. Observe star spectral line from Earth. Measure Doppler shift in spectral line. Max wavelength corresponds to star moving away from Earth. Min wavelength corresponds to star moving towards Earth. Doppler shift accept red shift (1) in spectral line (1) Doppler shift (∆𝜆) – at max or min - can be used to determine orbital speed (𝑣) as: ∆𝜆 /𝜆 = 𝑣/ 𝑐 (1)
Orbital period is determined from one cycle of Doppler shift e.g. from one max to the next max. (1)
(iii) The velocity calculated in part (a)(ii) is small. Explain why small velocities are difficult to measure using red shift. [2]
) Redshift is small or the redshift is proportional to velocity or small Δλor shown using the Doppler equation (1)
Small wavelength change is difficult to measure or shifted wavelength is too close to the original (1)
(ii) In fact the planet has a slightly elliptical orbit. Explain how conservation of energy applies to this elliptical orbit. [2]
Realising KE is involved (1)
Correct variation with distance for KE or velocity or as PE increases KE decreases or converse (1)
conserved
