1st place Mousetrap Car Ideas
– This is a mousetrap car. (funky music) They're coming for
competitions in high school physics classes, just like
the egg drop challenge or building toothpick bridges. The goal is to build a car
that travels the furthest or goes the fastest, but in
either case, the only power provided to move the car is
from a single mouse trap. So today I'm gonna show
you how to win first place by building some cars with
the world record holder.
And then we're gonna go to the
West Coast championships to see all these principles in action. And, don't leave. I know that 99.7% of you have
never nor will ever make one of these, but I will break
down in simple terms how I know this car will go
twice as far as this one and then I'll prove it and then we'll discuss why you see these DVD wheels so often. But do they work and why
do some winning cars have wheels that look like this? But before we fly all
the way out to Texas to meet the world record holder,
I need to lay the foundation for the one overarching
fundamental physics principle behind the mousetrap car. It's called mechanical advantage. And to do that, I'm gonna
need my niece and nephews. I'm gonna bet you guys I could
lift my car off the ground using just my pinkies.
If I can't do it, can
have this crisp Benjamin, but if I can you, guys
have to grab me ice cream. All right, deal? – I said nothing else but your pinkies. – I am just using my pinkies. – No, just your pinkies. – That's what I'm doing. This is really good you guys. Thank you. If you're willing to
move a greater distance you're able to reduce the amount of force by a proportional amount. I can't lift 500 pounds
worth of car one time but I could lift 10 pounds 50 times. A mechanical advantage is the ratio of the output force over the input force. So in this case it's 50. That means my hand had to
travel 50 times further than just lifting the car in one shot but the weight was 50 times less so it was totally worth it.
This principle of mechanical
advantage is everywhere. Let's take a look at a few examples. If I have four pulleys, that
means I have to pull the rope down four times further
than the dumbbell goes up. But in exchange, it
feels four times lighter. So this has a mechanical
advantage of four. For the ramp, you look at the ratio of the length to the height. Your mechanical advantage
therefore is 2.2. That means I have to
travel a little further but the brick should feel
2.2 times lighter pulling up the ramp versus just
pulling the brick straight up. And sure enough, if you
measure each with a scale this is exactly what you see. If you think about it a screw is just a ramp
wrapped around a nail. So here you look at this as traveled around the thread and
divide by the space in between the threads to get a
mechanical advantage of nine. And as you know, if you really
wanna multiply your force use a ratchet wrench. Now the distance your hand travels for one full rotation is 300
times longer than the distance the screw moves vertically
between one thread.
The total mechanical advantage is 300. It's like a really long short ramp. So if this scale reads six pounds the actual clamping force
would be 300 times more or nearly a ton. And with wheels and axles,
it's the same story. Since this wheel diameter
is twice what this one is as you could probably guess by now this weight weighs twice as much. So now we're balanced with a
mechanical advantage of two. And you'll also notice if I move this the lesser weight travels twice as far. And finally we have levers which is where we started
with my niece and nephews.
Here, if you compare the ratio of the distances from the pivot point we have a mechanical
advantage of four, which of course means I have to move
this end four times further. But it's super easy because it's one fourth
the weight on this side. And in all of these examples,
which you see everywhere around us, you trade lower
force for more distance travel. This is how humans built amazing things before all these fancy machines
with engines came around, human muscles are totally
strong enough as long as you're willing to spend
a little more distance to do the task.
And so this principle mechanical
advantage is at play over and over again with the
mousetrap cars only in reverse. It works both ways. In other words, I don't
want the full force of the spring acting over this
tiny distance to act directly on the wheels or they would spin out. That would be a very inefficient transfer of energy from the spring. So we use mechanical advantage and make the main lever
arm 15 times longer than the spring lever arm. And then the wheel
diameter is 24 times bigger than the wheel axle. So then if we multiply them our total mechanical
advantage is one over 360. That means the force is
360 times less right here on the output at the wheels to the floor versus right here on
the input on the spring. It also means we'll
travel 360 times further than the distance this spring arm rotates. Alright, so that's enough
of a foundation for now let's go to Texas and meet up with my buddy Al to build some race cars. (upbeat music) – [Man] U.S.A.
