Physics 111: Fundamental Physics I: Why did the indestructible glass windows of Tesla’s Cybertruck break during the reveal event?

In 2019, Tesla released the Cybertruck which was marketed as powerful, rough, and very durable. Elon Musk and Franz von Holzhausen revealed the truck’s “indestructible” properties to the public in a live premiere event. In the event, the truck’s stainless-steel body and windows were tested. The truck’s steel body was hit with a sledgehammer and was undented, however the truck’s glass windows broke when a metal ball was thrown at them. Watch the two-minute clip below of the live event if you have not already seen it.

https://www.youtube.com/watch?v=6Q3uaepRCl4

Prior to Franz throwing a metal ball on the truck’s window, his assistant dropped a metal ball on Tesla’s armor demo glass. And, Tesla’s armor demo glass did not crack. So, I wanted to use physics we learned in class to investigate why the truck’s windows broke but not the demo glass.

If the force exerted on the truck’s window was larger than the demo glass’s, it would make sense that the window on the truck broke. So, I calculated the force the dropped and thrown metal balls exerted on Tesla’s glass. I used the Youtube video above to derive my quantities, so my calculations required a lot of estimations but bear with me. Using kinematics, I first calculated the final velocity of the metal ball on the demo glass and the truck’s windows. To calculate the final velocity of the dropped metal ball, I estimated the height the ball was dropped from. The man who dropped the ball looked in the video to be about the same height as Franz who is around 6.2” (1.8796m). The man climbed up a ladder which was around 1.5x his height to the glass. So, the change in height of the ball was approximately 2.8194m. Neglecting air-resistance and assuming the ball was falling perfectly straight down (only in the y-direction) with constant gravitational acceleration, I used the kinematics equation vf^2= vo^2 +2 g Δy. The final velocity of the dropped metal ball was 7.43 m/s. To solve for the final velocity of the metal ball thrown, I again assumed no air-resistance, constant acceleration and that the ball was only moving horizontally in the x-direction at no angle. Since the Youtube video was only in seconds, I used a millisecond stopwatch to calculate the time the metal ball traveled when thrown which was 350 milliseconds (0.350s). I approximated that Franz was around one meter away from the car’s window. I first solved for the acceleration of the metal ball by using the equation Δx=vot+ ½ at^2. The acceleration was 16.3 m/s^2, and I solved for the final velocity by using the kinematic equation vf=vo+at where the final velocity was 5.71 m/s. Both the final velocity for the metal ball thrown and dropped were similar and only around 2 m/s away from each other.

I used Newton’s second law in the momentum form (∑FΔt= Δp) to calculate the force the dropped and thrown metal balls exerted on Tesla’s glass. I am assuming that the same metal ball (same mass/dimensions) was used both for the drop and throw, and I estimated the ball to be 1 kg. Tesla’s website suggests that the glass can withstand a 9mm bullet, so I searched online the weight of a 9mm metal ball, and it was around 1 kg. So, I was happy with my really rough estimation. To calculate the force on the demo glass, I estimated the time the ball just barely was above the glass to when the ball fully stopped moving on the glass. I used a millisecond stopwatch for the calculation, and it was around 5 milliseconds (0.005s). The ball bounces off of the demo glass which gives the ball a longer amount of time to fully come to a stop on the glass. The final velocity of the dropped ball just before it hits the glass became my initial velocity in this equation. The force exerted by the ball was calculated to be -1486N (around 334 pounds). From Newton’s third law, the force exerted on the demo glass is equal in magnitude and opposite in direction to the force exerted on the ball. So, the force exerted by the demo glass on the ball was 1486N. To calculate the force on the truck’s window, I estimated the time to be 1 millisecond (.001s). The ball stopped moving almost immediately when hitting the window, so I used the smallest time estimate I felt appropriate. Again, the final velocity of the thrown ball just before it hit the window I used in this equation as my initial velocity. The force of the ball hitting the truck’s window was 5710N, and the force of the window on the ball was -5710N (around 1200 pounds). The force the ball exerted on the truck’s window was far greater than the force the ball exerted on the demo glass, so it makes sense that the truck’s window broke.

After the failed premiere, Musk posted proof on his Twitter that the truck’s window did not shatter when the metal ball was thrown at it just before the event. The link to the Twitter post is below. Glen Meyerowitz wrote an article on Medium and investigated this same question I posed above. Instead of calculating the amount of force of the demo glass in the drop, he calculated the amount of force of the window in the video below. He calculated the force of the window to be around 1700 N in which the truck’s window did not break. Although I did not investigate this specific scenario, his calculation makes sense compared to mine. The force of the demo glass when the ball dropped was less than 1700N and the window did not break. And, the force of the truck’s window when broken during the premiere was way higher and broke.

https://twitter.com/elonmusk/status/1198090787520598016?ref_src=twsrc%5Etfw%7Ctwcamp%5Etweetembed%7Ctwterm%5E1198090787520598016%7Ctwgr%5Eshare_3&ref_url=https%3A%2F%2Fwww.theverge.com%2F2019%2F11%2F25%2F20981489%2Ftesla-cybertruck-glass-window-fail-elon-musk-explanation

On Twitter, Elon Musk explained why he thought the truck’s windows broke. A sledgehammer was hit on the truck’s door prior to the metal ball being thrown at the truck’s window. So, Musk claimed that the sledgehammer may have caused the base of the window to crack before the metal ball was thrown. However, Franz also threw the same metal ball on the rear window and that window broke. The video does not show Franz whacking the sledgehammer on the rear door, yet maybe that did occur in the actual event. I would suggest that Tesla’s mechanics should try to focus on the mounting of the car to make the truck’s glass more flexible like the demo glass. This will create a longer amount of time for the metal ball (or any object) to stop moving once it comes in contact with the glass. Ultimately, this will reduce the force needed for the window to stop the metal ball (or any object). This demo also reminds us, like Glen Meyerowitz mentioned, how important actual testing in real scenarios are.