Answer 1:
Let's think first about what happens when we
are standing (not running) on hard surface, like
concrete for example. When we are standing,
gravity is pushing down on us (because we have
mass) and the concrete is pushing back on us
through our feet...right? Don't believe me?
Stand up and try it! It works!
Okay now let's start moving, but not running
just yet. What happens when you jump? You
use your leg muscles to push on the ground, and
the ground pushes back to propel you towards the
sky! The amount that the ground pushes back
depends on the material that the ground is made of
and how that material is organized.
How does this affect us when we run?
Well if you tried to run on a very thick block of
concrete, like a sidewalk for example, you
wouldn't have to think twice about running because
the force with which the concrete pushes back is
equal to how hard you are hitting the ground while
you run. If you tried running on a very thin block
of concrete though--let's say as thin as a
popsicle stick--then you would have to worry,
because that amount of concrete probably won't
push back on you! Instead, it will crumble under
the force of your feet and you'll just fall right
through it!
The concept is similar for other materials. If you
can, try running on sand, concrete, and then the
track at your school and compare how each of them
feels. Here's what I can tell you about each of
these experiences, but you should seriously try it
for yourself:
1) Dry sand usually slides around when you try
to run on it--so it's taking away some of the
force that your feet are putting into the
ground--and it feels like you're going nowhere
even though you're running really hard!
2) Concrete doesn't slide around, but it
also doesn't change it's shape when you run on
it--so running on it feels pretty normal!
3) If your school has a track covered in special
rubbery plastic called polyurethane, then when
you run on it you might feel a little faster!
This is because the material is a little bit
"bouncy", so it pushes back on your feet
with even more energy than what you put into it
with your feet.
So this is what I hope you learned from this:
the surface of the ground affects a runner's
speed because the material and structure of the
surface influences how much force the ground
pushes back on their feet with.
Change the material and structure of the
ground and you can either decrease (e.g. sand) or
increase (e.g. rubber) the runner's speed!
Some companies like Adidas (see Adidas
Springblade) have even tried to change the
material and structure of their shoes to increase
runner's speed.
Hope this helps! | 
