Lesson 1 of 7 · Motion and Forces
Inertia Is Measured by Mass
🎢
🎯 Your mission
Run the experiment. Find out what really happens.
⚡ The twist
The opposite of what you expect is often what happens.
What you'll learn
Newton's first law is usually read as a rule about what objects do, but it is better read as a definition of inertia. Inertia is measured by mass in kilograms, so a 2,000 kg car resists a change in its motion exactly twice as strongly as a 1,000 kg car under the same net force. Nothing in the law says an object prefers to stay still: it says that with zero net force the acceleration is zero, whatever the velocity already was.
Key words
inertia
The property, measured by mass in kilograms, that sets how strongly an object resists a change in its velocity.
net force
The single force equivalent to every force acting on an object once they are added as vectors.
Make a hypothesis first
Predict: what happens if you double the force?
Say your guess out loud — then find out if you were right.
Get ready
🧰 What you need
- dynamics trolley
- 1 kg masses
- elastic band
- stopwatch
- tape measure
Get ready
⚠️ Safety first
- Keep feet clear of the run-off end and stop the trolley before it leaves the bench.
Step 1 of 4 · Read
Think about what you just learned: say what "inertia" and "net force" mean in your own words before you start.
Step 2 of 4 · Do
Load a trolley with 0, 1 and 2 kg of masses and pull each with the same stretched elastic band, timing it over a fixed 1 m. Plot mass against time taken. Then predict the time for 3 kg from the pattern, test it, and say how close your prediction came.
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 3
Question 2 of 3
Question 3 of 3
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 3 right.
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✨ 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.”
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Inertia Is Measured by Mass
Run the experiment. Find out what really happens.
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Print the whole Motion and Forces track
All 8th Grade Motion and Forces lessons as a printable PDF, with worksheets and answer keys.