# kinetic energy

### A thought experiment for the relativity skeptics

"Great spirits have always encountered violent opposition from mediocre minds." -Albert Einstein It may be hard to believe, seeing as how it's been our leading theory of gravity for nearly a century now, but Einstein's General Relativity is possibly the most frequently challenged scientific idea of all-time. Of course, it's emerged victorious from each and every one of those challenges, making a slew of unintuitive predictions that have been spectacularly confirmed each time they've been tested. Image credit: Miloslav Druckmuller (Brno U. of Tech.), Peter Aniol, and Vojtech Rusin. This…

### What if everyone jumped?

Might as well jump. Jump. Go ahead, jump. - Van Halen Suppose everyone in the world got together and jumped. Would the Earth move? Yes. Would it be noticeable? Time for a calculation. Note: I am almost certain that I have done this before, but I can't find where. Starting assumptions. 7 billion people. Average weight: 50 kg (you know, kids and stuff) Average vertical jump (center of mass): 0.3 meters - and I think that is generous. Mass of the Earth: 6 x 1024 kg Gravitational field near the surface of the Earth is constant with a magnitude of 9.8 N/kg Ignore the interaction with the Sun and…

### Turn or go straight? Quick!

This is a classic problem. You are in a car heading straight towards a wall. Should you try to stop or should you try to turn to avoid the wall? Bonus question: what if the wall is not really wide so you don't have to turn 90 degrees? Assumption: Let me assume that I can use the normal model of friction - that the maximum static friction force is proportional to the normal force. Also, I will assume that the frictional coefficient for stopping is the same as for turning. Stopping I am going to start with the case of trying to stop. Suppose the car is moving towards the wall at a speed…

### Collisions: Kinetic energy or momentum?

In the last episode of MythBusters, they wanted to see if a tornado could make some glass cut off a person's head. The first attempt was just to take some glass and through it at a simulated human neck. Clearly, this wasn't quite the same as a tornado. So, here was their plan. If they want to simulate glass moving at 300 mph, they could get a bigger piece of glass and put it on a truck moving at 80 mph. The result would give a piece of glass with the same kinetic energy as a smaller piece moving at 300 mph. Their calculations look to be correct. However, the question is: would this make…

### Ask a ScienceBlogger: Evaporating Water

Here is another question from Ask a ScienceBlogger. Reader Uday Panta asks: How does water evaporate in the seas? Doesn't water evaporate at 100 C? There were some very good responses in the comments where the question was, but I am going to answer it with some more details. Small Particle Model This is where we need to start - the small particle model of liquids and gases. This model treats the liquid as being made up of a lot of particles (well, obviously). If there is a gas (or liquid) at a certain temperature, then there are particles moving around at different speeds. Often it is…

### How hot would the space jumper get?

A new video from the Red Bull Stratos Jump guys came out. Here it is: This reminds me of an unanswered question about the Stratos jump that I didn't address on my last post on this topic. Commenter Long Drop asked about how much Felix would heat up as he falls from 120,000 feet. This is a great question. The first, off the bat answer is that he won't heat up too much. Why do I say this? Well, when Joe Kittinger jumped from over 100,000 feet and didn't melt. Still, this is a great thing to calculate. How do you calculate something like this? I will look at this in terms of energy. For…

### MythBusters' energy explanation

I already mentioned the MythBusters' crashing two cars episode where they correctly doubled the speed of a pendulum type object. Overall, this was a very visual (although expensive) demo. There was one part that left a sour taste in my mouth - the final explanation from the narrator. First, they showed this. And then they had an explanation that went something very similar to to this (after restating what the sign above said) "Although the two-car crash doubles the speed, the energy the crash is transferred to twice the mass resulting in a crash that looks like just one car hitting a wall…

### MythBusters and double the speed

In the last episode of MythBusters, Adam and Jamie wanted to test something that Jamie had said earlier: "Two cars crashing head on at 50 mph is the same as one car crashing into a wall at 100 mph" Jamie was wrong, but that is not what I am going to talk about. Instead, I am going to talk about Adam's small scale test of this situation. Really, it was a nice set up. Basically, he wanted to collide something into a wall at one speed and then double that speed. Then he was going to collide two things together at the lower speed. He had a cool way of measuring the collision. He put a piece…

### Some People Running

This video seems like it is getting popular, but maybe that is because it is so awesome. Maybe it is just me, but I find this video very visually satisfying. I love the way they compare the different runners. Anyway, there is some physics here. Commenter Ben sent me the link to this video (thanks!) with some great questions. Which of the runners has a greater kinetic energy? What about the power? These aren't too difficult to answer, but the first thing is to get the data. There are several options (including just using a stop watch). But no, that is not good enough for me. Instead, I…

### What could you do with 54,000 watts?

I already looked at ESPN's Sport Science episode where they calculate that Marshawn Lynch produces 54,000 watts when pulling some tires. Yes, that is way too high. However, what would happen if some was actually that powerful? What could that person do? How fast could they run 100 meters? That is what I am going to calculate. First, I am going to assume that Marshawn has a mass of about 100 kg. Also, let me say that he can produce 54,000 watts no matter what his speed. Take a short time interval. During this time, Marshawn will increase his speed from say v1 to v2 this would be a…

