Make a Balance
Learn how you can use a plastic hanger and some paper cups to make a balance and compare the mass of different objects!
What You Need:
- Plastic coat hanger
- String or yarn
- Two paper cups
- Wooden skewer
What You Do:
- Ask an adult to carefully poke two holes in each cup using the wooden skewer. The holes should be a little below the cup's rim and directly across from each other.
- Cut two pieces of string about two feet long. They need to be the same length.
- Make a cup handle: Use the skewer to push an end of one piece of string through one of the holes in one cup. Tie the end in a knot so it is securely attached to the rim of the cup. Push the other end through the hole on the opposite side of the cup and tie it.
- Do the same thing with the other cup and piece of string. The loops of string on each cup need to be exactly the same length so that the cups will hang evenly on your scale. Before you tie the second side, check to make sure they are the same.
- Find a place to hang your scale. You need a place where it can hang freely without bumping into anything. A shower curtain rod works well. Have an adult tie a piece of string to the hanger's hook and tie the other end around the curtain rod so that the hanger is easy for you to reach.
- Now hang the loop of each cup on one of the small clothing hooks on each side of the hanger. The hanger should be balanced and the cups should hang down at equal levels on each side.
- Now you can experiment with your balance! What happens if you add an object to the cup on one side but not the other? Can you find an object to put in the other cup that will make the cups balance again?
You just made a balance. You can use it to compare the the mass of different objects. 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.
How does the balance work? Since the paper cups are the same size and made from the same material and the strings you used were the same length, the hanger balanced evenly because each side had the same mass. Notice that if you take one cup off, the balance tips so that the side without the cup goes up in the air! That's because the mass from the other cup is pulling down on the hanger. When you put the cup back on, the hanger is balanced again and the cups are level.
If you place a quarter in the cup on the left, the balance tips. The coin adds more mass to the left side, so it tips down and the right side with the empty cup goes up. If you put a dime in the cup on the right, its mass will push the cup down. It has less mass than the quarter, though, so the right side will still be higher. If you add a penny to the cup on the right, the mass will change even more and the balance will move again. This time the cups should balance. Now the mass in each cup is the same (or almost the same) and the cups balance each other again. A quarter has the same mass as one dime plus one penny! You can compare the mass of lots of different objects with this balance.
Do you know the difference between mass and weight? Mass is a measure of how much matter is in an object, but weight is a measure of how much gravity is pulling on the object. Gravity is a force that affects us all the time. (You'll learn more about it later. For now, you just need to know that there is less gravity on the moon than there is on Earth.) When you stand on a bathroom scale, it tells you how much you weigh. It doesn't tell you how much mass you have. A scale measures how much force is pushing down on it. When you stand on it, it measures how much gravity is pulling down on you while you are pushing down (in other words, standing) on it. If you could go to the moon and stand on your bathroom scale, you would find that you weigh much less than you do on Earth, because there would be less gravity pulling down on your body as you stand on the scale.
So, what do you think would happen if you could use your hanger balance in space, where there is less gravity pulling on objects? Since your balance only compares the mass of objects, not their weight, you would get the same results on the moon as you do on earth! Even on the moon, a quarter on one side of your balance would still have the same mass as a dime and penny on the other side of the balance. The coins would weigh less on the moon, but their mass would not change! Objects still have the same mass--amount of "stuff" in them--no matter how much or how little gravity pulls on them.
Ball vs. Feather
Which object do you think will fall to the ground faster, a ball or a feather? Test it out and learn why with this experiment.
What You Need:
- a small ball
- a feather or a tissue
- two sheets of paper
What You Do:
- Hold the ball in one hand and the feather or tissue in the other.
- While standing up, hold your arms out in front of you with the backs of your hands facing up.
- Open both of your hands at the same time and watch the objects fall. Which one reaches the floor first?
- Now try dropping the ball and a sheet of paper (hold your hand flat under the paper and then pull your hand out to let it drop). Which one makes it to the floor first?
- Crumple one sheet of paper into a ball. Drop the paper ball and the full sheet of paper at the same time. What happens?
- Now drop the ball and the paper ball at the same time and notice what happens.
Even though you dropped both objects from the exact same height, the ball hit the ground much sooner than the feather (or tissue). You probably found that the ball also reached the floor before the sheet of paper. Can you explain why? In step 5, you probably found that the paper ball hit the floor several seconds before the sheet of paper did. Both pieces of paper had the same mass, so why did one get to the floor before the other? You can try it again if you like, to see if you can get the sheet of paper to reach the ground at the same time as the ball of paper, but you will find that the ball always gets there first! The results of step 6 might have surprised you even more. The paper ball reached the floor at the same time as the regular ball! How is that possible?
Even though it seems like heavier objects (or objects with more mass) would fall to the ground fastest, that isn't always true. Mass and weight do not determine how quickly an object will fall to the ground. It's easy to think that the ball will fall first because it has more mass. To understand this, you need to know what makes objects fall. The force of gravity is what causes objects to fall. If you throw or kick a ball into the air, it will eventually come back down, because gravity is pulling down on it. All objects actually fall at the same speed, because gravity pulls on them equally, no matter how heavy they are! That explains why the paper ball and the regular ball landed on the floor at the same time, but why didn't the sheet of paper, tissue, or feather fall as quickly?
Well, it turns out that objects will only fall at the same speed if no other force is acting on them. So they started out falling at the same speed, but after falling a few centimeters, the air started pushing up against the objects just as gravity was pulling down on them. Since a ball is round and smooth, the air couldn't resist it very much and the force from gravity that was pulling down on it was still stronger than the force of the air pushing up against it. However, the feather, tissue, and paper were affected by air resistance. Air that was caught underneath the objects pushed up against them and their fall was slowed down.
The shape of an object has a lot to do with how much air resistance will affect it. Think about a parachute falling to the ground. Why do you think it falls to the ground slowly enough to keep a person from getting hurt when he or she lands on the ground? It's because of the air that gets caught under the parachute and pushes back up against the force of gravity that is pulling it down. The air actually slows the parachute down as it is falling!
To learn more about mass, weight, and gravity, visit this Teaching Tip.