Science · Grade 4 Science
Chapter 1: Energy and Motion
Twice as fast is four times as much.
Watch first. The explanation comes later.
A cart rolls into a block and shoves it a little way. The same cart at twice the speed shoves it four times as far.
How much more energy does something have when it goes faster?
Keep the mass at 1 and try every speed from 1 to 6.
Step 1 — Predict
At speed 2 the energy is 4. What will it be at speed 4?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Now the explanation, after you have seen it happen.
A moving object carries energy simply by moving, and you can tell because of what it does on arrival: it pushes whatever it hits. A rolling cart knocks over a block; the same cart sitting still knocks over nothing.
Energy of motion The energy a thing has because it is moving.
A ball sitting still has no energy of motion at all.
energy What makes things move, heat up, or light up.
Speed counts twice over. Double the speed and the energy is not doubled but four times as big; treble it and it is nine times. That is why a small increase in speed makes such a large difference to how hard something hits.
speed How fast something moves.
Double the mass and the energy just doubles. Mass and speed do not behave the same way.
mass How much stuff a thing is made of.
Doubling the speed of a moving object gives it four times the energy of motion, not twice. That is why a small increase in car speed makes a crash so much more damaging.
A heavy object moving slowly can carry as much energy as a light one moving fast. Both mass and speed count, which is why a lorry at 30 is more dangerous than a bicycle at 30.
Lots of people think
“Going twice as fast gives you twice as much energy.”
The same idea somewhere new.
A car at 60 km/h does not have twice the crash energy of one at 30. It has four times. That single fact is behind most of the speed limits near schools, and it is the same rule you just found in the table.
Practice makes it stick.
The Rolling Cart
Problem 1 of 2
A cart at speed 2 has 4 units of energy. What does it have at speed 4?
The Heavier Cart
Problem 2 of 2
A cart of mass 1 at speed 3 has 9 units. What does a cart of mass 2 at speed 3 have?
1 of 4
You double the speed. The energy becomes...
2 of 4
You double the mass. The energy becomes...
3 of 4
A cart of mass 1 at speed 5. How much energy?
4 of 4
Which changes the energy more — doubling the speed, or doubling the mass?
Show what you know.
Question 1 of 1
At speed 3 the energy is 9. What is it at speed 6?
Claim, evidence, then reasoning.
The question
Using your table of every speed at mass 1, write an argument answering the question: when you double the speed, does the energy double too?
Evidence you can use
Fill in all three boxes. The reasoning box is the one that matters most.