Organizing IdeaEnergy — Understandings of the physical world are deepened by investigating matter and energy.
Guiding QuestionIn what ways can interactions lead to physical change?
Learning OutcomeStudents analyze forces and relate them to interactions between objects.
This lesson covers: Elastic or spring force and how it is exerted on an object in contact with a stretched elastic object; how stored energy in a stretched elastic transfers to a projectile when released; external forces changing the position of objects; forces and interactions leading to physical change.
1
What is energy?
A force can push or pull, but to actually do something — to
speed a thing up, lift it, stretch it, or break it — the force needs
energy. Energy is the fuel that lets forces get to work.
Energy is never destroyed — it just changes from one kind to another.
Pull a slingshot back and you store energy in the stretched rubber. Let go,
and that stored energy becomes a flying rock.
2
Three flavours of energy in our slingshot world
Kinetic energy
The energy of motion. A flying rock has it. A walking kid
has it. Faster + heavier → more.
Elastic potential energy
Energy stored in a stretch. A pulled-back rubber band, a
squished spring, a bent bow — all coiled up, waiting to spring back.
Gravitational potential
Energy stored just by being up high. Apples in a tree
have it — gravity is waiting to turn it into kinetic energy.
the rule
Energy can't be made or destroyed, only transformed
from one kind to another. The slingshot game below is a perfect place to
watch it happen.
3
The slingshot
Drag the rock back from the slingshot pouch, aim, and let
go to fire. Knock as many apples off the tree as you can. Toggle
show forces to see every push and pull on the slingshot, the rock,
and the apples. Use the chips below the canvas to turn individual
forces on or off — try removing gravity or air drag and watch what
changes.
energy
Elastic
0
Motion
0
Height
0
apples knocked0 / 6
shots0
stretch0%
Click any force to turn it off and see what changes:
how the energy moves
Pull back — your arm's muscle energy goes into elastic
potential energy in the stretched rubber. Release — that
elastic energy becomes the rock's kinetic energy. Mid-flight
— the rock trades a little kinetic for gravitational potential as it climbs,
then trades it back as it falls. Hit! — kinetic energy
passes to the apple (Newton's third law in action), the apple breaks free of
the stem, and gravity converts its potential energy back into kinetic
on the way down.
4
Spot the forces — every arrow explained
The game shows up to six different forces, each in its own
colour. They're all forces we've met in earlier lessons — here's the cast list.
Applied force (red) — the pull from
your hand on the slingshot pouch. An external force from outside the
system.
Spring force (orange) — the
stretched rubber pulling back. The harder you pull, the bigger this gets.
Also called elastic force — same as the rubber band in lesson 7.
Gravity (grey) — Earth's pull, always
downward, always there. Acts on the rock, on the apples, on every part of
the tree.
Air drag (blue) — friction from the
air. Always points opposite to the way the rock is flying — a small
force that slowly slows it down.
Tension (purple) — the pull inside
an apple's stem holding it to the branch. As long as tension is bigger than
gravity, the apple hangs on. A hard knock breaks it.
Normal force (green) — the push-back
from a surface. The ground pushes up on the slingshot's base; the pouch
pushes up on the rock while it's still resting in it.
action and reaction everywhere
Every arrow you see has an invisible partner pointing the
other way. Your hand pulls the rubber → the rubber pulls your hand back.
The rock smacks the apple → the apple smacks the rock back (which is
why your shots wobble after a hit). Newton's third law is hiding in every
interaction.
5
The whole notebook in one diagram
This little game is hiding every idea from the rest of the notebook.
The tug-of-war from lesson 2 (attractions vs motion) is
right there in the apple stem — tension fights gravity until you knock it
loose.
The elastic / plastic idea from lesson 7 picks the
slingshot's rubber: stretches and snaps back without staying deformed.
The internal forces from lesson 7 are inside the rubber
(pure tension!) and inside the slingshot frame (compression where it joins).
Newton's three laws from lesson 8 run the whole show:
inertia keeps the rock flying once it's free, F = m·a sets how much the rock
accelerates from your pull, and action–reaction pairs hide behind every arrow.
And way down at the smallest scale, every collision, stretch, and bounce
is just particles pulling on particles — which is exactly
where the notebook started.
in one sentence
Energy is what forces use to do their job, and a slingshot shot is
a tour through every kind of energy and every kind of force we've learned
about.