(upbeat music) – [Mark] Not only is he the mousetrap car world record holder but he also kind of
started the whole thing and he was Texas high school
physics teacher of the year. And since my dream job
is to one day switch from working as an engineer
in the private sector to go teach high school physics somewhere, I made him show me all his cool demos. – I came up with this idea
back in 1991 and since that time I have literally
built thousands and thousands of mousetrap cars myself. I've seen every possible
engineering design you could ever come up with. – And there's lots of different variations for rules for a mousetrap car race. Let's talk about how to build the best
long distance car first. For our testing, we started
with three identical cars. The only difference was the
length of the leather arm. So one was short, one was
medium, and one was long. And I've calculated each of
their mechanical advantages which you can see written here. And given what we know
about mechanical advantage what do you think is about to happen? – Ready?
– Yep.
As you might've guessed,
the short lever arm car takes a strong early lead. This makes sense because it
has the largest mechanical advantage, therefore the
highest force where the wheels and ground meet. The downside is that it's a
short-lived burst and the medium and long lever arm cars pass
it once it's quickly used up all its energy. In the end, this is how far they each
traveled with the longest lever arm car going the slowest,
but making it all the way to 30 feet. This brings up the first
principle for the long distance car, to win you want the
smallest possible force over the longest possible distance. In other words, the smallest fraction for mechanical advantage possible. You want your car to be barely
creeping forward to waste as little energy as possible.
You can think of the
total energy of the spring as this amount of water
in this cup, and then this cup represents the
amount of energy that's passed onto your car to move it forward. If you just quickly
dump in all the energy, a ton spills and splashes out,
this will be due to losses from extra heat generated or
even drag force from the wind which is proportional to
your velocity squared. But if you do it slowly and more controlled, much more energy goes to actually moving your car forward. The next thing we tested was
adding graphite to the axles on all three of the cars
and then we race them. This made a huge difference and now they went this far, again the longest lever arm car won because it was the slowest.
But this shows the importance
of dealing with friction. It's definitely your biggest enemy with these cars and the
friction comes from two spots. You have the rolling friction between the wheels and the ground and then the biggie is between
your axles and the car body. This is why we put the
lubricating graphite powder there. To take our testing a step further we took the long lever arm car and added ball bearings
in place at the graphite and that set a new
record for us at 50 feet. So if you only have one
hour to make your car and you wanna have a good showing you can use a long lever
arm like this in conjunction with the CD wheels to give
you a mechanical advantage of about one over 360 and then use ball bearings at the axles or just apply some graphite and
you're gonna do pretty well. Next, we figured if long lever
arms make it travel slower and therefore further, we should
do a super long lever arm. But it only made it to here which was worse than even
the short lever arm car. The problem was that it
didn't coast very well because we had to make it really big which means it's more heavy,
which means more friction.
You know this already intuitively because it's harder to push a heavy object on a table than a light object because there's more
friction resisting you. So principle three is
to make it lightweight. I love this example though because it shows you need
to balance these principles. If you take any one of them too far than
another principle will creep in and start penalizing you. It's an optimization
problem and that's what makes the mousetrap racers
such a great project. That's also why testing is so important. So tweaking and testing different things like Al and I did is critical for honing in on the sweet
spot for your specific design. Next, we tried this big wheel design which is a popular approach.
The strategy here is the
wheel is 56 times larger than the wheel axle. So when you combine it with the lever arm you get a built-in mechanical
advantage of one over 840 and that's the equivalent
of a lever arm that's two and a half feet long, but
without needing the big heavy car that seems like a good deal and as such it was our best car yet and
made it all the way to here. The downside is that it takes energy to get a
big wheel like that rotating. It's called rotational inertia. Here's the demo I built to
showcase this principle. These two wheels are identical except this one has the
steel weights placed at the outer edge of the
wheel versus near the axle. This means it has a
higher rotational inertia. So when we spin them both identically the one on the right
starts spinning faster and will reach a higher max speed but the one on the left
will coast for longer.
