Why didn't the wrecking ball smash his head?

[Chenkel]

TL;DR Summary

In a Walter Lewin lecture the wrecking ball was released at the height of the glass and ended up breaking the glass

[Mentor Note -- post appears to have been edited based on post #2]

Hello everyone! I was watching a Walter Lewin lecture and saw him do something very dangerous, he released a wrecking ball at a certain height from the ground, upon returning the ball did not smash his head. If you watch the video, you see him touch some glass with the ball, release it at seemingly 0 velocity, and the ball comes back, and smashes the glass! Wouldn't you expect the ball to not touch the glass on the return journey? That's what I'm confused about, here's the video, the segment is about 46:36 into the video

 

[kuruman]

I would like to help you but I am not going to watch a 1-hour long video. Please provide a time reference so that I (we) can fast forward to the event that you are describing without having to go through the other stuff.

 

[Chenkel]

I would like to help you but I am not going to watch a 1-hour long video. Please provide a time reference so that I (we) can fast forward to the event that you are describing without having to go through the other stuff.

I believe it's 46 minutes and 36 seconds in. After watching the video again, it looks like he does add a little velocity, let me know if you guys see the same, thank you!

 

[berkeman]

In one of my undergrad Physics classes, the prof did a similar demonstration in the lecture hall (probably 300 students attending). He pulled the rope and bowling ball weight up as he climbed a ladder on one side of the stage, then held the ball to his chin and released it to swing away and then back at him.

Everybody in the building expected the experiment to be a success and the ball to stop and turn around at the same point right in front of his chin, but unfortunately for the professor, he was not careful controlling his body position (and his associated head position) during the few seconds of the swing. It turned out that he had drifted an inch or two forward, and...

He was not injured, but his pride sure was!

 

[Chenkel]

In one of my undergrad Physics classes, the prof did a similar demonstration in the lecture hall (probably 300 students attending). He pulled the rope and bowling ball weight up as he climbed a ladder on one side of the stage, then held the ball to his chin and released it to swing away and then back at him.

Everybody in the building expected the experiment to be a success and the ball to stop and turn around at the same point right in front of his chin, but unfortunately for the professor, he was not careful controlling his body position (and his associated head position) during the few seconds of the swing. It turned out that he had drifted an inch or two forward, and...

He was not injured, but his pride sure was!

That's amazing he didn't die because bowling balls are very heavy! He sure is one lucky guy

 

[Office_Shredder]

That's amazing he didn't die because bowling balls are very heavy! He sure is one lucky guy

An inch of acceleration is not very much.

 

[Vanadium 50]

He was not injured, but his pride sure was!

It is best to do this with your back to a wall. Otherwise, it is natural to lean back when holding the ball, and to stand up straight after releasing it, This is sub-optimal.

There is good news - you're getting smacked in the face when the energy is mostly potential and the ball is at its slowest.

 

[Ibix]

That's amazing he didn't die because bowling balls are very heavy! He sure is one lucky guy

The point of this experiment is to correct that kind of intuitive "this is going to hurt" judgement. Lewin's version uses a fairly shallow arc, but imagine one where the suspending cable starts at 45°. If you lean forward 5cm (~2in) the ball hits you when it only has 5cm to climb before stopping, so its kinetic energy is the same as the remaining gravitational potential ##mgh##. Even if the ball weighs 10kg (an estimate on the heavy side, I suspect) that's only 5J, and it can't push your head back more than 5cm. That won't be fun but it's unlikely to be lethal - I don't know how reliable this site is, but it suggests your average untrained guy can deliver around 100J with a punch - which is eminently survivable or the pubs at the dodgy end of town would be strewn with dead bodies on a Saturday night.

Also, look at Lewin's video again - the ball comes to rest quite a long way short of his face. He's smart enough to use a fairly lossy system so he has quite a safety margin due to friction slowing the ball, and he could lean forward an inch or two with no risk. So a minor posture change like @berkeman's lecturer made probably barely brought him into contact - but even a light tap on the jaw is quite shocking, especially if you weren't expecting it.

 

[Chenkel]

The point of this experiment is to correct that kind of intuitive "this is going to hurt" judgement. Lewin's version uses a fairly shallow arc, but imagine one where the suspending cable starts at 45°. If you lean forward 5cm (~2in) the ball hits you when it only has 5cm to climb before stopping, so its kinetic energy is the same as the remaining gravitational potential ##mgh##. Even if the ball weighs 10kg (an estimate on the heavy side, I suspect) that's only 5J, and it can't push your head back more than 5cm. That won't be fun but it's unlikely to be lethal - I don't know how reliable this site is, but it suggests your average untrained guy can deliver around 100J with a punch - which is eminently survivable or the pubs at the dodgy end of town would be strewn with dead bodies on a Saturday night.

Also, look at Lewin's video again - the ball comes to rest quite a long way short of his face. He's smart enough to use a fairly lossy system so he has quite a safety margin due to friction slowing the ball, and he could lean forward an inch or two with no risk. So a minor posture change like @berkeman's lecturer made probably barely brought him into contact - but even a light tap on the jaw is quite shocking, especially if you weren't expecting it.

I believe where the force is applied is also significant when considering potential injury. I heard of a story, if I recount correctly, a man was swimming in a pool, and as he swam to one side, he failed to notice the wall approaching; I believe his head was aimed slightly upward, and a relatively small force pushed his head backward, and he ended up with a broken neck. The story was told to me by my father, so I'm not sure how much force was involved, but I've heard of many broken necks every year from accidents in pools, so it's something to keep in mind.

 

[Ibix]

A man weighing 70kg doing 1m/s (~2mph, which a quick Google suggests is a typical human swimming speed) has 35J to dissipate, and would tend to keep going where the pendulum is constrained to stop after 5cm. I'd be surprised if 35J was enough to kill, but I'm told there are angles where necks dislocate quite easily. But we're talking about 5J with 5cm of follow-through, much of which would be absorbed by the jaw.

I'm not suggesting stepping in front of swinging wrecking balls is good for your health, but I think you'd have to take a fairly large step forward for this experiment to be risky. There's a reason Lewin used glass and not anything tougher when he wanted to show something breaking!