~ the forces you push, and the forces hidden inside ~
1
Two big families of force
Every force is a push or a pull (we saw that in the last lesson). But forces
also come in two big families, depending on where they
come from:
🖐 External forces
Come from outside the object. Something else in the world
pushes or pulls on it.
Applied — you pushing a box
Friction — the floor scraping back
Spring / elastic — a stretched rubber band pulling on your hand
🔩 Internal forces
Happen inside the object — between the particles that hold
it together — when something tries to pull it apart, squish it, slide it,
or twist it.
Tension — being pulled apart
Compression — being squished together
Shear — being slid sideways
Torsion — being twisted
the connection
Whenever you apply an external force to a solid object, the
particles inside push back on each other to keep the object together. Those
invisible push-backs are the internal forces. No external push
→ no internal stress.
2
External forces — coming from outside
These are the ones you can usually see and feel. They act
between two separate things — your hand and a box, a wheel and the ground, a
spring and a wall.
Applied force
A direct push or pull from a person or another object.
e.g. shoving a shopping cart, kicking a ball
Friction
A pull from the surface that fights motion. It always
points opposite to the way the object is sliding.
e.g. a sled slowing on snow, brakes on a bike
Spring / elastic
Stretched or squished elastic things pull back toward
their natural shape — the more you stretch them, the harder they pull.
e.g. rubber band, bungee cord, trampoline
all of these need something else
Notice how every external force has two things involved: the
object, and something else doing the pushing or pulling. That's why
they're called external — the source is on the outside.
3
Internal forces — hidden inside the material
When you pull on a rope, it doesn't just stretch — every chunk of rope along
its length is pulling on the next chunk. That tug between
particles is an internal force. There are four big shapes it can take.
Tension
Particles being pulled apart. The material resists by
holding itself together.
e.g. tug-of-war rope, suspension bridge cable, your tendons
Compression
Particles being squished together. The material pushes
back to keep from being crushed.
e.g. pillar holding up a roof, your bones when standing
Shear
Layers sliding sideways past each other — like cutting
with scissors.
e.g. scissors, wind pushing a tall building sideways
Torsion
The material is being twisted around its own axis. The
two ends rotate in opposite directions.
e.g. wringing a wet towel, turning a screwdriver, an axle
why this matters to engineers
Different materials are good at different jobs. Concrete is
amazing in compression but cracks in tension. Steel cables are
the opposite — strong in tension, useless in compression (they just buckle).
That's why bridges combine both: concrete piers push up, steel cables pull
across.
4
Try it: external in, internal out
Pick how you want to load the bar below. The red arrows are
the external forces you're applying. The blue arrows inside
show the internal force the material has to deal with.
External: pulling apart → Internal: TENSION
cause and effect
The shape of the external force decides the kind of internal force.
Pull → tension. Push → compression. Slide ends sideways → shear. Twist ends
in opposite directions → torsion.
5
The bigger picture
External forces are the cause. Internal forces are the
response. When the internal forces get bigger than the
material can handle, things bend, snap, or shatter — exactly the behaviours
we saw with the spring, the clay, and the glass back in the forces lesson.
And it goes all the way down. At the very smallest scale, the
internal forces inside any solid are just the attractions between
particles we studied in the very first lessons of this notebook.
The same tug-of-war, just zoomed way out.
in one sentence
External forces act on an object from the outside; internal forces
are the object's particles holding on to each other in response.