- Good Morning.
My name is Billy and this is
my video project for Flipping Physics.
♫ Flipping Physics ♫
- Today we are going to answer the question,
"Do anti-lock brakes use a static
or kinetic friction to slow down a vehicle?"
Most modern automobiles are made with ABS brakes,
which stands for anti-lock braking system.
These brakes are designed to prevent
the wheels from locking up.
In physics terms, this means the brakes
are designed to keep the wheels rolling without slipping.
In the absence of anti-lock brakes,
the wheels would lock up, stop spinning,
and in physics terms be rolling with slipping.
I will now use a bicycle to show the difference.
Notice when I ride the bicycle normally,
the wheels roll without slipping, which means
the bike tires do not slide relative to the ground.
When I accelerate the bike quickly,
the rear tire slips relative to the ground
and rolls with slipping.
When we take a closer look at the tires
while riding the bike normally
where the wheels roll without slipping,
you can see rolling without slipping
uses static friction because the two surfaces
are not sliding relative to one another.
And when we take a closer look at the tires
while accelerating the bike quickly,
where the tires roll with slipping,
you can see rolling with slipping
uses kinetic friction because the two surfaces
are sliding with respect to one another.
Again, rolling without slipping uses static friction,
and rolling with slipping uses kinetic friction.
Now let's talk about ABS,
or anti-lock brakes on an automobile.
The whole idea behind the anti-lock braking system
is the car has sensors in it
to identify when the tires start to slip,
and when a tire does start to slip,
the ABS brakes automatically decrease the braking pressure
to prevent the wheel from slipping relative to the ground.
I borrowed Mr. P's Flipping Physics mobile
in order to show this concept.
Thanks, Mr. P.
- Wait, what?
- The automatic decrease and increase in braking pressure
is the pumping sound you hear and feel coming from
the brakes when the anti-lock brakes are being engaged.
ABS brakes try to keep the wheels rolling without slipping
to keep the friction static rather than kinetic.
That's because the coefficient of static friction
is greater than the coefficient of kinetic friction.
Here is a video at real speed
of just one tire on snow
while the anti-lock brakes are being applied.
As you can see, it is very difficult at real speed
to see if the tire rolls with or without slipping.
So here is the same video at 1/4 speed.
(tire grinds erratically)
And now I will loop a short section
where the tire rolls with slipping.
You can see while the tire is rolling with slipping,
the tire does not rotate as much
and is sliding relative to the ground.
Now let's watch the entire braking period
for just that tire and I will number each
of the frames where the wheel rolls with slipping.
As you can see, even at 1/4 speed,
it is very difficult to see
when the wheel rolls with slipping.
And of course, a car has four tires.
So here are all four tires
shown as the car is slowing down,
with the number of frames where the tires
used kinetic friction rather than static.
Let's watch that again,
and this time I've added the total frame count.
(tires grind erratically)
The number of frames per tire is 404,
and with four tires that means the total number
of frames was four times 404,
or 1,616 total frames.
And the total number of frames
where the tires were rolling with slipping was 44.
Therefore, 44 divided by 1,616 times 100
is the percentage of the time
the tires were rolling with slipping,
and therefore using kinetic rather than static friction.
So this experiment showed 2.723
or roughly 3% kinetic friction,
and therefore 97% static friction.
I think we can conclude that, at 97% static friction,
anti-lock brakes are almost entirely static friction.
And remember, static friction is more effective
at slowing the vehicle down because
for any two surfaces the coefficient
of static friction is greater
than the coefficient of kinetic friction.
Thank you very much for learning with me today.
I enjoyed learning with you.