Click on the following link: https://phet.colorado.edu//. Search for Gravity and Orbits (HTML5). Click Play and then Model.

**Please note that in this simulation mass is represented by the size of the Sun or Planet. Mass and size do not mean the same thing!A very massive object can be quite small, as can a very large object have very low mass. For example, a solid metal pellet can be more massive object than a Styrofoam ball. The equivalence of size and mass in this simulation is intended to provide a visual representation of differences in mass.**

Check the Pathand Grid boxes. Play the Star and Planet simulation.

i. An orbital period is the time it takes for an object to make one full revolution around another. What is the orbital period of the Earth around the Sun?

ii. What is the shape of the Earth's orbit around the Sun? (Hint: Study the Grid closely.)

a. Circular

b. Slightly elliptical

c. Highly elliptical

iii. Now play the Planet and Moon simulation. What is the orbital period of the Moon around the Earth?

iv. What is the shape of the Moon's orbit around the Earth?

a. Circular

b. Slightly elliptical

c. Highly elliptical

v. Reset the simulation. Check the Pathbox. Play the Star, Planet, and Moon simulation. Which of the shapes best represents the Moon's orbit around the Sun?

vi. Which of the following images represents the relative motion of the Earth, Moon, and Sun?

vii. Reset the simulation. Check the Gravity Force box. Compare and contrast the motion of the Earth moving around the Sun with the Moon moving around the Earth.

viii. Please note that the length of the gravity force arrow corresponds to the relative magnitude of the gravitational force; the greater the force, the longer the arrow.

ix. Complete the following statement: "The gravitational force of the Earth on the Sun is __________ (less than / equal to / greater than) the gravitational force of the Sun on the Earth."

x. Complete the following statement: "The gravitational force of the Earth on the Moon is __________ (less than / equal to / greater than) the gravitational force of the Moon on the Earth."

b. the chart below provides six situations that change the gravitational force between objects. Observe what happens to gravitational force in each simulation by checking either the "Gravity Force Increases" column OR the "Gravity Force Decreases" column. After each situation, reset the simulation.

ACTION	Gravity Force Increases	Gravity Force Decreases
Bring Star and Planet closer together.
Increase Star Mass.
Increase Planet Mass.
Decrease Star Mass.
Decrease Planet Mass.
Put Star and Planet farther apart.

i. Complete the same situations in question (9) between the Earth and the Moon. How do those situations compare with what you observed between the Star and the Planet?

Reset the simulation. Check the Velocity and Gravity Force boxes.

ii. Complete the following statement: "The direction of velocity is always __________ (in the same direction as / perpendicular to / in the opposite direction as) the gravitational force."

iii. While the simulation plays, turn off Gravity. What do you observe?

Reset the simulation. Check the Velocity and Gravity Force boxes. Increase the Star Mass to 2, which is double the mass of our Sun. Click Play.

a. What do you observe?

iv. Reset the simulation. This time decrease the Star Mass to half. Play the simulation. What do you observe?