Lesson 2 of 12 · Forces
Newton's Second Law: F = ma
🏋️
🎯 Your mission
Set it up. Test it. Be amazed.
⚡ The twist
Energy never disappears — it just hides somewhere else.
What you'll learn
The resultant force on an object equals its mass times its acceleration. Doubling the force doubles the acceleration; doubling the mass halves it. Force is measured in newtons, where one newton accelerates one kilogram at one metre per second squared.
Key words
newton
The SI unit of force; 1 N accelerates 1 kg at 1 m/s².
mass
The amount of matter in an object, measured in kilograms.
Make a hypothesis first
Before you start: which will move faster, and why?
Say your guess out loud — then find out if you were right.
Get ready
🧰 What you need
- dynamics trolley or toy car
- spring balance
- masses
- stopwatch
- metre rule
Get ready
⚠️ Safety first
- Keep fingers clear of the trolley's path and stop it at the bench edge.
Step 1 of 4 · Read
Think about what you just learned: say what "newton" and "mass" mean in your own words before you start.
Step 2 of 4 · Do
Pull a trolley along a bench with a spring balance held at a constant reading and time it over a metre. Repeat at double the force, then repeat at the original force with a second mass on board. Tabulate force, mass and acceleration.
Step 3 of 4 · Observe
Record what actually happened, including anything that did not match what you expected.
Step 4 of 4 · Record
Write a conclusion that answers the question this lesson opened with, and state your uncertainty.
Question 1 of 2
Question 2 of 2
Mind = blown
🎢 You feel weightless on a roller coaster because you and the seat are falling at the same speed.
Nice work!
You got 0 of 2 right.
Next Smart Experiment
We'll pick an experiment that matches exactly how you're thinking right now.
✨ More to explore
🌍 Where you see this in real life: You use this every time you flip a light switch, ride a bike, or kick a ball.
🏆 Stretch challenge (try it this week): Time how long a small ball rolls down something, then change the slope and try again.
👨👩👧 For the dinner table: “Show me something at home that uses this kind of energy.”
Share this experiment
Send it to a parent who's looking for a 10-minute kitchen science win.
Ready?
Newton's Second Law: F = ma
Set it up. Test it. Be amazed.
Full screen — no distractions
Done with science? Try something else
Print the whole Forces track
All 9th Grade Forces lessons as a printable PDF, with worksheets and answer keys.