Gravity

 

If you push something, the law of inertia says it should keep moving, but friction stops it. If you hold something in the air and let go, the law of inertia says it should stay there, but it falls. The reason it falls is gravity. Gravity is the force that pulls things toward Earth.

If something is in free fall, it has no forces acting on it except for gravity. Gravity is an unbalanced force. Because unbalanced forces can cause an object to accelerate, an object in free fall accelerates. Every second something is in free fall, it accelerates by 9.8 meters per second. So if an object free falls for two seconds, it would be traveling at 19.6 meters per second.

A demonstration of how air resistance can affect how fast a piece of paper and a ball fall to the ground

If a piece of paper and a marble are in free fall, they will fall at the same speed, so they should hit the ground at the same time. If you test this by just dropping a marble and a piece of paper you will find it is not true. This is because the objects are not in free fall. To be in free fall, gravity has to be the only force acting on the objects. When you just drop something, there is also air resistance. Air resistance is a type of fluid friction. Because friction acts in the opposite direction of the object’s motion, air resistance of an object falling downward is an upward force. This is because a falling object is coming down, so the opposite direction is up. If air resistance were equal for every object, objects would still fall at the same rate. Since we know they do not fall at the same rate, we know air resistance is different for different objects. The amount of air resistance acting on an object depends on the object’s surface area. If an object has a small surface area, it will have little air resistance. Because the piece of paper has a larger surface area than the marble, the marble will have less air resistance than the piece of paper. Just because an object has more air resistance than another object doesn’t mean it will fall slower. This is because of weight, the force of gravity on an object at the surface of the planet. Some people confuse weight and mass, they are different. Mass is how much matter is in an object, and weight is the force of gravity on an object. If the object with the high air resistance had a high weight, it might fall faster than the object that has little air resistance, like if you had a wrecking ball and a piece of paper. Even though the wrecking ball had more air resistance than the piece of paper, it would fall faster because the wrecking ball weighed more than the paper.

Terminal velocity

As the speed of a falling object increases, the air resistance does too. If an object falls long enough, the force of the air resistance will equal the force of gravity. Now the forces are balanced so the object will stop accelerating. When this happens it is called the terminal velocity. If a skydiver jumped out of a plane, he would start accelerating. After falling a while, the force of air resistance would equal the force of gravity. The forces are balanced, so the skydiver stops accelerating. This does not mean he will stop falling, he will just stop continuously gaining speed.

Universal Gravitation

Object

Weight in Pounds
Our Sun 26,933
Mercury 35
Venus 86
Earth 100
Mars 38
Jupiter 263
Saturn 111
Uranus 106
Neptune 141
Pluto 13
Our Moon 17
The force of gravity acts between all objects. This is the law of universal gravitation. This means you are attracted to everything else in the universe. You don’t notice it because the force is not very strong. The strength of the force depends on the masses of the objects. You are attracted to Earth because Earth has such a great mass. There are other things that have great masses, such as the moon and the sun. Why aren’t you attracted to these? The reason is the force of the gravity also depends on the distance between the objects. If you were on the moon, you would be close enough for it to attract you. Still the force of gravity on the moon is about 1/6 of what it is on Earth. On the moon you would weigh about 1/6 of what you do on Earth. Because the different planets have different masses, you would have a different weight on each.

Force Newton's First Law Newton's Second Law Friction Gravity Newton's Third Law Momentum

Introduction
 
Space in the Spotlight
Novi Meadows Elementary 2002

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All pictures courtesy of NASA unless otherwise noted