Pick up a book and then drop it on the floor. Why did it fall? It fell because of gravity. Gravity is a very strong force that pulls on objects. All objects experience the force of gravity pulling down on them all the time. On the earth, gravity pulls things down towards the center of the earth. It doesn’t push things up, only down. When you throw a ball in the air, it might travel up for a little while because of the force of your arm throwing it, but eventually the force of gravity will pull it back down to the ground. The amount of gravity that pulls on objects on earth always stays the same. However, on the moon, in space, and on other planets, the pull of gravity is different. The amount of gravity that pulls on objects on the moon is much less than the amount of gravity that pulls on objects on the earth. Each planet has its own gravitational field that pulls objects towards its center.
If the book hadn’t hit the floor, what do you think would have happened to it? Would it have kept falling? An object will continue to fall and be pulled closer to the center of the earth until it is stopped by something, like the floor or a table. Even once it stops, gravity is still pushing down on it, but there is nowhere else for it to go, so it stays still.
Since the earth is round, have you ever wondered why we don’t fall right off of it? What keeps you from floating through the air when you jump up? It’s because of gravity, which pulls us constantly towards the earth, keeping us from floating away into space! Gravity keeps our feet on the ground so we can walk instead of floating randomly through the air. It is also what allows rain and snow to fall to earth.
Gravity helps us in a lot of different ways. It is what keeps objects in place when we set them down, which allows us to eat and drink without making much of a mess. Gravity keeps food on our plates and liquids in our cups. Eating and drinking in space where there is less gravity can be quite an interesting experience! You can learn more about food in space here.
Mass and Weight
All things are made up of matter. Mass is a measure of the amount of matter that an object has, or how much “stuff” it is made up of. Weight is a measure of how much gravity pulls on a mass or object. On the moon, there is less gravity pulling on objects, so they weigh less. For example, a rock that weighs one pound while on Earth will weigh less than that if it is taken to the moon. But does its mass change just because it is on the moon? No, it is still the same size and looks the same, it just has less gravity pulling on it, so it weighs less. That is why objects that seem heavy to us on Earth end up being light enough to float around if they are taken into space.
Something similar happens in very high places, like tall mountain peaks. There is less gravitational pull in high places, so you would weigh a tiny bit less on the top of Mt. Everest (the highest mountain peak in the world), but your mass would still be the same! This is because the peak of a mountain is higher and farther away from the center of the earth, so the pull of gravity in that spot is weaker than at the bottom of the mountain. Remember, even if you weigh less because of a change in gravity’s force on your body, your body’s mass is still the same.
As your body grows, you will have more mass, which also means you will weigh more. That’s because when you’re on the earth, the amount of gravity that pulls on you stays the same. So when your mass changes, so does your weight!
Mass– the amount of matter than an object is made of. Mass does not change with gravity.
Weight– the amount of gravity acting on (pulling down on) an object (or mass).
Gravity– a natural force that pulls objects downward. Earth’s gravity pulls us and all objects downwards towards its center.
For science projects to use with this topic, visit our Mass & Weight Science Projects page.
Click here for a worksheet about gravity and weight.