1. Unit 2 Review Questions

2. 1. Draw an ellipse. Label the parts of the ellipse.
A generic ellipse is shown to the right
An elliptical orbit is shown to the right

3. The eccentric of an ellipse measures the flatness of an orbit
The eccentric of an ellipse measures the flatness of an orbit. Eccentricity varies from zero to one.

4. Kepler’s First Law:All planets move in elliptical orbits, with the sun at one focus.

5. Kepler’s Second Law: A line that connects a planet to the sun sweeps out equal areas in equal times.

6. p = orbital period in years a = avg. distance from Sun in AU
Kepler’s Third Law: The square of the period of any planet is proportional to the cube of the semimajor axis of its orbit.
p2 = a3
p = orbital period in years
a = avg. distance from Sun in AU
This means more distant planets orbit the Sun at slower average speeds, planets closer to the Sun orbit at faster speeds.

7. 2. Describing Motion
For objects in freefall the acceleration is called the acceleration due to ________gravity___________________. It is given the symbol _____g_____. And the value of the acceleration is _______10 m/s2________. This means that an object speeds up by ______10 m/s_________ for each second that it falls. So an object that was dropped from a tall building is going 40 m/s after 4 seconds.

8. 3. Momentum and Force
Momentum is like the inertia of a moving body. Momentum is mass x velocity. So all bodies that have mass and are moving have momentum.

9. Force is push or pull acting on something
Force is push or pull acting on something. A force must be applied over a time interval to produce a change in the momentum of an object.
Net force = the total force acting on an object.

10. Angular momentum is the inertia of a body that’s spinning and orbiting
Angular momentum is the inertia of a body that’s spinning and orbiting. Its found by mass x velocity x radius.

11. Explain the difference between mass and weight.
Mass is determined by the type and amount of matter an object is made up. Weight is a measure of the force of gravity on that mass.
W = mg

12. Explain why astronauts are weightless in space.
Astronauts are weightless because they are in constant free fall. Their sideways speed keeps them from hitting the earth as they fall.

13. 4. Newton’s Laws of Motion.
Newton’s first law: objects at rest stay at rest and objects in motion stay in motion, moving at the same speed in the same direction.
A spacecraft in motion in space keeps moving because there is no force to bring it to rest.

14. Second Law: The acceleration of an object is proportional to the net outside force acting on the object
Force = mass object  acceleration of object
Formula: F = ma

15. Use Newton’s 2nd law to describe the acceleration of a planet orbiting the sun
In the case of an orbiting planet the force in newton’s 2nd law is the gravitational force between the sun and the planet.
This force is what keeps a planet in orbit around the sun

16. Third Law: For every force, there is always an equal and opposite reaction force.

17. Newton’s 3rd law - Boulder
Action force – earth’s gravity pulls boulder down
Reaction force – boulder’s gravity pulls earth up.
The boulder has a small mass so it will have an acceleration of about 10 m/s2
The earth has a large mass so its acceleration is negligible.

18. Newton’s Law of Universal Gravitation
the gravitational force between two mass is proportional to the product of the masses and inversely proportional to the distance between them squared.

19. 5. Conservation Laws What is the law of conservation of momentum?
The law says that the total momentum of a system of interacting objects is constant.

20. What is the law of conservation of angular momentum?
In a closed system (like a planet and the sun) the total angular momentum (mass x velocity x radius) is conserved.

21. Kepler’s 2nd law implies planets speed up and slow down.
Use the law of conservation of angular momentum to explain why Kepler’s 2nd law is true.
Kepler’s 2nd law implies planets speed up and slow down.
The mass of the planet doesn’t change so only velocity or radius change.
On the right, the radius is big so the velocity must be small
On the left, the radius is small so the velocity must be big.

22. What is the law of conservation of energy?
The law says that energy can’t be created or destroyed. It can change form or be exchanged between objects.

23. Kinetic energy is the energy associated with motion.
KE = ½ mv2
m = mass, v = velocity
Potential Energy is stored energy. Gravitational PE depends on:
object’s mass (m)
Acceleration due to gravity (g)
distance object could fall
PE = mgh
The car has the most potential energy at point A

24. The planet has more potential energy at aphelion than it does at perihelion
Radiative Energy is the energy from light

25. 6. Energy, Orbits and Tides
Explain how gravity causes tides
Because the gravitational force is weaker with distance, the moon’s attraction is stronger on Earth’s oceans closer to the moon, and weaker on the oceans farther from the moon. This slightly elongates the earth and causes two high tides on opposite sides of the earth.

26. Tides & Phases
Spring tides occur when the moon, earth and sun are in a line
thia produces the largest tidal range
the tidal effects of the Sun and Moon reinforce each other
at new moon and full moon

27. Tides & Phases
Neap tides occur when the moon, earth and sun form a right triangle
this produces the smallest tidal range
The tidal effects of the Sun and Moon partially cancelled each other
At first quarter and third quarter