Note: This is a series of posts with my favorite physics labs. These labs are intended for the algebra-based college level course (but clearly could be adapted for other courses). Feel free to use this in a way that makes you happy.
Objective
This is the first lab of the course, so the goal is two part:
Practice with measurements and uncertainty.
Practice making graphs (linear graphs)
The graph thing might seem silly, but students need work on this and it’s used in pretty much every lab for the semester.
Measurementsand Uncertainty
Here is my version of this whole thing. VERY SHORT.
We like to measure stuff in physics – however, we can never measure stuff perfectly. One way to account for error in measurements is to include an uncertainty. Suppose I measure a length, it could be reported as L = 0.13 +/- 0.01 meters. This says the length is most likely between 0.12 and 0.14 meters.
There are three ways to get this uncertainty.
Approximation. If you can only record a value once (or repeating won’t give you anything new) then the best option is to just be a human and assign a reasonable value. The uncertainty can’t be smaller than half the smallest division on your measuring device.
Standard deviation. If you can repeat the measurements, you can calculate the average and the standard deviation. The standard deviation would be the uncertainty. Note: make sure you get at least 5 values or you will just be kidding yourself.
Calculation. If you want to get the area of a table, you measure the length and width and then calculate the area. This means you will have to calculate the uncertainty also. You can do this with the “crank three times” method. Use your uncertainties to find the smallest and largest possible values. The uncertainty is then halfway between these values.
Here is a short lecture on measurements.
Now for some practice.
Take an aluminum block (or really any block). Find the volume and density with uncertainty.
Reaction time. Take a ruler and hold in vertically near another human’s fingers. Drop it and measure how far it falls before it is caught. Repeat 10 times, get the average and standard deviation.
Graphing
In physics, we like to build models. If the model is expressed as an equation, what better way to show the model is legit than to make a linear graph. OK, let’s just get to it. Here is a quick tutorial on graphing.
Now for some practice. Find a bunch of circular objects of different size (at least 4 different sizes). Measure both the circumference and diameter. Now create a plot of circumference (vertical axis) vs. diameter (horizontal axis) and find the slope. Recall that we have the following relationship:
I’m kicking myself. I should have been writing these MacGyver notes for each episode as they aired on TV. But no. I had to make things more complicated. Well, here I am—starting the notes for season 2.
Let’s just jump in. Note: there’s some pretty good stuff in this one.
Blade from a button.
MacGyver takes a button from his shirt and breaks it in half. From this half button he sharpens the edge so that it can cut through a rope.
I wouldn’t normally include this one in the blog since there’s not much science to talk about—but I’m just excited.
Flare-based harpoon and winch.
MacGyver takes a metal rod and puts it in an empty dip stick tube from an engine. He adds the powder from a road flare to act as a propellant. This would then launch the improvised harpoon forward.
Once the harpoon is stuck into a fleeing car, Mac wraps the cable around the horizontal axle on the sidecar to a motorcycle. As the axle spins, the cable wraps around it and pulls the motorcycle closer to the car.
Mini gun as a starter motor.
They need a car. MacGyver finds one—but it’s missing a starter motor. Really, there is nothing super special about a starter motor. It’s just a DC motor that is strong enough to turn over the engine so that it can turn itself (using gasoline).
The mini gun also uses an electric motor. In this case, the motor spins a combination of gun barrels so that the fire rate can be higher than a normal machine gun.
Could you use one motor for another application? Theoretically, yes. The only tough part might be mounting the motor (which MacGyver does with some wire—wire is often better than duck tape). The other problem is making sure the gears on the motor match up with the gears on the car to turn over the motor—but it’s still possible.
Run up a wall with a pole
Sometimes, there are hacks that look too crazy to be true—but in fact would totally work. Here is an example of such a hack.
MacGyver and friends take a long pole. MacGyver gets on one end near a wall and the other two push the pole towards the wall. This allows MacGyver to walk up the wall.
But wait! Here is a video of this trick being used in real life by a Vietnamese SWAT team.
Here is my super short explanation.
If you push someone against a vertical wall, there will be a frictional force pushing UP.
If the push is great enough, this upward frictional force can be equal to the gravitational force.
Boom. That means a person can walk up a wall.
Here is a force diagram of that situation.
Oh, I also made a video to show you how this works.
Ok, you can put a camera in a ball and kick it through a window. Not really much of a hack, but clever.
Improvised diver propulsion vehicle.
MacGyver needs to get through a submerged passage—it’s long enough that he can’t swim the whole way. This means he needs some scuba stuff. In particular, he needs the following:
Some type of air supply with a regulator.
