~ the forces you push, and the forces hidden inside ~
Alberta Grade 6 Science
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: Internal forces within an object: tension, compression, shear, and torsion; external forces acting on an object from outside: applied force, friction, and elastic or spring force; how external forces cause internal forces within an object; plasticity (permanent shape changes) and elasticity (temporary shape changes) as properties of materials.
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 force 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, every chunk of rope along its length is
pulling on the next chunk to hold the rope together. 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
very strong in compression but cracks in tension. Steel cables are
the opposite — strong in tension, but in compression they buckle very easily.
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
Bouncing back vs staying bent
When the external force stops, what does the material do?
Does it spring back to its old shape, or stay in the new one? That depends on
whether it was stretched elastically or plastically.
Elastic — rubber band
Stretches when you pull, then snaps right back to its
original shape the second you let go. The internal forces between
particles never let them stray too far.
also: springs, balloons, trampolines, your skin
Plastic — paper clip
Bend it open and it just stays in the new shape. The
particles slide past each other to new spots and settle there — they
don't slip back.
also: clay, dough, soft metal, a crushed soda can
the elastic limit — where one becomes the other
Most materials are both. Bend a paper clip a
tiny bit and it really does spring back — that's elastic. Push it
past its elastic limit and it stays bent — that's plastic.
Push even harder and it snaps. Even a rubber band, stretched
too far, gives up and breaks. Every material has its own limits before the
internal forces can't hold the particles in their original spots any more.
6
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.