A force is just a push or a pull. Forces happen whenever
two things interact. You push a door open, you pull a sock on,
the Earth pulls an apple down — those are all forces.
two flavours of force
Contact forces happen when objects touch each other —
pushing, pulling, friction, kicking. Non-contact forces
happen across empty space — gravity pulling the apple down, magnets
pulling each other together, even from a distance.
2
Forces change motion
A force can make a still thing start moving, make a moving
thing stop, make something speed up, or make
it change direction. Try it — push the cart left or right,
or hit the brakes!
notice
Once the cart is moving, it keeps moving on its own — you
don't need to keep pushing. That's because of inertia:
moving things want to keep moving, still things want to stay still. The cart
only slows down because of friction from the wheels.
3
Forces can also change shape
Forces don't just move things — they can also squish, stretch,
bend, or break them. Different materials respond very differently
to the same push.
three ways materials respond
Elastic (the spring, the balloon): spring back to their
original shape when the force stops.
Plastic (the clay): keep their new shape after being
squished.
Brittle (the glass): can't bend at all — they just
crack and break.
4
What happens when forces fight?
When more than one force acts on the same thing, they add up.
If they're equal in opposite directions, the forces are
balanced — the object doesn't move. If one side is
stronger, the forces are unbalanced — the
object moves toward the stronger side.
Blue team5
Red team5
⚖ BALANCED — no movement
balanced doesn't mean nothing's happening
Both teams are pulling really hard — there are huge forces
on the rope. But because they're equal and opposite, the
net force is zero, so nothing moves. This is why a wall
you're leaning on doesn't fall over: gravity pulls down, the floor pushes up,
balanced.
5
Forces transfer energy
Here's the big idea that ties this whole notebook together: when a force
moves something, it gives that thing energy.
That energy is called kinetic energy — the energy of motion.
And it's the same kind of energy we saw in the very first lesson —
the energy that makes hot particles wiggle faster than cold ones. Whether
it's a giant cart on a track or a tiny particle in a bulb, motion =
kinetic energy.
the bigger picture
Forces cause physical changes. They change motion (Step 2),
change shape (Step 3), and when forces are unbalanced they cause acceleration
(Step 4). They transfer energy from one thing to another
(Step 5). And way back at the particle level, the pull
between particles and the motion of particles is the same
tug-of-war we've been studying since lesson 1.