A mask and some type of mouthpiece to breath.
A light.
A DPV—diver propulsion vehicle.
Please forgive me, but I’m going to go over more detail in this case that you would like. I can’t help it. MacGyver is basically cave diving—this is something that I used to do quite a bit.
Just to show you what that was like, here is an older picture of me. It’s not cave diving, but it’s using all of the same gear (it was practice).
Let’s start with the scuba gear. MacGyver doesn’t have a regular scuba tank, so he uses an oxygen tank used for welding. You don’t normally want to use oxygen for scuba—you want to use air (which is only 21% oxygen). You see, oxygen is actually toxic. If you breath oxygen at high pressure, it can do bad stuff to you. Fortunately, MacGyver is going to use this at very shallow depths—he should be fine. Also, he won’t need to much gas as you consume much more at greater depths (for open circuit systems like scuba).
What is a regulator? Suppose you have a pressurized tank at 1000 psi. You can’t really breath air (or any gas) at that pressure (although there are some tricks—ask me later and I can tell you about this). That’s where the regulator comes in. It takes pressure from the tank and reduces it to the ambient pressure. That’s really important. It has to deliver the pressure at the same pressure around the human. If it was too low, you wouldn’t be able to expand your lungs and breath.
Luckily, they have regulators for welding stuff too. You need a second regulator to let air out only when you breath—but it’s possible to build one of these (they are much simpler).
Now for the DPV. These things are very useful in cave diving. How are you supposed to get 5,000 feet back in a cave if you have to swim the whole way? The early DPV (or scooters as we called them) were essentially trolling motors from a bass fishing boat connected to a battery.
Just about any electric motor with a battery could work. Ideally, the motor should be sealed so it can run underwater—but it doesn’t have to be perfect. It only has to work for a short time.
I really like the scooter in the episode. It really looks like a home built scooter.
OK, you can’t see it too well in that image—just trust me. Or better yet, watch the episode.
I also like how MacGyver side mounts his tank. Even if you have done normal scuba diving, you might be surprised at how these tanks behave underwater. Just because they are heavy out of water doesn’t mean they will pull you down underwater. Very often we would bring extra tanks (stage bottles) in a cave and carry them on our side just like MacGyver did.
Parkour Wall Jump.
MacGyver gets to run up a wall twice in this episode. Just like the wall run with the pole, this case also uses friction. It’s your classic parkour wall run-jump.
If you run towards a wall and push yourself back, there is a force between you and the wall. The faster you run, the greater the force. If this force is great enough, there will be a large enough upward frictional force so that you can get an extra upward jump.
It’s not a super Mac-hack, but it works. The only thing MacGyver does is to put a toothpick in place a grenade pin. It only works for a little bit before the grenade explodes. However, the physics discussion is pretty good. Let’s go over some of the terms.
Tensile strength. This is essentially the maximum force a material can withstand when being pulled apart. Just imagine a rope—how hard can you pull on the rope before it breaks. That would be the ultimate tensile strength. Yes, wood has a pretty high tensile strength.
Compressive strength. How hard can you squeeze the thing before it fails? Something like concrete has a very high compressive strength, but not so much with tensile strength. Wood could have a good compressive strength if it’s wide and short. Long skinny boards of wood tend to buckle.
Sheer strength. This is the maximum force an object can withstand when two forces are pushing in opposite directions but not directly at each other. Think of scissors.
What is a dirty bomb?
This is another non-hack. However, I just want to describe the difference between a nuclear bomb and a dirty bomb. A nuclear bomb uses a nuclear reaction (usually started with conventional explosives) to make a massive boom.
The dirty bomb is NOT a nuclear explosion. Instead, it uses conventional explosives to spread radioactive material around. It’s dirty.
Electromagnet
MacGyver builds a strong electromagnet to move a bolt inside a locked door. Yes, this is possible. You would need a strong electromagnet—that means high electric current and thus thick wires. You also need a fairly beefy battery to get this much current.
Oh, one possible problem. If the bolt is ferromagnetic (steel) and so is the door, then it’s going to be difficult to get that bolt to move. However, if the door is aluminum or some type of non-ferromagnetic material then this would work.
Wall walk
There are two methods to get over the pressure sensitive floor (they end up not using this though). There is a wall-walking stilt method and a rolling sled method. Both have the contact point with the wall at an angle—this is needed in order to work (because of physics).
Let me just start with a setup that would only barely work. Here is a view of a person using completely horizontal stilts along with the forces on the person.
The first problem is that the stick the reaches across the hall would have to fit perfectly. The harder it is pushed against the wall, the greater the frictional force. And it is this upward frictional force that balances the weight pulling down.
