The physics of motion

1. The physics of motion

2. If you see the Moon setting and it is full, what did the Moon look like when it was on the eastern horizon 12 hours earlier?

3. Discussion Suppose it is the first day of autumn in the northern hemisphere. What phase is the Moon if the Moon is located at the position of the vernal equinox?

4. Discussion What time of the year can the full Moon be its highest above the horizon? Explain.

5. Lunar eclipse

6. Discussion What phase must the Moon be in for there to be a lunar eclipse?

9. Discussion Why don’t we have a lunar eclipse every time there is a full Moon?

12. Total Lunar eclipse of July 16, 2000

13. Discussion Why do you think the Moon appears red during a total lunar eclipse?

14. Discussion The Moon goes through its phases in 29.53 days. This is called the synodic month, i.e. the month relative to the Sun. The actual orbital period of the Moon is 27.32 days, the sidereal month. Why is there a difference in the synodic and sidereal months?

15. In the 27.32 days that the Moon has taken to orbit the Earth the Earth has moved in its orbit relative to the Sun. The Earth moves about 1 degree per day so that after 27 days the Moon needs to move through an extra 27 degrees in the sky to get back to the same position relative to the Sun.

16. Synodic and Sidereal months

17. Discussion If the Earth moves faster in its orbit does the length of the sidereal month change?

18. Discussion If the Earth moves faster in its orbit, does the synodic month get longer or shorter?

19. Discussion The Moon always keeps the same side facing the Earth. Does this mean the Moon does not spin on its axis? Explain?

20. Synchronous Rotation If the Moon did not spin on its axis we would see the Moon’s entire surface over the month. The Moon therefore does rotate but the rotation period is exactly equal to its orbital period and rotates such that we always see the same face. Thus, the Moon’s rotation period is synchronous with its orbital period.

23. Discussion What is the length of the sidereal day and mean solar day on the Moon?

24. Mean solar day on the Moon 29.5 Earth days, the Sun rises in the east sets in the west 14.75 days later, followed by 14.75 days of darkness. The Moon’s sidereal day is its actual rotation (and revolution) period of 27.32 days.

25. The motions of the Planets Each planet follows the same diurnal motion as the Sun, Moon and stars, rising in the east and setting in the west each day. Like the Sun and Moon, each planet moves west to east with respect to the stars. This is called direct motion. In addition, all the planets stay close to the ecliptic.

26. Path of Mars

27. Retrograde motion

28. Kepler’s first law of planetary motion The orbit of a planet about Sun is an ellipses with the Sun at one focus.

31. Kepler’s Second law of planetary motion A line drawn from the planet to the Sun sweeps out equal areas in equal intervals of time.

34. Kepler’s third law of planetary motion The square of the sidereal period is equal to the cube of the semimajor axis of the orbit.

35. Example An asteroid has a period of 8 years. 8  8 = 64 = 4  4  4 So the semimajor axis of this asteroids orbit is 4 AU.

37. Galileo

41. Galileo’s Physics • Inertia: A moving object will stay in motion unless acted upon by a force. • Rate of falling: Gravity accelerates all objects, regardless of weight, by the same amount

42. Speed Example: a car moving at 60 miles/hour

43. Velocity Velocity refers to not only how fast an object is moving but its direction as well. Example: a car moving 60 miles/hour due west.

44. Acceleration Acceleration refers to any rate of change in the velocity of an object. An acceleration can mean a speeding up, a slowing down, or simply a change in the direction of motion with no change in speed.

45. Units of acceleration

46. Example A car accelerates from a stop light at 10 m/sec2 following a straight path. So, at time t = 0 the car’s speed is 0 m/sec. After one second of acceleration, the car’s speed is 10 m/sec (velocity 10 m/sec south). After two seconds, the car’s speed is 20 m/sec.

47. Discussion After one minute of accelerating at 10 m/sec2 at what speed is the car moving?

48. Discussion If I had two identical inclined planes placed so that they faced each other and I rolled a ball down one of the planes. How high up the second inclined plane will the ball get before it stops rolling? Explain.

49. Discussion What if the second inclined plane is replaced by one which is half as steep as the first inclined plane. How high will the ball reach on this plane?