Forces, energy and waves for high-school physical science: study cards over three tiers, the tricky ones, a practice quiz at three levels — Foundation, On-Level, and Challenge — and a lab where you graph a car's velocity–time trip, label the electromagnetic spectrum, sort twelve energy transfers, and argue from crash-test numbers why a seat belt cuts the force five-fold. Work here, or print it and use a pencil — both count.
A velocity–time graph. The line climbs, so the speed is increasing — this is acceleration.
On a velocity–time graph, HEIGHT is speed and SLOPE is acceleration. A flat line means constant speed — only a line sitting at zero means stopped. Reading flat as stopped is the single most expensive mistake on these graphs.
What people get wrong
⚠️People often think…
Newton’s third-law pairs cancel each other out, so nothing can ever accelerate.
The two forces act on DIFFERENT objects — you push the wall, the wall pushes you. Forces only cancel when they act on the same object. Draw the free-body diagram for one object and the pair splits up.
Third-law pairs act on different objects.
⚠️People often think…
In a car crash, kinetic energy is conserved.
MOMENTUM is conserved in every collision. Kinetic energy is only conserved in an elastic one — and a crash is the opposite of elastic. Most of that energy goes into heat, sound and crumpled metal, which is exactly what crumple zones are for.
Momentum always. Kinetic energy only if elastic.
⚠️People often think…
Double the speed and you double the stopping distance.
Kinetic energy goes with speed SQUARED. Double the speed and you have four times the energy to get rid of — so roughly four times the stopping distance. Sixty to thirty is not twice as safe; it is four times.
KE goes with v squared. Double speed, four times.
⚠️People often think…
A higher-frequency wave travels faster.
In one medium the speed is FIXED by the medium. Since v = fλ and v is stuck, raising the frequency shortens the wavelength instead. All EM waves move at the same speed in vacuum; they differ only in frequency.
Speed is set by the medium. f up, λ down.
Force, momentum, energy, waves
1Watch one
Why do cars have crumple zones instead of being built rigid?
In the crash, the car’s momentum has to change by a fixed amount.
Force × time produces that change. The change is fixed; the time is not.
A crumple zone stretches the stop out over more time.
More time means less force on the people inside. A rigid car stops too fast. ✓
2Do one with me
Fill in what is conserved and what is not.
On a v–t graph, a flat line above zero means constant
The quantity conserved in every collision is
Doubling the speed multiplies kinetic energy by
💬One sentence, then you move on
Why do third-law pairs not cancel each other out?
3Try one
In one medium, the frequency goes up. What happens to the wavelength?
I want a hint first
The medium holds v still. If f grows and the product must stay the same, λ has to do the opposite.
💬Last one — then you're done here
Why is space silent but not dark?
Where this goes
Where this lives
Seat belts and airbags, why a speed limit drops near a school, why a microwave and a radio station are the same kind of wave at different frequencies.
What this feeds
Next unit is Earth and space — the same physics at an enormous scale.
Name one safety feature that works by stretching out a stop.
One card at a time — tap “Show me” to check yourself, then Next. Start at Foundation; when those feel easy, climb.
Helpful Hints
🪜 Forces, energy, waves — the ladder
No net force, no change in motion → F = ma sets how much the motion changes → forces come in equal-and-opposite pairs on two objects → momentum p = mv is conserved in collisions, and impulse F × t is the change in momentum, so more time means less force → energy is conserved as it changes form: ½mv², mgh, and finally heat → waves carry energy at v = fλ → the EM spectrum is one family from radio to gamma, sorted by frequency, with energy rising along it.
Force changes motion; momentum and energy are conserved; waves carry energy.
🤝 The whole idea
Two quantities are never lost — momentum and energy — and almost every problem in this unit is bookkeeping on one of them. Waves are just energy on the move without any matter going with it.
Keep the books on momentum and energy.
⚠️ Tricky ones
On a v–t graph, FLAT means constant speed, not stopped. Only a line AT ZERO is stopped.
Third-law pairs act on DIFFERENT objects. They never cancel each other on one object.
In a crash, MOMENTUM is conserved; KINETIC ENERGY is not — much becomes heat and crumpled metal.
KE goes with speed SQUARED. Double the speed, four times the energy and stopping distance.
v = fλ: MULTIPLY. In one medium the speed is fixed, so f up means λ down.
All EM waves move at the SAME speed in vacuum. They differ only in frequency — and energy follows frequency.
Sound needs a medium. Light doesn’t. That is why space is silent and not dark.
🧮 The equations — the chart
Quantity
Equation
Units
What it tells you
force
F = ma
newton (N)
how hard a push changes motion
momentum
p = mv
kg·m/s
how hard something is to stop; conserved in collisions
impulse
F × t = Δp
N·s
the same change can be gentle over more time
kinetic energy
KE = ½mv²
joule (J)
energy of motion; speed squared
potential energy
PE = mgh
joule (J)
stored by height; g ≈ 9.8 m/s²
work
W = Fd
joule (J)
energy moved by a force over a distance
power
P = W ÷ t
watt (W)
how fast energy is spent
wave speed
v = fλ
m/s
frequency times wavelength; fixed in one medium
Eight equations; every problem in the unit is one of them.
🎯 How the test will ask
A v–t graph — “find the acceleration in each interval and the total distance.”
Two carts collide and stick — “find the final speed” (conserve momentum).
“Explain, using impulse, why airbags reduce injury.”
A coaster at height h — “find the speed at the bottom” (mgh = ½mv²).
“A wave has f = … and λ = …; find v” — or the reverse.
The EM spectrum with blanks — “order these by frequency and mark the most energetic.”
✅ Can you…
State all three of Newton’s laws with one example each?
Read slope and area off a v–t graph?
Conserve momentum through a collision and explain why KE isn’t conserved?
Explain seat belts with F × t = Δp?
Trade mgh for ½mv² on a coaster and say where the energy ends up?
Use v = fλ, and place a frequency on the EM spectrum?
Pick your level
Look back at anything you missed — the hint that appeared is exactly what to reread tonight.
How sure did you feel?
Lab
Work like a scientist: label the diagrams, turn the table into a graph, then write a claim with evidence and reasoning. Every lab checks itself on the spot, and hints are free.
Your practice never leaves this device. There is no account and no sign-in. Your work is saved in this browser only, and you can erase it whenever you want.
Your practice record — saved on this device
This is your record of the module on screen — it stays here and goes nowhere. Independent means you got it right on the first tap; supported means you got it after the explain-and-retry, or marked ‘I had it’ on a revealed answer. Both count, and neither is a grade. If your teacher asks, copy the row or show them this screen.
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The answer key is for a teacher: it prints only from here, for the unit on screen. Print the study packet prints the study pages and a blank quiz — never the answers.