Episode 3 ·
Why Does a Spring Snap Back? Hooke's Law
Peko asks and Nora explains: deformation, elastic force and Hooke's law, made simple.
🔬 Today's friend: Nora
The lesson
Peko stretched a metal spring, let go, and it snapped back and tapped his cap. "Ouch! Nora, why does it snap back?" Nora smiled and said they would find out together.
First, a new word. When an object changes its shape, we call that deformation. Squash clay and it stays squashed: that is plastic deformation. Stretch a spring and it returns to its shape: that is elastic deformation.
So what pulls the spring back? The elastic force. It always acts against the stretch. If you pull the spring to the right, the elastic force points to the left.
The further you stretch the spring, the stronger this force becomes. Stretch it twice as far and the force doubles. This rule is called Hooke's law, and it is written F = −kx.
Each letter has a job. F is the elastic force. x is how far the spring has stretched. k is the stiffness: the higher it is, the harder the spring is to stretch. The minus sign tells us the force goes the opposite way from your pull.
Then they tested it. Nora hung a 1 kilogram weight on a spring. The weight pulled down, the spring pulled up, and when the two forces balanced, the weight stopped. The spring had stretched almost 5 centimetres.
Peko guessed that 2 kilograms would make it four times longer. It was only twice as long, almost 10 centimetres. A spring twice as stiff would stretch only half as far.
But there is a limit. Stretch a spring too far and it will not come back, just like the clay. This same physics is at work in trampolines and spring scales. As Peko says: the more you pull, the harder it pulls back!
💛 What we learn
The more you stretch a spring, the harder it pulls back.
💬 Talk about it together
- What is the difference between how clay and a spring change shape?
- If you pull a spring to the right, which way does the elastic force point?
- What happens to the force when you stretch a spring twice as far?
- Where at home or at the playground can you find a spring?
🔬 Did you know?
Robert Hooke first published his idea in 1676 as a scrambled puzzle of letters, so that nobody could claim it. Two years later he revealed the answer, a Latin phrase meaning "as the extension, so the force".
✏️ Try it at home
Ask a grown-up for a rubber band, a small bag, some coins and a ruler. Hang the bag from the rubber band and measure how long the band is. Add the same number of coins again and measure once more. Did the stretch double? Stop before the band is pulled very tight.
What Peko says in the video
Ouch! Nora, why does it snap back?
Good question, Peko! Let's find out.
When an object changes shape, that's deformation.
Does clay deform too?
Yes, but it stays squashed. That's plastic deformation.
A spring returns to its shape. That's elastic deformation.
But what pulls it back?
The elastic force. It acts against the stretch.
Pull the spring right, and the force points left.
What if I stretch it more?
Stretch it twice as far, and the force doubles.
That's Hooke's law: F equals minus k x.
What's x?
How far the spring has stretched.
And k?
Stiffness. The higher it is, the harder it is to stretch.
Why the minus?
The force goes the opposite way from your pull.
Let's test it!
Let's hang a weight on the spring.
The weight pulls down, the spring pulls up.
The forces balance, and it stops.
It stretched almost five centimetres.
Two kilograms… so four times longer?
Let's see!
Only twice as long! Almost ten centimetres.
A spring twice as stiff stretches half as far.
But stretch it too far, and it won't come back.
Just like the clay!
That's how trampolines and spring scales work.
Got it! The more you pull, the harder it pulls back!
What if we join two springs? Find out next time!
