Students will be able to explain what a gravitational field is and where they exist. Students will be able to describe how the gravitational fields change within a massive object. Students will be able to explain weightlessness.
Explanation of gravitational fields and how they change within the Earth. Using elevators we are able to increase our weight and decrease out weights as well as become completely weightless.
Fields are a concept in physics that explains the direction of unseen forces as they act on objects in the universe. More relevant to our studies are the gravitational fields that are created by any object that has mass. The Earth is a good example of an object that has a gravitational field.
The image on the left shows an illustration of the field lines that surround the Earth. The lines are infinite in number and completely cover the surface of the planet. Each of the lines point towards the center of mass which is the center of the Earth and have no starting point because they can stretched out to the end of the galaxy. Objects that are further from the Earth will feel less of an affect from gravity but no matter how far away it is it will still be feeling some force from the mass of the Earth. The further away from the Earth you get the less dense the field lines become. The only way to not feel the affects of gravity from Earth is to be at its core.
If you were able to drill a whole through the Earth (and assuming it was actually uniform in shape and density) and then drop a ball through the hole it would fall through the core only to pop up the other side at the exact same height you dropped it from. The ball would continue to fall through the earth and pop out again, without something to slow it down, it would continue to fall forever.
If you sitting in a cave at the center of the Earth you would not feel any forces of gravity. When sitting at the center of a uniform mass you are pulled in all directions at once so the resulting force is canceled out.
What is weight? When you talk about how much something weighs you are actually describing the force that gravity exerts on an object. The term weight is often used incorrectly as a description or assessment of the amount of mass an object has. You may say that the Earth has a weight but then you would have to describe that the weight is being generated by or measure against.
When you stand on a scale your body is being pulled towards the ground and the amount of mass that you have will affect the amount of force. So people who have more mass are said to weigh more. When we talk about weights and use the units of pounds to describe your weight it is actually a way of quantifying the forces due to gravity on your body in simpler terms.
Imagine that you step into an elevator with your favorite household scale. You set the scale on the floor and get on. For a person who weighs 170 lbs the scale would read 170 lbs. When you press the button to make the elevator go to the lobby you notice that the scale now only reads 150 lbs and then after a moment returns to 170 lbs. What just happened here?
When the elevator began to accelerate towards the ground floor you began accelerating in the same direction as gravity. So if the elevator does some of the work for gravity the scale believes you weigh less than you actually do but then the elevator reaches maximum velocity so the scale returns to normal. Now that you are on the bottom floor you press the button for the top story of the building. You read the scale as the elevator begins its accent and discover that you now weigh 190 lbs. You are traveling in the opposite direction as gravity and the elevator is resisting the forces already pulling on you which is why you feel heavier. As the elevator reaches maximum velocity the scale again returns to your normal weight.
After you reach the top floor the cable for the elevator snaps and you begin falling. Now the scale reads zero. You do not have a weight as you fall because you are traveling with gravity and are not resisting any of the forces. You do not feel the forces as they act upon you, that is... until you get to the lobby again.