# Motion in the Universe

Section 1: Understanding Motion in the Universe

• The Universe is in constant motion, with galaxies, stars and planets moving in different ways.

• Motion refers to an object’s change in position relative to a point of reference.

• The speed of an object refers to how fast it’s moving, usually measured in metres per second (m/s) or kilometres per hour (km/h).

• The velocity of an object includes its speed and direction. It’s a vector quantity, meaning both magnitude and direction must be stated.

Section 2: Understanding Rotational Motion

• Rotation is when a celestial object spins around an internal axis. For example, Earth rotates on its axis, causing day and night.

• Revolution is when an object moves around another in a path known as an orbit. Earth’s revolution around the Sun causes the change of seasons.

• Orbital period is the time taken for one complete orbit around another object. Earth’s orbital period around the Sun is one year.

Section 3: Newton’s Laws and Astronomical Motion

• Newton’s first law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.

• Newton’s second law reveals that the force acting on an object is equal to the mass of the object multiplied by its acceleration (F=ma).

• Newton’s third law indicates that for every action force, there is an equal and opposite reaction force.

Section 4: Kepler’s Laws of Planetary Motion

• Kepler’s first law (law of ellipses) states that all planets move in elliptical orbits, with the Sun at one focus.

• Kepler’s second law (law of equal areas) suggests that a line joining a planet and the Sun sweeps out equal areas in equal times.

• Kepler’s third law (law of periods) illustrates a relationship between the orbital period of a planet and its average distance from the Sun.

Section 5: The Doppler Effect and Redshift

• The Doppler effect is a shift in frequency and wavelength of waves from an object moving relative to the observer.

• This effect explains the red and blue shifts observed in light from distant celestial objects.

• Redshift is a shift towards longer wavelengths, interpreted in astrophysics as evidence for the expansion of the Universe.