I usually preserve an eye on impending videos with appealing physics. In this scenario, it’s Captain The us: Civil War. The most recent trailer is wonderful, but permit me look again at an earlier clip that shows Captain The us jump-kicking some dude on best of a automobile. Of course, there is physics below.

Momentum and Collisions

Even even though Captain The us is kicking another person, the interaction can be regarded as a collision. We can represent this interaction as a force. Because forces are an interaction among two objects, the force that Cap pushes on the dude (which is his official title now) is the very same force that the dude pushes again on Captain The us.

If the collision is short, then this kick force is the only major force (gravity does pull on both of those, but the time interval is short). Searching at just Captain The us, I can use the Momentum Principle in the x-route to say:


In this article I am applying px2 as his momentum just after the kick and px1 as the momentum just before. Assuming that his mass does not alter through this interaction, I can write the momentum in conditions of his velocity.


So here’s the approach. If I measure the velocity of Captain The us just before and just after the kick, I can discover his alter in momentum. Because the dude has the very same force for the very same time, he will have the very same (but opposite) alter in momentum—if there are no exterior forces. Let’s just see what occurs.

Oh, but what about the electricity from the kick? This just indicates that Captain The us will include electricity to the program so that the complete kinetic electricity just after the kick could be increased than the kinetic electricity just before the kick. It is like an explosion—but momentum is still conserved.

Video clip Analysis

I require to get velocity values from the video. The ideal way to do this is with video analysis. My chosen instrument is the totally free Tracker Video clip Analysis software program. The standard notion is to look at the locale of an object in each and every frame of the video to get placement and time facts. There is one compact dilemma with this video—the digital camera moves. This indicates I must account for the motion of the object with regard to the track record. The good news is, Tracker Video clip Analysis has a method for working with these conditions applying calibration details. All I require to do is to scale the video (based on the measurement of Captain America’s shield), then transfer the coordinate program in each and every frame.

There are two objects to consider—Captain The us and the dude. Let’s begin with a plot of Captain America’s vertical placement as he jumps onto the truck.


By fitting a quadratic perform to this facts, I can get the vertical acceleration. The time period in front of t2 would be 1/2 the acceleration. This offers a vertical acceleration of about twelve m/s2—which is greater than the predicted value of nine.8 m/s2. I’m not upset. For a video like this, which is not much too terrible. Also, perhaps my scale for the shield is off. Let’s transfer on.

What about the horizontal speed of Cap just before and just after the kick? Here’s what I get.

Data Tool

Oh, consistent velocity. Which is interesting, suitable? The slope of this line is 4.09 m/s (nine.fifteen mph)—so that looks fair. But at what issue did Captain The us collide with the dude? It is not at the conclude, it’s at the locale with the compact arrow. Of course, his speed didn’t even alter. Was that mainly because he kicked the dude? No. If he kicked the dude, then the dude would have pushed again on him (in the negative x-route) and this need to minimize his x-momentum. But it didn’t. Okay, let’s look at the motion of the dude.

Data Tool

From this, the dude has a recoil speed of 8.fifteen m/s (eighteen mph). The recoil speed isn’t a dilemma, it’s the complete momentum. Suppose that the dude and Captain The us have the very same mass (it’s shut plenty of to be true). In this scenario, I can write the following expression for the x-momentum both of those just before and just after the kick.

La te xi t 1

Actually, the only way Cap’s velocity can be shut to the very same just before and just after the kick is if the dude has no mass (or if Cap’s mass is substantially substantially much larger than the dudes). So evidently there is a dilemma below. I will repair this by analyzing the suitable closing speed of Captain The us. Employing the equation over, I can remedy for vc2 (the c is for Cap) and I get:


Captain America’s kick would force him again in the opposite route.  He wouldn’t just preserve moving in the very same route.

Modeling a Kick

You really don’t truly recognize something until finally you make a product. In this article is a python product of Captain The us kicking the dude. I set the mass of the both of those objects to 75 kg, but this is something you need to alter and enjoy all-around with. The product starts off with Captain The us moving via the air. When he will get shut plenty of to the dude, there is a force pushing them apart. Oh, there isn’t a gravitational force on the dude just to make it look like the clip. (Just click on the enjoy button to see the code operate.)

You can see that if momentum is conserved, Captain The us need to recoil again just after the kick. The only way to repair this would be to have some exterior force pushing him ahead through this kick—maybe like a rocket pushing him. Okay, there is an additional clarification. Perhaps the dude has a wire that pulls him again when Captain The us kicks him. Of course, I know this is only a motion picture and not real lifetime. That does not suggest I should not evaluate it.

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