# A block of cheese hits a steel wall

• mrspeedybob
In summary, when a block of cheese impacts a steel wall at a speed greater than the speed of sound in cheese, the leading molecules of cheese are unable to transmit the force of impact backward due to the limitation of the speed of force propagation. This results in the molecules moving either sideways or into the steel. If the steel is strong enough, the only option is sideways, which may violate conservation of momentum. However, this is not the case because momentum is a vector and the resultant momentum from all the sideways motion will be zero. The transmission of sound is also the transmission of force, so the idea of force propagation being faster than sound propagation seems contradictory. It is likely that a shock wave can propagate backwards through the cheese and the cheese will

#### mrspeedybob

...at greater then the speed of sound in cheese. The leading molecules of cheese cannot transmit the force of impact backward because the speed of force propagation is limited by the speed of sound in cheese so the molecules must move either sideways or into the steel. If the steel is strong enough then the only option is sideways. The area of impact may be large enough that the sideways velocity needs to be larger then the incoming velocity, but this would violate conservation of momentum. I can see the following possible solutions to this dilemma, but I don't know which, if any, are correct.

1. As the cheese hits the steel it compresses, forming a more dense medium through which force can be move rearward. I don't like this idea because it raises the problem of which moves rearward through the cheese first, force or compression. Compressing the cheese requires force, but the force cannot move backward because the cheese is moving forward faster.
2. I cannot treat the steel wall as rigid. It will deform to make room for the compressed cheese and/or cheese molecules will be forced in between metal atoms creating a cheese/steel mixture at the interface. I don't like this idea because replacing the steel with a stronger substance or replacing the cheese with a softer substance returns me to the original problem. Surely there is some combination of substances in which the wall can be treated as rigid and impermeable
3. There are conditions in which the propagation of force through a medium can be faster then the propagation of sound through the same medium. I don't like this idea because the transmission of sound is the transmission of force, it seems self contradictory.

So. Is there any truth to any of these? What is really going on at the cheese/steel interface?

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mrspeedybob said:
If the steel is strong enough then the only option is sideways. The area of impact may be large enough that the sideways velocity needs to be larger then the incoming velocity, but this would violate conservation of momentum.
Momentum is a vector, not a scalar. The resultant momentum from all the "sideways" motion, in different directions, will be zero, however large the sideways velocities might be. So conservation of momentum is not violated by sideways motion of the cheese.

On the other hand, you didn't ask what happened to the momentum of the cheese perpendicular to the wall, which might seem to be reduced to zero. That is not the case because the momentum is actually transferred from the cheese to the earth, but the very large mass of the Earth means its change in velocity is too small to notice.

Note that the transmission of sound is the speed of the transmission of a compression wave in the limit of very low amplitude. A high amplitude compression wave will travel more quickly than the speed of sound (also known as a shock wave). So, a shock wave can propagate backwards through the cheese. In addition, the cheese will likely compress rather significantly at the cheese-wall interface,so it won't just flow directly outward at higher than the incoming velocity (which would violate conservation of energy, though it would actually not violate conservation of momentum).

cjl said:
(which would violate conservation of energy, though it would actually not violate conservation of momentum).

No violation if only a part of the cheese ends up going faster than the incoming lump. And this is what we observe in many collisions: parts fly off very much faster than the original objects. And that is in spite of the fact that most real-life collisions are not actually elastic.

Quick question. What is the speed of sound in cheese? Does it matter if the cheese is Camembert or Roquefort?

I imagine the speed wouldn't be very different except for the really hard cheeses. Cheese would almost certainly have very high sound absorbency. Good for recording studio walls, no doubt and handy for a spot of lunch too.

sophiecentaur said:
I imagine the speed wouldn't be very different except for the really hard cheeses. Cheese would almost certainly have very high sound absorbency. Good for recording studio walls, no doubt and handy for a spot of lunch too.

Silence your neighbors AND have a handy snack!

Of course unless the studio is very chilly, the walls would gradually melt and rot, particularly in the summer. I think I would rather annoy my neighbors than attempt to record music in either a refridgerator or a room surrounded by purifying cheese.

But, of course the cheese compresses. Portions of the cheese block that are behind the impact front do not know they should slow down, stop, or move sideways, and so they pile up into the cheese in front that has been notified that it needs to change speed and direction. The cheese continues to move forward unimpeded until it crosses the back-propagating compression wave (a shock in the high-amplitude limit).

This thread deserves to be memorialized for all to experience the "cheesiness".

sophiecentaur said:
No violation if only a part of the cheese ends up going faster than the incoming lump. And this is what we observe in many collisions: parts fly off very much faster than the original objects. And that is in spite of the fact that most real-life collisions are not actually elastic.

Very true. It's fairly likely that a small portion of the incoming cheese will fly off much faster than the initial velocity, but it must be a relatively small portion such that the overall kinetic energy of the debris is less than the initial kinetic energy of the block of cheese.

I was thinking of this myself, recently, although with a metal bar rather than cheese. Very interesting...

## 1. How does a block of cheese hitting a steel wall affect the cheese?

When a block of cheese hits a steel wall, the impact causes the cheese to compress and deform. The force of the impact can also cause the cheese to break or crumble.

## 2. Why is a block of cheese used in this experiment instead of another object?

A block of cheese is often used in this experiment because it is a soft and pliable material that can easily deform upon impact. This allows for a clear observation of the effects of the collision.

## 3. What factors can affect the outcome of this experiment?

The outcome of this experiment can be affected by the size and shape of the block of cheese, the speed and angle at which it hits the steel wall, and the material and thickness of the steel wall itself.

## 4. What can we learn from studying the collision between a block of cheese and a steel wall?

Studying this collision can help us understand the principles of physics, such as momentum and energy transfer. It can also provide insights into the behavior of different materials under impact and how they respond to external forces.

## 5. How can this experiment be modified for further study?

This experiment can be modified by using different types of cheese, changing the speed or angle of impact, or using different materials for the wall. It can also be replicated with other objects to compare the effects of different collisions.