Science With Mr. Milstid

Let’s start off with something neat:

What exactly is gravity then? We experience it every day – take it for granted even – but most of us aren’t really all that familiar with the facts about it.

So how about a definition:
Gravity: the force that causes two objects to pull towards each other.

Something to note is that all objects have gravity. Even you attract other objects to you because of gravity. But you have too little mass for the force to be very strong.
Gravity only becomes noticeable when there is a really massive object like a moon, planet or star involved.

Also important to note is that gravity is predictable.
In small scale physics, we work off a law of gravity that predicts how it behaves.
This law, called the Law of Universal Gravitation, was created by Isaac Newton and is incredibly simple for tackling such a remarkable force as gravity.

The Law of Universal Gravitation states:

• Gravity increases as the masses of the objects involved increases
• Gravity decreases as the distance between the objects increases

Basically, this law states that larger objects have more gravity, and the further away from an object you get, the less you feel its gravity.

For instance:
The Earth has more mass than the Moon, so there is less gravity on the moon than on earth.
The gravitational force is greater on the Earth’s surface than it is in space, high above the Earth, so the Earth’s gravitational force pulls objects towards the center of the Earth.

Another important thing to note about gravity is that, it not only pulls things toward each other, it also affects their behavior and certain characteristics.

Gravity affects weight:
Weight is simply a measurement of the amount of force pulling an object downward – or gravity. Gravity affects weight in (surprise!) predictable ways. As the gravity on an object increases, its weight increases; as gravity decreases, weight decreases.

So if you move to a planet on which there is only 1/5 the gravity of earth, your weight will only be 1/5 of what it was when you left earth.

A fun example of how gravity affects weight can be found here: Weight on Other Worlds.
*Note that, if you can observe how gravity changes on another world (as in the website linked above), you can also infer characteristics of that world, such as mass and the relative amount of gravity.

One important side note to the fact above is that gravity does not affect the mass of an object. No matter where you are in the universe in any given moment, your mass remains the same.
This means that if you were able to be immediately transported to the world mentioned above (where your weight would be reduced to 1/5 of its original weight on earth), you would stay the same size. Your mass – or the amount of matter that you are composed of – would remain constant.

Another important concept to master is that gravity is constant for any object in a given environment. In other words, gravity pulls on all objects by the same amount.
So, neglecting air resistance, all objects fall toward the ground at the same rate.
A famous example of this is Galileo’s hammer and feather hypothesis: if you were to drop a hammer and feather at the same time, they would be accelerated toward the ground at the same rate.

Proof!


Clearly this doesn’t work on earth, where air resistance opposes the motion of falling objects, but it does prove that gravity acts on all objects in a similar fashion.

So what is this constant rate?
All objects are accelerated downward due to gravity at a rate of 9.8 m/s². So then, for every second that an object falls, its downward velocity increases by 9.8 m/s.

This rate is shown in the table below:

We can use this constant rate to determine how fast, and how far a falling object moves.

To determine the velocity of a falling object, we simply multiply the constant for gravity (9.8 m/s²) by time(s).
Or: v = g*t

To determine the distance a falling object travels, we multiply 1/2 the constant for gravity (.5*9.8 m/s², or 4.9) by the time of the object’s fall squared (t²).
Or: d = .5g*t²

An interactive example of calculating the distance of falling objects is shown below:

Posted in Forces & Motion, Physics & Physical Science