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Ancient History of Astronomy
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ē Why is the summer longer than winter? Because of the precession of the Earthís axis of rotation.

ē Because of the moonís 5o inclination with respect to the Ecliptic.

ē Because the Earth is rotating around its axis more slowly in the summer (→ longer days!).

ē Because the Earth is closest to the sun in January and most distant from the sun in July.

ē Because the Earth is closest to the sun in July and most distant from the sun in January.

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Ancient History of Astronomy

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Keplerís Third Law

Keplerís Third Law

A planetís orbital period (P) squared is proportional to its average distance from the sun (a) cubed:

(Py = period in years; aAU = distance in AU)

Orbital period P known → Calculate average distance to the sun, a:

aAU = Py2/3

Average distance to the sun, a, known → Calculate orbital period P.

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It takes 29.46 years for Saturn to orbit once around the sun. What is its average distance from the sun?

It takes 29.46 years for Saturn to orbit once around the sun. What is its average distance from the sun?

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Think critically about Keplerís Laws: Would you categorize his achievements as physics or mathematics?

Think critically about Keplerís Laws: Would you categorize his achievements as physics or mathematics?

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Keplerís Third Law

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Keplerís Third Law

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Keplerís Third Law

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Keplerís Third Law

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Velocity and Acceleration Acceleration (a) is the change of a bodyís velocity (v) with time (t):

Velocity and Acceleration

Acceleration (a) is the change of a bodyís velocity (v) with time (t):

Velocity and acceleration are directed quantities (vectors)!

Different cases of acceleration:

Acceleration in the conventional sense (i.e. increa sing speed) Deceleration (i.e. decreasing speed)

Change of the direction of motion (e.g., in circular motion)

Slide 33

A ball attached to a string is in a circular motion as shown. Which path will the ball follow if the string breaks at the marked point?

A ball attached to a string is in a circular motion as shown. Which path will the ball follow if the string breaks at the marked point?

Slide 34

Newtonís Laws of Motion (II) The acceleration aof a body is inversely proportional to its mass m, directly proportional to the net force F, and in the same direction as the net force.

Newtonís Laws of Motion (II)

The acceleration aof a body is inversely proportional to its mass m, directly proportional to the net force F, and in the same direction as the net force.

Slide 35

Newtonís Laws of Motion (III) To every action, there is an equal and opposite reaction. The same force that is accelerating the boy forward, is accelerating the skateboard backward.

Newtonís Laws of Motion (III)

To every action, there is an equal and opposite reaction. The same force that is accelerating the boy forward, is accelerating the skateboard backward.

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