The second problem is with these horizontal arms. When they attach to the person, there is no upward force. This would be like trying to hold a rope with a weight in the middle perfectly horizontal. It won’t work.
Here is a better option.
This setup fixes both of the problems. The sticks can be longer than the hallway (and not fit perfectly) and there is now an upward component from the wall that helps support the human.
Mercury Switch
What is a mercury switch and how can you build something similar? Here, I made a video for you.
Smartphone Radiation Detector
MacGyver uses some smartphones (as usual) to detect radiation from the dirty bomb. This is essentially real.
A smartphone camera has a sensor that is normally used to detect light. However, this same sensor can be set off by other types of radiation—like the stuff that is produced by radioactive stuff.
In order to actually detect this radiation, you need to block the light from getting to the camera—electric tape over the lens will do the trick.
But wait! There is a real project that uses normal human smartphones to detect cosmic radiation. Check it out—the CRAYIS Project. https://crayfis.io/
MacGyver gets a broken spray can of something (it really could be any aerosol can) and attaches a cigarette lighter to it. He then makes it so the lighter burns while the spray sprays. When he throws it, boom.
Yeah, these spray cans can ignite stuff. This is plausible.
DIY hot air balloon for Jack’s phone
Someone needs to make a super clip of all the times Mac says “Jack, I need your phone”. I think that’s funny.
In this case, the idea is to build a mini hot air balloon to lift up the phone so that they can see a “bird’s eye view” of the city. Here’s how it works.
Get a thin plastic trash bag.
Get some fuel—in this case it’s that stuff that burns to keep food hot for a buffet or something. Oh, they put it in aluminum foil.
Hang the phone and light the fuel.
Boom. That’s it. Yes, it’s real. The basic idea is that the fuel heats up air that fills the bag. Hot air has a lower density than cold air—this means that the weight of the air inside the bag is less than the weight of air outside of the bag. This gives a net upward buoyancy force on the bag.
OK, but would this be enough to lift a phone? It would be tough, but it’s at least plausible. It depends on the weight of the phone and fuel, the size of the bag, the temperature of the inside and outside air. So, it’s possible.
Here is one you can make yourself.
Bullet proof paper
OK, it’s not bullet proof paper. It’s a calculation of how much paper you would need to stop a bullet. I love how well this turned out.
Bullet proof shield
This one is simple. Yes, if you tape a bunch of kevlar vests to a door it will be fairly bulletproof. MacGyver’s calculation is great (I should know). OK, it’s not perfect—but it’s a good example how to make a basic estimation.
Personally, the dialogue gets to the basic point and the animations and graphics are really nice. LOVE IT.
Let’s go over some of the details.
You need some basic values—like the speed and mass of a bullet from an AK-47. I googled this, but maybe MacGyver just knew it.
From there, you want to somehow model the interaction between a bullet and paper. The first idea is to think of it like a drag force (just like a bullet going through air or something). Of course this causes a problem because that makes it a velocity dependent force and therefore VERY difficult to deal with.
But what if there is a constant force on the bullet during the interaction with the paper? In that case, we can use the work-energy principle (which MacGyver says—YAY!).
With a constant drag force, you can then find the distance over which this force needs to do work to stop the bullet.
For the constant drag force, I estimated the density of paper (a little bit lower than the density of water) and assumed this was the constant force. Of course this is wrong—but it’s just a place to start. You have to start somewhere.
In order to fend off the attackers, MacGyver makes some improvised explosives to shoot marble cannon balls. I don’t want to go into the chemistry of explosives so I will just put my normal explanation.
Any time you mix two or more chemicals, it is plausible that it could make an explosion. The end.
Intercept radio transmission
MacGyver wants to figure out what the bad guys are saying on their radios. He uses a yaghi antenna to get a directional signal and then he connects it to an AM/FM radio and picks up the signal.
OK, they probably don’t broadcast on the AM-FM frequency range. However, it’s possible he could modify the tuner in the radio to pick up their frequency. It would help if he knew their frequency. Also it’s hopeful that they aren’t using encrypted radios.
Dish soap to slide a safe
This is basic physics. Dish soap can indeed decrease the frictional force—especially for smooth surfaces. This would make a great physics problem.
Sugar putty bomb
Again with the bomb thing—using sugar for an explosive. Well, you can make a rocket from sugar (again—from the MythBusters)
Radio detonator
In order to detonate the explosives, MacGyver takes apart one radio such that it makes a spark when receiving a signal (instead of making a noise). This is fairly plausible.
There are a couple of other things, but I will stop here.
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