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Use the Force! Two forces combine to keep our planets revolving around the sun; inertia and gravity. Without these forces we would fly into space traveling farther and farther from the sun! In this investigation we will discover how these forces combine to keep us at just the right place in space. Objective: Students will be able to define the forces of gravity and inertia in order to describe how they combine to maintain the orbits of the planet. Newton’s first law of motion states that an object in motion will stay in motion in the same direction with the same speed unless acted on by another force. This is known as the law of inertia. Inertia is the force resistant to change. For example, if you have ever had to push a heavy object, it is always hardest to get it moving. Once it’s moving the force of inertia helps to keep the object moving making it easier to push. Keeping that in mind, consider the following questions. 1. Raven’s quarterback, Joe Flacco, throws a pass to Todd Heap. However, the object in motion, the football, does not stay in motion with the same speed. Predict reasons why the football defies Newton’s first law of motion and write them below. 2. If Flacco threw the pass to Todd Heap in space 1 Everything that exists creates a force called gravity. Gravity is an attraction force or pulling force. However, we are not sucked into the gravity of lockers, students, or even buildings. Why do you think that is? The strength of gravity is dependent on two things, distance from the object and mass (weight) of the object. We can observe this on a scale. If we weighed an infant on a scale and a full grown adult we would expect that the adult would weigh more. That is because the more mass they have the stronger they are attracted to Earth. Similarly, all objects in space are pulled into our sun because they are too far away to be affected by our gravity. Observe the pictures and circle the object that would have the greatest amount of gravity. 1. 2. Circle the “X” that would be most affected by the flag’s gravity. X X X X 2 Our elliptical orbits are caused by a combination of forces, inertia and gravity. The sun’s gravity pulls the planets inward and the inertia of the moving planets pulls the planets in a perpendicular direction (see image). Neither force is great enough to overcome the other. Therefore, the planets are locked in orbit around our sun. Analysis 1. When researching our planets we noticed that the inner planets were primarily made of heavy rocks and metals while the outer planets were made of gasses. Using evidence from the investigation, explain why this may have happened. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ 3