You climb the last few rungs of the ladder and step inside. “Wow, is this cool!” Your friend is behind you. “I told you so.” It’s your first time in your friend’s tree house. It has just about everything. Two comfy chairs, a table, a bookshelf, games, even a TV.

You plop down in a chair. Then you realize that you’re hungry. “Got any food up here?” “I knew I forgot something,” your friend says.

Just then, your friend’s grandmother comes out of the house carrying two lunch bags.

“Yay, Grandma, you read our minds! Now, if only there was a way to get the bags up here.” “Hmm . . .,” says Grandma. She stares at a thick branch near the doorway of the tree house. “I have an idea. I’ll be right back.”


A few minutes later, Grandma comes back with a rope and a bucket. She ties a big knot in one end of the rope.

She swings the knotted end a few times and lets it fly. The rope sails over the branch. The knot dangles just a few feet above the ground. She undoes the knot and ties the end to the bucket handle.

“Oh, I get it!” you say. “What a great idea!” Grandma places the lunch bags in the bucket. She pulls the other end of the rope. With each pull, the bucket rises closer to the tree house.

At the top, it’s easy to reach out and grab the bucket . . . and lunch.

It's Simple

Grandma used the rope and branch as a simple machine. Simple machines move things faster, farther, or more easily than we can without them.

Simple machines have no more than a few parts. Some are made of only one part. Combined in different ways, they make work easier.

In the Groove

How can a rope and a branch be a machine? Together, they form a pulley. A pulley is a wheel with a cord wrapped around it. The cord might be a rope, a chain, or even a steel cable. The cord fits in a groove around the wheel. That way, the cord doesn’t slip out. The wheel turns as the cord is pulled through the groove.

The branch doesn’t have a groove. And it doesn’t turn like a wheel. But the idea is the same. Grandma pulled down on one end of the rope. Then the other end moved up. The pulley made it easier to lift the bucket. It changed the direction of the force. A flagpole works the same way.


Pull down on this end of the rope.


The rope moves through the pulley.


The flag moves up with this end of the rope.

How to raise a flag with a pulley