Hi, my name's scientist Rick. I'm an educator at the Ontario Science Centre.
I also teach grade 12 physics and I'm lucky enough to teach at the Ontario
Science Centre Science School. Welcome to my home,
where we're going to do some science at home using some materials that I found
around my house. We understand that there are forces acting on this object. The
force of gravity, and that gravity is pulling the object down towards the Earth.
But because the books are not moving, we know
that there must be another force that's acting equal and opposite. We define that
force as the normal force, but we don't understand how the normal force knows
how much force is needed to push back with. Building a model is supposed to be
about understanding how and why objects interact the way they do. Taking the
normal force as a given, and assuming it's constant, is not going to be good enough.
And in physics, when we run into a situation where our model no longer is
able to help us understand the why or the how, we need to improve that model.
And we're going to do some experiments to try to improve that model today.
We can change the mass and change the force. Now there's more gravity pulling
these objects down, there must be more normal force pushing those objects up. But what
change with my surface. If you look really close, you might be able to see
that the surface is actually bending. That bend might be what's determining
how much force the normal is pushing back with. When we place the four
textbooks on the first board, it didn't seem like it bent at all. Or maybe we
just didn't see the bend because it was so small. But if we add some more weight,
that bending and elastic property starts to become a lot more visible. We might be
able to get a better idea of what's happening if we use this yoga mat that
I've wrapped with tape. If I apply a force, my yoga mat is going to bend and
what we will be able to see is is that these parallel
lines seem to get further apart at the bottom and closer together at the top.
When I remove the weight, the yoga mat goes back to its original shape.
We had some compression at the top and then that snapped back, and we had some
stretching at the bottom and that snapped back. We might just be able to
model what's happening using something else that stretches and snaps back.
This is a paper towel roll. On the bottom, it's attached together with elastics. if
there's a load placed on top of the roll, the roll will bend similar to our
surface. The more weight, the more bend and what's holding that surface together
is the elastics at the bottom. The more the elastic stretch, the more force
pulling our surface back together and the more weight that it can support.
We might be able to use these elastics and how much they stretch to determine
what the normal force is going to be. But what is this elastic force? We're now ready to
do a science at-home experiment and I have a banana stand. You really need
anything that can hold on to an elastic. The elastic is the most important thing
that we're going to use today and if you don't have an elastic, you can use a hair
tie, some nylons or even a sock - anything that stretches a little bit under some force. I'm going to attach some mass to my elastic and I'm going to use a container
to hold on to that mass. I've made this container out of construction paper and
some tape and my masses are going to be nickels so I can keep track of my mass
in nickel units. If you don't have nickels,
anything that you find that you can put into a container that has the same mass.
We know that an elastic is anything that stretches or compresses when you apply
a force and snaps back when that force is removed. The more the stretch, the more
the force. What we're going to do is we're going to see if we can come up
with an equation for that force and we're going to do that using an elastic,
a stand to hold our elastic, a safety pin and a container
to hold masses, assortment of masses whichever you like, and a ruler to
measure the stretch of the elastic. We're going to record our results. Now your
elastic is going to be different than mine. You can use a string, an elastic, a
hair clip, nylons, anything that stretches and bounces back to its original shape
when it's done. We know that if we have a hanging mass, the gravity can be
calculated as F equals mg and we also know that if acceleration is zero if our
mass isn't moving, that means that the elastic force has to be equal and
opposite in direction. So the more mass that we add, the greater our elastic
force is going to be and it's going to be the exact same as gravity. We can
always measure the elastic force but now we're going to be able to calculate it,
based on how much stretching the elastic does. Now the spring that you're using is
different. The masses that you may be using are different as well.
Your data is not going to be the exact same as my data. You can carry out your
own experiment and record your own, while I do this one in fast time.
Now if you were to graph that data, what you would notice is that you have a
pretty clear line of best fit that is a straight line. So that means that the
spring force or the elastic force should look something like KX: some constant
times some stretch distance. And just like, that we have an expression for
elastic force, similar to what we have for gravity. So we've successfully come
up with an equation to calculate spring force in a similar way in which we do
gravity. We know that if we have a string or a spring, we pull on it, it will
stretch and while it's stretching the force increases linearly with how much
we stretch it. So are we done? No, because if we carry out the experiment
further, we eventually reach a point where we can stretch the spring no
longer and then it breaks. And that breaking point is important because, just
before that breaking point, the formula F equals KX no longer holds and we're
going to need a new model and new physics in order to explain what's
happening. But, during that period where it's stretching in a linear way, we can
gain some valuable information and in fact that is what material scientists do.
They take materials and they exert forces on it and they watch how much it
stretches and bends. Materials physicists use information about how much an object
bends before it breaks in order to determine what materials to use when
they're building.
Or how much bend an object can withstand before it becomes deformed.
They're looking for the elasticity and the breaking point of those materials and
that helps architects and civil engineers decide what materials to build
with. So there's another fun application to understanding how much more can bend
before it breaks using spring forces. You can also use inelastic collisions,
conservation of energy and your knowledge of spring forces to figure out
how hard that you need to hit a board in order to break it, just like a martial
artist. Shall I give it a try? I'll save that for another time.
So I hope that you've had fun sciencing at home with me in my home. There's
always room to improve on models that we use to understand the universe.
We've improved on our model of classical physics by explicitly understanding that
forces do depend on position, but we know that the spring force is already not
enough to explain things like deforming and breaking. We need something else.
What could that possibly be?
Maybe those spring forces holding our materials together has something to do
with the things that are holding the balloon to the wall. Lots to discover and
lots more experiments to do to find out. If you have some materials around your
house, you can explore all of these things that we talked about today and
improve your understanding about the world around you, just by using objects
at home. I hope you had fun and go forth and science at your own homes.