⚖️ Change, measure, keep the same: the variables card

What you change

In the ramp project you change how high the ramp is. You try it at 3 levels: 1 book high, 2 books high and 3 books high.

Scientists call this the independent variable. It's the one you choose, on purpose, before each try. In a fair test, only one thing gets changed.

What you measure

You measure how far the car rolls, in cm. That's the number that goes in your results table, one for every try.

Scientists call this the dependent variable, because it depends on what you changed. You don't choose it. You let go, wait and see what it does, then write it down straight away.

Measure it the same way every single time: from the bottom of the ramp to the front of the car, with the tape flat on the floor. If you measured to the back of the car on some tries, the numbers would wobble for a reason that has nothing to do with the ramp.

What you keep the same

Here is everything the ramp project keeps the same:

• The same car.
• The same floor.
• Letting go without a push.
• The same start line on the ramp.

Scientists call these the controlled variables, or the controls. They're what make the test fair. If you used a different car at each height, and the car on 3 books went farthest, was it the ramp or the car? You'd never know.

Letting go without a push matters more than it looks. A tiny push on one try adds speed the ramp didn't give, and that one number can fool you.

The card, filled in

On the hypothesis sheet, the variables card is three boxes. For the ramp it reads:

• I change: how high the ramp is.
• I measure: how far the car rolls, in cm.
• I keep the same: the same car, the same floor, letting go without a push and the same start line on the ramp.

Fill in the card before your first try, not after. It's your plan, and it stops you changing something by accident halfway through. Print it blank, or filled in from your project, from the hypothesis sheet.

Check yourself

Two quick checks catch most mistakes before you start:

• Say what's different between your first try and your last try. If the answer is more than one thing, fix it.
• Point to the number you'll write down. If you can't say its unit (centimetres, seconds, pennies), you haven't decided what to measure yet.

Older students test a second variable, but never at the same time as the first. The ramp project's Going further line says how: "Change a second thing: run the 3-book ramp onto carpet and onto a hard floor, and draw both lines on one graph."

Judges ask about your variables more than anything, so practise saying all three out loud with the judge's questions. Then try the card on another project, like Which floor makes a ball bounce highest?.