Making a Simple Machine

1What a simple machine really does

A simple machine does not do the work for you, and it does not make the stone lighter. The stone weighs exactly the same as before.

What a simple machine does is let you use your own push in a much easier way. That is all. And that is enough to move a stone that beat three grown men.

2Machine 1: the lever

A lever is a stiff bar that turns on one fixed point. That point is called the fulcrum. Say it: ful-crum.

Look again at what Gatete's uncle did:

• The long stick is the bar.
• The small rock under the stick is the fulcrum — the stick turns on it.
• The big stone he is lifting is the load.
• His push on the far end is the effort.

You already know a lever: a see-saw at school is a lever, and the bar in the middle is its fulcrum.

3The secret of the lever — a worked example

Uwase tries to lift a heavy stone with a stick that is 100 cm long. She wants to know the best place for the fulcrum.

Try 1. She puts the fulcrum right in the middle. Her side of the stick is 50 cm long. She presses down hard... the stone lifts, but it is heavy work.

Try 2. She slides the fulcrum close to her own hands. Now her side is only 20 cm long, and the stone's side is 80 cm. She presses with all her weight. The stone hardly moves! This is worse.

Try 3. She slides the fulcrum right up close to the stone. Now the stone's side is only 10 cm, and her side is a long 90 cm. She presses gently — and up comes the stone.

Do you see the rule she just discovered?

Put the fulcrum close to the load, and stand at the far end. The longer your side, the easier the lift.

Levers are everywhere: a crowbar, a bottle opener, a hoe, a pair of scissors, a wheelbarrow, the oar of a boat on Lake Kivu.

4Machine 2: the wheel and axle

Drag a full jerrycan of water across the yard. It scrapes and catches and fights you every step. That scraping is called friction — the rubbing between two things that slide past each other.

Now put the same jerrycan in a wheelbarrow. It glides.

Why? A wheel does not scrape. It turns on a rod called the axle, and it rolls. Rolling has far less friction than dragging.

Roll it, don't drag it. That is the whole idea behind carts, wheelbarrows, bicycles and every moto in Kigali.

5Machine 3: the ramp

One more, quickly. To load a heavy drum onto a lorry, you do not lift it straight up. You lean a plank against the lorry and roll the drum up the ramp. It is a longer journey, but a much gentler push. A ramp trades a hard, short job for an easy, long one.

6Now build it: your own lever

You need: a flat stick or a 30 cm ruler; a round pencil or a thin round stick; a stone; a small book.

1. Lay the pencil on the table. That is your fulcrum.
2. Lay the ruler across it, like a see-saw.
3. Put the stone on one end. That is your load.
4. Press the other end down with one finger. Up goes the stone!
5. Now test the rule. Slide the pencil close to the stone and press again — easy. Slide the pencil close to your finger and press again — hard, maybe impossible.
6. Write down what you found: "The closer the fulcrum is to the load, the less effort I need." You did not read that in a book. You proved it with your own finger.

7Now build it: your own wheeled cart

You need: a piece of strong cardboard (as long as your hand); 4 bottle tops; 2 straight thin sticks longer than the cardboard is wide; 2 short tubes of tightly rolled paper; tape; string; a stone to carry.

1. Plan. Draw your cart. Mark the body, the four wheels, the two sticks, the string.
2. Gather. Collect every part before you start building.
3. Wheels. Ask an adult to make a small hole in the middle of each bottle top. This part is sharp — it is an adult's job, not yours.
4. Axles. Push a stick through one bottle top, then through a second one, so a wheel sits at each end. That stick is now an axle with two wheels. Do the same with the other stick and the other two tops.
5. Fit them. Tape the two paper tubes under the cardboard — one near the front, one near the back. Slide an axle through each tube. Tape the tube to the cardboard, never the axle. The axle must be free to spin.
6. Handle. Tape a string to the front of the cardboard.
7. Test. Put the stone on the cart and pull. Then take the stone off and drag it along the same ground with your hand. Which one is easier?

8Fix it when it goes wrong

Your first cart will have a problem. Every first cart does. Do not throw it away — find the reason.

• A wheel will not turn? The hole is too tight for the stick, or the tape has stuck to the axle. Widen the hole a little, or move the tape.
• The cart tips over? The stone is too high or sitting at one end. Put it low and in the middle.
• The wheels fall off? Wrap a small ring of tape on the stick outside each wheel to hold it on.

Fixing is not failing. Fixing is what makers do.

Try it yourself

Answer each one in your head first.

1. Ishimwe is lifting a heavy log with a long pole. Should he put the fulcrum close to the log, or close to his hands?
2. What do we call the point that a lever turns on?
3. Mutesi must move a sack of potatoes from the gate to the kitchen. Which is easier: dragging it along the ground, or wheeling it in a wheelbarrow? Why?
4. Keza says, "My wheelbarrow made the potatoes lighter." Is she right?
5. Your cart is finished, but the back wheels will not turn at all. Name one thing you would check.

9Answers

1. Close to the log. Then his own side of the pole is long, and a long side needs only a small push. Close to his hands would make it much harder.
2. The fulcrum.
3. The wheelbarrow. Its wheel rolls instead of scraping, so there is much less friction fighting her.
4. No. The potatoes weigh exactly the same. The wheelbarrow only makes them easier to move — that is a different thing from lighter.
5. Any one of these: the hole in the bottle top is too tight for the stick; the tape is stuck to the axle instead of the tube; or the axle is bent.