This is a mousetrap car.
They're common 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
mousetrap. So today
I'm going to show you how to win first place by building some cars with the
World-record holder. Then we're gonna go to the West Coast Championships to see all these principles in action, and wait 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 the 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. "Imma bet you guys I could lift my car off the ground
using just my pinkies. If I can't do it, you can have this crisp Benjamin
but if I can, you guys have to buy me ice cream. Deal?"
"I said nothing else but your Pinky!"
"I am using just 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 and 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 4 pulleys that means I have to pull the rope down 4 times further than the dumbbell goes up.
But in exchange, it feels four times lighter, so this has a mechanical advantage of 4. For the ramp
you look at the ratio of the length to the height. Your mechanical advantage therefore is 2.2. And 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 has traveled around the thread and divided by the space in between the threads to get a mechanical advantage of 9
And as you know if you really want to multiply your force use a ratchet wrench
Now that is since 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 6 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 2
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 4 which of course means I have to move this end four times further
But it's super easy because it's 1/4 the weight on this side, and in all of these examples
Which you see everywhere around us you trade lower force for more distance travelled
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 1 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 it will 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 racecars
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
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 lever 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?
As you might have 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 could 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 on to your car to move it forward if you just quickly dump it 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 raced them
This made a huge difference and now they went this far
Again the longest lever arm car one 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 their axles and the car body
This is why we put the lubricating graphite powder there and to take our testing a step further
we took the long lever arm car and
Added ball bearings in place of 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 want to 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 that you need to balance these principles if you take any one of them too far
Then 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 Allen
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 a demo I built to showcase his 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 near 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 wait less to reduce the energy given to rotational inertia
so the final test we ran was Al's world record car which travelled an
Astounding 600 feet when he set the record he did some crazy things like using jeweler's 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 four thousand six hundred and eight it's the equivalent of a 16-foot
Lever arm or 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 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 of one, the 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 waste of 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 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 well 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 these cars weigh next to nothing
It's only gonna slow you down
I should also mentioned that my buddy Al has an amazing website called www.docfizzix.com or you can buy all the parts
I show today to experiment and come up with your own unique design. I also put that link below here we go
For this competition the objective was to travel forward 15 feet
And then returned back and stopped 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 use ball bearings small foam wheels and they made their car body out of
Aluminum it weighs more than balsa wood, so it did cost more energy to friction
But there's plenty of energy in the spring for only traveling thirty 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's just awesome to hang out and see all the various design approaches
Hopefully you learn 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
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