How Does Quadrupling Playback Speed Affect the Mach Number in Film Effects?

[Idioticsmartie]

Speed of Sound and Mach

Homework Statement

You are hired as the physics consultant for a large movie studio. The producer is way over budget and wants to perform some special effects that will save him money. He decides to shoot the dramatic action boat chase scene with the boats going at 1/4 the speed of what will be seen on film. He quadruples the speed of playback. Explain why or why not this will work.

Homework Equations

Mach # = velocity of boat/ speed of sound
Speed of sound = 331 + 0.60*T
T = temperature

The Attempt at a Solution

I don't even know where to begin. I don't think it will work, but maybe if someone could point me in the right direction?

 

[learningphysics]

I think the idea is that the sound won't be right. What happens to the frequency of the sounds heard when playback is quadrupled?

 

[ogb p]

As a physics consultant for the movie studio, it is my responsibility to ensure that all special effects are both scientifically accurate and visually appealing. In this case, the producer's idea of shooting the boat chase scene at a slower speed and then quadrupling the playback speed may seem like a cost-saving measure, but it is not a feasible solution.

Firstly, let's consider the concept of Mach number, which is the ratio of an object's speed to the speed of sound. In this scenario, the boats are moving at 1/4 the speed of what will be seen on film, which means their Mach number is 0.25. However, when the playback speed is quadrupled, the boats will appear to be moving at 4 times their actual speed, giving them a Mach number of 1. This is well above the speed of sound, which can cause significant issues.

When an object travels at or above the speed of sound, it creates a shock wave known as a sonic boom. This can be extremely loud and can even cause damage to nearby structures. In the case of the boat chase scene, the boats appearing to move at a Mach 1 speed would create a deafening sonic boom, which would not only be unrealistic but also potentially dangerous for the actors and crew on set.

Furthermore, the speed of sound is affected by temperature, as shown in the equation provided. This means that if the temperature on the day of filming is different from the temperature when the playback speed was quadrupled, the boats would appear to be moving at a different speed than intended. This would result in an inconsistent and inaccurate portrayal of the boat chase scene.

In conclusion, while the producer's idea may seem like a cost-saving measure, it is not a scientifically accurate or safe solution for the boat chase scene. It is important to adhere to the laws of physics and ensure that all special effects are realistic and safe for everyone involved.