# Physics is Phun! – Gravity and Mass

Introduction

I’m going to have a series of posts about some science classroom activities starting with this post. Most of these activities are things you could do at home with your kids, but some require a large group of kids to do the demonstrations. I’ve tried to direct this at year 3 and 4 (third and forth grade) students, but I think many of these would be fun for older students as well. You could make some of them a bit more involved for older students. For this age group, my main goal is to introduce these concepts and let the kids explore them through activities. Even if the kids don’t completely understand the concepts, hopefully they will have some memory of the activities when they are older and it will help them grasp the concepts at that time.

Forces

There are 4 main forces that act in the universe: gravity, elecromagnetism, the strong force, and the weak force. I am going to address all of these in a series of posts. This first post focuses on gravity and mass. The material in this post took about 30 minutes to get through including time for the kids to put information in their notebooks.

Gravity

First I showed a picture of the solar system and explained what it is. The picture was a cartoon looking down on the solar system so that it resembled the Bohr atom diagram. (I used this type of picture because I will have one class about atoms with them drawn like that.) Then I said that the sun pulls the planets toward it. That force is gravity.

Mass Bends Space

Often gravity is described as mass bending space. This is something Einstein hypothesized and it is a little abstract to think about. I used this video about gravity to explain it. Instead of just playing it though, I turned the sound off an only played (:34-1:12) and (1:45-1:59) while I narrated it. This is a really cool video, but it is too involved for little kids to watch the whole thing in class. Here is what I said about the video.  “See that when the ball is placed on the fabric, it bends it down? When two balls are placed on it they both bend it and then roll together. Now look what happens to the fabric when this other object is placed on it. Do you see that the fabric bends down more? Look how the marbles fall to it. Now can you see that the marble spins around the center object, just like the planets orbit around the sun.”

Mass

Now refer to the second object and ask why it pulled the fabric down more. The answer is that it has a greater mass than the balls, and objects with a greater mass pull objects toward them more strongly. But what is mass?

mass = density x volume

I asked the kids if they had heard the word “massive” before and asked them what they thought it meant. Then I told them that mass did have to do with size, but also with density.

Volume

Volume is fairly easy to explain as the amount of space an object takes up. You can measure it with a ruler for solids or a container for liquids.

Density

Density is a little trickier. I explained this by using two jump ropes to form a circle on the floor. I called up a few kids to walk around in the circle, then I walked through it. Then I called a few more kids up and had them all walk around the circle, and I walked through it again. Finally, I called up enough to pack the circle, so I couldn’t walk through. The circle became increasingly more dense. I explained that the full circle was like a solid. Then I had some kids sit down and explained that the density of those left was like a liquid. I has a few more sit down and said this was like a gas.  Now you have discussed  solid-liquid-gas densities and reminded the students that air is matter as well as liquids and solids. You have also directed them for the next task when they need to order all 3 states by density.

Activities

In the next part of this you can have kids break up into little lab groups or demonstrate it by calling a few kids up to do this in front of the class. I did these activities with a class of 60 kids so breaking up into small groups was a bit challenging with this large of a group. You can have any items for this, but try to get things that are obviously ordered by volume and density. One should be a blown up balloon and another should be a bag of water. The solid should be something dense like a marble or a rock. Make sure they can all be ordered easily by volume as well. The last comparison should be two solids. Try to find something that has air in it like styrofoam, a sponge or a wooden block that is about the size of the marble or rock that you used in the previous comparison.

Increasing Volume

If you print out pictures of the objects the kids can cut them out and paste them in the correct order in their notebooks. Have the kids order the pictures by increasing volume. Then call a few kids up to the front of the class to put the objects in the correct order and talk about if it is correct. Then have kids paste them in their notebook. Then do the same for density.

Increasing Density – Gas, Liquid, Solid

Increasing Density – Solids

For the last part, I wanted to make the point that density can vary among members of the same phase of matter. You could do this with liquids if you had a very viscous one verses water, but I was trying to avoid making a mess in class. Instead I decided to compare wood and a marble. I found a block and a marble that were approximately the same size. The kids will need to hold these in their hands to feel the difference.

In a separate class I discussed differences in the mass of air molecules after I introduced atoms and molecules.

Weight vs. Mass

If you have time at the end of class, you can talk about weight vs mass. This is a concept that commonly confuses students, so I thought introducing it early will help them learn the distinction eventually. An object’s mass will be the same no matter where it is (as long as it isn’t destroyed like it would be if it got too close to the sun for example), but the weight will vary depending on the object’s location. Weight is the measure of the force of gravity on an object. The more massive the body is that pulls on an object, the higher the weight will be.  An object will weigh less on the moon than on the Earth because the pull of gravity is less on the moon. You can remind kids about the astronauts bouncing around on the moon so they can get an idea of what that would mean. An object’s weight on Neptune would be higher than on Earth because Neptune has a greater mass than Earth.

For more STEAM activities, take a look at my Pinterest board.

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