By having bigger heavy wheels,
you're basically using them as a temporary storage for your energy and then you give it back
during the coasting phase. The problem with this is
anytime you transfer energy you lose some. Going back to the cups, a little splashes out each time you pour it, no
matter how slowly you do it. So instead of pouring your
spring energy into a big cup and then eventually getting
it back in the coasting phase it's better just to have
reasonable size wheels and just have one slow pour directly into the final cup of
making your car move. Additionally, big wheels like
this can be hard to steer. So principle four is to
reduce rotational inertia. This is also why you see people do this to their wheels sometimes. It's an effort to keep the wheels large in diameter to get that
built-in mechanical advantage but to make them weigh less
to reduce the energy given to rotational inertia.
So the final test we ran
was Al's world record car which traveled an astounding 600 feet. When he set the record,
he did some crazy things like using jewelers bearings on the axles but the real secret is this
pulley here in the middle. If we look at the ratios and calculate the mechanical advantage from the lever, to the
pulley, to the wheels we're looking at one over 4,608. It's the equivalent of a 16
foot lever arm or a back wheel four and a half feet in diameter,
but without the downside of the extra weight or
wasted rotational inertia, this thing barely crawls along. It's hard to even see the
spring lever arm moving as this back wheel spins. It's really hard to
beat a design like this. And now we'll quickly go
through the speed car principle since most of the same principles apply.
The biggest difference
is this time we want to access all the energy from the spring in a short burst right at the beginning because the finish line
is only 15 feet away. So it doesn't make sense
to have a really small mechanical advantage like the pulley car. Here we want it much
closer to the direct force of the spring itself, which would be mechanical advantage one. Problem is if we did that,
the wheels would slip. So you basically want to
incrementally increase your mechanical advantage by
making your rear axle thicker and thicker with tape until
your rear wheels start to slip. Slipping is bad of course because that's wasted energy
because your wheel is spinning without actually moving your car forward.
It's helpful to zoom
in and use the slow-mo on your phone to see if your
wheels are slipping or not. This means having good traction on your rear wheels is important because it means you
can have higher forces before you start to slip. These squishy foam wheels work great and just like with the
distance car, reducing friction by using bearings or graphite
will definitely help.
As will making it lightweight because Newton's second law teaches us that heavier things are
harder to accelerate. Just like you throw a baseball further than a heavy bowling ball and smaller diameter wheels not only help by keeping your mechanical
advantage closer to one but you don't have time to give energy to these big wheels and then
get it back through coasting. You want all that spring
energy to go directly into making your car go forward.
Okay, so those are the basic principles. Before we head to the West Coast
Mousetrap Car Championships I'll just mention I put a list of 10 practical quick build
tips in the video description. For example, you should soak your bearings in WD 40 to remove all the grease. The grease is useful if the bearings are actually
seeing a lot of load but since he's cars weigh next to nothing, it's only gonna slow you down. I should also mention that my buddy Al has an amazing website called docfizzix.com where you can buy all the parts
I showed today to experiment and come up with your own unique design. I'll also put that link below. Here we go. (upbeat music) – [Man] U.S.A. (upbeat music) – For this competition the objective was to
travel forward 15 feet and then return back and stop as close to the exact same spot you started in the least amount of time. Those moves may seem complicated but you can switch from forward to reverse by simply switching the direction you wrap up your axle halfway
through, and you can stop at a certain point by using a
wing nut on a threaded axle.
So given the rules your design choices should
foster both speed and precision. About half the designs relied
on preconceived notions and use CD wheels, which
are a real bad choice here because you're not looking for distance. They have more rotational
inertia and poor traction. The winning team car which I won't show here because
the finals are next month really focused on precision. They used ball bearings, small foam wheels and they made their car
body out of aluminum. It weighs more than balls of wood so it did cost more energy to
friction, but there's plenty of energy in the spring
for only traveling 30 feet.
So it was worth the trade off for the extra rigidity
and repeatability. They also told me they tested and tweaked their design for six months. It was awesome to hang out and see all the various design approaches. Hopefully you learned enough by now to give you a solid foundation for your own unique design
so you can build, test, tweak like crazy and then
dominate the competition. Thanks for watching. (upbeat music).