### More ESPN Sport Science Goodness

Let me be clear. I am not really an attacker. If someone wrote a report about ski jumping or something and misused the word "momentum", no big deal. However, if you have a show that claims to be about SCIENCE and you are obviously putting a lot of money into this show AND a whole bunch of people will see and think this is science - then you need to be a little careful. I think shows like ESPN's Sport Science are a good idea - you know, introduce some cool science ideas by using cool sports. This show just needs some help. Yes, I know I make mistakes. I try to correct them when I become…

### MythBusters, Falling, Stopping, and Integration

In a MythBusters episode some time ago, Adam and Jamie jumped off a building. There was some cool stuff in this, but I want to focus on the acceleration data they collected. Before jumping into a pit of foam, they first wanted to test the set up by dropping a dummy into it and measuring the accelerations. Lucky for me, they showed a quick screen shot of their data. Note: I previously posted the calculations for jumping and stopping off of a building. For me, I see this and think - numerical integration. Before that, let me look at the physics. Here is a diagram of someone jumping off a…

### MythBusters: How do you characterize a collision?

Have I not made it clear how much I like the MythBusters? Also, I am totally aware that they are not (nor do they claim to be) scientists. Really, this is what makes their show appealing (maybe?). So here is the problem now. And, it is not just the MythBusters - I see other shows making the same mistake. If two things are colliding, how do you characterize the collision? No one really gets this right. For this particular episode, the MythBusters were looking at the collision between a bullet and gun. They wanted to see how hard it would be to shoot a gun out of someone's hand. The…

### Energy in an exploding water heater

The more I think about the last MythBusters' exploding water heater, the more cool things I see. How about I look at the energy of the explosion. There are three things I can look at: How much energy went into the water heater from the electric source? How much kinetic energy did the water heater have right after the explosion? How much thermal energy did the water and water heater have? How much gravitational potential energy did the heater have at it's highest point? Hopefully, I can show that the energy in from the electric source is greater or equal to kinetic plus thermal. Also, the…

### Punkin Chunkin - they will never make a mile range

In my previous post on launching a pumpkin (punkin chunkin) I essentially just looked at what happens to the pumpkin after it is launched. How fast would you have to shoot it to go 1 mile? The answer seems to be around 1000 mph and they are currently shooting them around 600 mph. The question for this post, how fast can you launch a pumpkin so that it does not smash into smithereens? First, why would it smash at all? Here is a diagram of a pumpkin being launched while still in the tube. The pumpkin launcher works by releasing compressed air inside the tube. This means that the force…

### MythBusters and the first water heater

Last night I saw the newest episode of MythBusters. One of the myths they revisited was the exploding water heater. Well, it turns out that I had an analysis of this first explosion, but I didn't move it over when I switched software. So, here it is. In case you never saw the first episode of exploding water heaters, here is the important part: If you are impatient, here are the answers (from the video analysis): Time of flight = 11.8 seconds Max height = 167 meters = 548 feet Launch speed = 234 mph Speed on impact with the ground = 76 mph First, from re-watching the video, I can see (and…

### Snow Board Jump Help

I really shouldn't do this. I might be helping someone to set up something dangerous. But, I am going to anyway. Here is a question posted on some forum. (actually, it is from math help forum) "I'm anticipating a good winter this year, one with lots of snow. My yard is sloped quite a bit and it would be the ideal place for a huge snowboard jump, only problem is I need to calculate how fast I will be traveling when I hit the jump, how high and what angle the jump should be, and the distance and angle of the landing ramp to optimize my range." So, what am I going to do? I am going to give…

### Labs to not do - mechanical equivalent of heat

Note to self: don't do the mechanical equivalent of heat lab again. It doesn't really work that well and there are better labs to do. So, what is the mechanical equivalent of heat lab? It is actually a pretty cool idea. Take and object and drop it. What happens to the kinetic energy the object had right before it hit the ground? Most of it goes into thermal energy of the object and surroundings. In this lab, the students measure the change in gravitational potential energy for a falling object (where object is really lead shot or something) and then measure the change in temperature in…

### July 4th and an example of work-energy

July 4th can be fun. One activity my family enjoys is playing in the lake at my parents house. Along with this comes the jumping off the dock. Great fun, and great physics. Here is a short clip. Work Energy Example from Rhett Allain on Vimeo. Notice that I violated my own rules for making videos. In particular, the camera was not perpendicular to the motion. Also, I can handle panning cameras, but not when there is nothing but sky in the background. This video is therefore not appropriate for a video analysis. That is ok. I don't need it to talk about physics. So, here is the…

### How much would "free" energy cost you?

This was on my 'to do' list, but Tom at Swans on Tea beat me to it. Basically, this grocery store has these plates that when depressed produce electrical energy. Tom does a good job pointing out that this is not free energy (the original article says this also). Clearly, the energy comes from the cars. How much would this cost the cars? As always, let me start with some assumptions. The original article says that the bumps will generate 30 kW of energy every hour. That is an odd thing to say. I am going to interpret that as 30 kW of power for all hours (every hour). They couldn't have…