(FOM) Gravitational Forces Lesson

Gravitational Forces

Let's look a little closer at gravity. It is important to note that every object in the universe attracts every other object in the universe.  However, the force of gravity is a weak force and is dependent on both the mass of the object and the distance between the objects.  Gravity is most clearly seen with very large massive objects such as the Earth.  It is important to know that the more massive the objects the more gravitational force each will experience. In addition, the closer the objects are to each other the more gravity they will experience.  Whereas, the further the objects are apart the less gravitation force they will experience.  

Acceleration Due to Gravity

It's a good thing this mountain climber's safety gear is working because it's a long way down to the ground! If he were to fall, he'd be moving really fast by the time he got there. The higher any object starts falling from above Earth's surface, the faster it's traveling by the time it reaches the ground. Do you know why? The reason is gravity.  

As stated above, gravity is a force that pulls objects down toward the ground. When objects fall to the ground, gravity causes them to accelerate. Gravity causes an object to fall toward the ground at a faster and faster velocity the longer the object falls. In fact, its velocity increases by 9.8 m/s², so by 1 second after an object starts falling, its velocity is 9.8 m/s. By 2 seconds after it starts falling, its velocity is 19.6 m/s (9.8 m/s + 9.8 m/s), and so on. The acceleration of a falling object due to gravity is illustrated in the picture below.

In summary, when an object is dropped, it accelerates toward the center of Earth.  Galileo was instrumental in showing that, in the absence of air resistance, all objects fall with the same acceleration (which is denoted by a constant g).  Consider an object with mass falling downward toward Earth. It experiences only the downward force of gravity.  Knowing the gravitational acceleration constant and the mass of the object can provide us with the weight.  To calculate weight, the following equation is used:

w = m x g  

(On Earth, the gravitational constant (g) is 9.8 m/s²)

What if you were to drop a bowling ball and a soccer ball at the same time from the same distance above the ground? The bowling ball has greater mass than the basketball, so the pull of gravity on it is greater. Would it fall to the ground faster? No, the bowling ball and basketball would reach the ground at the same time. The reason? The more massive bowling ball is also harder to move because of its greater mass, so it ends up moving at the same acceleration as the soccer ball. This is true of all falling objects. They all accelerate at the same rate due to gravity, unless air resistance affects one object more than another. For example, a falling leaf is slowed down by air resistance more than a falling acorn because of the leaf's greater surface area.

When the net external force on an object is its weight, we say that it is in free fall. That is, the only force acting on the object is the force of gravity. In the real world, when objects fall downward toward Earth, they are never truly in free fall because there is always some upward force from the air acting on the object.  However, in true free-fall (neglecting air resistance) all objects accelerate to the Earth at a constant rate of 9.8 m/s².  Now, the value of g decreases the farther away from the center of Earth an object gets. This means the weight of an object would decrease if it was placed on top of a mountain or put into space.

What happens in the presence of air resistance, as is most commonly the case on Earth? When an object accelerates toward the Earth due to gravity, air resistance is opposing the motion.  Eventually, the downward pull of gravity equals the upward force of the air resistance.  When this occurs, there is a balanced net force which means that the object is no longer accelerating. The object will remain going at a constant speed toward the Earth.  This final, constant velocity is called terminal velocity.

Weight vs. Mass

Mass and weight are often used interchangeably in everyday language. However, in science, these terms are distinctly different from one another. Mass is a measure of how much matter (how much "stuff") is in an object. Mass is typically measured in kilograms. Weight, on the other hand, is a measure of the force of gravity acting on an object. Weight is equal to the mass of an object (m) multiplied by the acceleration due to gravity (g). On Earth's surface, the gravitational field strength is a constant of 9.8 m/s². Like any other force, weight is measured in terms of Newtons. Assuming the mass of an object is kept intact, it will remain the same, regardless of its location. However, because weight depends on the acceleration due to gravity, the weight of an object can change when the object enters into a region with stronger or weaker gravity. For example, the acceleration due to gravity on the Moon is 1/6 that of the gravity on Earth.  If you measured your weight on Earth and then measured your weight on the Moon, you would find that you "weigh" much less, even though you do not look any skinnier. This is because the force of gravity is weaker on the Moon. In other words, a 180-pound man would only weigh 30 pounds on the Moon.  In fact, when people say that they are "losing weight," they really mean that they are losing "mass" (which in turn causes them to weigh less).  

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