Lab 3 ASTR101

In this lab we will be exploring Kepler’s Laws and Orbital Motion using the “Gravity and Orbits” PhET simulation and the Newton’s law of universal gravitation using another simulation from PhET, the “Gravity force lab”. Use the links below to access the simulations, https://phet.colorado.edu/sims/html/gravity-and-orbits/latest/gravity-and-orbits_en.html https://phet.colorado.edu/en/simulation/gravity-force-lab . Look at the explanatory video via YouTube: https://youtu.be/m6e2y4fef1I Learning Objectives ___________________________________________________________ With this lab we will • Investigate the shape of planetary orbits, • Relate how planetary orbits link to Kepler’s first two laws of planetary motion and • Explore gravity and Newton’s third law of motion . Part I Kepler laws and orbits 1 Astro 101 Elena Campuzano Fabila Lab 3: Kepler’s Laws, Orbits and Gravity Lab 3

1. Is the orbit of a planet circular? - Press the TO SCALE option at the bottom of the screen with the star and planet chosen, see opposite image. - Turn on the path/grid option ON, see green circle . - Allow the planet to move through 360 o . - Turn on the measuring tape from the tool bar, green circle . - Measure the horizontal distance from the path line on the left of the star. Write the measurement in the table below. - Now do the same from the star to the path line on the right hand side. • What do you notice about the distances? Distance from left side is longer than right side ___________________________________________________________ • What does this data say about the orbit of the planet, discuss? ________ Orbit is not perfect because the dictance on both sides are not the same ___________________________________________________ ___________________________________________________________ 2. Linking planetary orbits to Kepler’s Laws? - Go to model - Click sun and planet - Press PLAY , blue circle (with path/gravity/ velocity/grid ON , green circle ) and leave for one cycle then PAUSE . • Screen shot trajectory 2 1475336635Kilometers 15326148 kilometers Distance (miles) Left side from path to star Right side from star to path

- Now increase the velocity of the planet to a large extent by extending the red ‘v’ arrow. • What happens to the planet in the orbit? ____ _ The planet's orbit expands to such an extent that the gravitational force can no longer keep it tethered, causing it to gradually move apart. ____________________ - Now decrease slightly the velocity of the planet by diminishing the red ‘v’ arrow, green circle . • What happens to the planet in the orbit? ____ The orbit is smaller _________________________________ • What is the shape of the orbit when increased? ________ Circular _________________________________________ ______ - Now decrease the velocity of the plant to a large extent by moving the ‘v’ arrow in. • What happens to the planet in the orbit? _____ Due to a significantly reduced orbit, the planet collides with the star, leading to a collision. ________________________________________________ __ • Now try to explain this in terms of v and gravity? ____ As the velocity (v) of the planet increases, the gravitational force weakens, resulting in the planet drifting away because gravity lacks the necessary strength to retain it. Conversely, when the velocity decreases (v decreases), gravity intensifies. ______________________________________________ _____ • Is the velocity constant throughout the journey? No 4

• Which one of Kepler’s laws does this relate to? _____ Kepler’s 2 nd law_________ - With the star and planet chosen and path/grid on, increase the size of the star by sliding the controller to 1.5/1.75/2.0, green circle . • What do you notice about the orbit? ________ Orbit is smaller when the star size gets bigger • What happens to the velocity on the path? _________________ Velocity increases as the planet is closer to the star ____________ - Now do the same but change the planets mass 1.5/1.75/2.0, blue circle . • What do you notice about the orbit? ______ It doesn’t change ______ • Why do you think the observation for your previous answer occurs? __________________ Is smaller than the star still _____________ Part II: Gravity Go to http://phet.colorado.edu/en/simulation/gravity- force-lab And select RUN Qualitative Observations. 1. Move the masses closer. When they move closer the force between them becomes ( Greater/Less/the same) 2. Move the masses further apart. When the masses move away the force between them becomes ( Greater/ Less/the same) 5