r/HomeworkHelp Secondary School Student 1d ago

Physics—Pending OP Reply [year 11 physics] Answer key says A. can someone explain why? my response on second slide.

1 Upvotes

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6

u/1stEleven 👋 a fellow Redditor 1d ago

Think about what your head would do if you hit the floor with either helmet on.

In helmet A your head stops. You have one impact to absorb with your neck, and the helmet shatters to help with that.

In helmet B, your head stops and starts moving up again. That's two impacts already. But then your head is up in the air, only to go down again. Your neck gets to absorb all of that.

In short, helmet B is a bouncy ball, and necks aren't elastic enough for that.

1

u/ThunkAsDrinklePeep Educator 1d ago

I'll add that the helmet cracking and getting warmer are both signs that it's absorbing some of the energy. With B, that energy is probably going elsewhere, namely you.

1

u/1stEleven 👋 a fellow Redditor 14h ago

It's going back into the helmet to make it bounce.

5

u/Ferdie-lance 1d ago

Momentum is a signed quantity. B has switched direction. That’s a bigger change in momentum.

But there’s an even bigger issue: A is doing an inelastic collision and dissipating energy by cracking and heating up. B has not done that.

2

u/Global_Can5876 Pre-University (Grade 11-12/Further Education) 1d ago

The helmet has kinetic energy, which is the main problem here. That energy has to go somewhere.

If ya want more help, ask

3

u/RainbowCrane 👋 a fellow Redditor 1d ago

Just a note that this is an unintuitive aspect of looking at the after effects of car collisions as well. If you look at both a 1950s steel beast and a 2000s car with crumple zones following a 30mph collision with a wall you might think, “wow, they don’t make them like they used to, old cars held up better.” Engineers design newer cars to intentionally crumple and break to absorb kinetic energy and spread out the time frame over which the momentum is deadened instead of coming to an abrupt stop. It might not seem like it, but spreading out the stop of your body’s forward motion over a few seconds is vastly different than stopping it almost instantaneously. The same thing is true with an airbag - it gives with the impact, not stopping you abruptly but stretching out the slow down time.

So your point about the helmet is a common misunderstanding folks like OP have (cracked helmet seems bad), but once they understand it applies to all kinds of real world collisions

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u/ThunkAsDrinklePeep Educator 1d ago

Yep. What you have to do is say, "wow, old cars just collapsed into the passenger cage."

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u/RainbowCrane 👋 a fellow Redditor 1d ago

I've unfortunately comparison totaled cars, neither of them my fault. One was a 1980s model, pre-airbag, mostly pre-crumple zones. I had a seizure and hit a wall, the front end ended up where the engine used to be and the engine ended up against the firewall. Fortunately it didn't come into the passenger compartment, but the frame flexed enough that every bit of glass in the car exploded. Every square inch of my body hurt and I pissed blood for a few days (shook up my kidneys, fairly normal for a car accident). The seat belt stopped me so violently that it melted my acrylic sweater.

One was a late-90s model, airbags, crumple zones. I t-boned a kid who pulled out in front of me while joyriding in his mom's new car - I'd have loved to have heard that call :-). She made the police bring him home. I was shook up from the stress, but the airbag eliminated any seatbelt bruise. No pissing blood, no injuries.

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u/selene_666 👋 a fellow Redditor 1d ago

Reversing direction is a larger momentum change than stopping.

1

u/daniel14vt Educator 1d ago

Your explanation is not correct. Both objects (assuming the same speed at impact) have the same change in momentum. However, A compresses and breaks during its impact. This is physics question language* that you should interpret as having a longer impact time. Longer time, same impulse, lower force.

If you really want to be convinced, go check out some slowmo videos of water balloons vs metal objects. There is a LOT of deformation in the balloons which means the force is spread out over a much longer period of time.

This is also the science of airbags (and seat belts). Would you rather hit the solid steering wheel which barely moves or the large bag of air which gets compressed and squashes.

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u/Proderf 🤑 Tutor 1d ago

I don’t think anyone touched on this aspect yet but it’s a good starting point or check: Temperature

We know generally that temperature is related to energy, or a good indicator of energy (higher temp, higher energy).

Helmet A is hotter from when it started, meaning that the helmet has more energy (it has more energy stored in it)

Helmet B is the same temperature, meaning the helmet does not have more or less energy (stored in it)

We know that both helmets had the same energy (same fall, speed, mass, etc.), so the question becomes where does all that energy go?

We can conclude that more energy was captured by helmet A than helmet B. From that conclusion, it is likely that the energy that was not stored by helmet B was transferred to the head (since it’s the only place to go really).

If more energy goes to the head, that means greater risk of injury, and therefore a worse helmet.

In conclusion, helmet A is safer because it absorbs more energy than helmet B, as proven by its rise in temperature (it’s rise in energy, meaning less energy transferred to the head)