The discussion centers on the physics of collisions, specifically the interaction between a falling mass and a knife point. When a 50lb ball of clay falls from 10 meters onto a knife, the knife tip does not experience the full force of the clay due to the deformation of the clay upon impact. The concept of impulse, which considers the force exerted over time, is crucial in understanding the collision dynamics. A steel ball, being less deformable than clay, would exert a higher force upon impact, leading to greater stress on the knife.
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What does this mean?Decaff said:
I don't know a whole about physics, so bear with me.Dale said:
What if the ball were made of high strength steel instead of clay? Can you say what you think that difference would make?Decaff said:
Well, I'm not very experienced with physics, and this topic comes from a conversation I had with a friend. I know F = M*A, so the falling clay has Force. My friend thinks the knife point would be hit by all that force, while I thought the smaller area of the knife point would only subject it to some of the force before it digs into the clay.berkeman said:Welcome to PF.
What are your thoughts so far? What do you know about collisions between deformable materials?
I think the hardness and density of the steel ball would be too much for the knife to handle.berkeman said:
I assume the knife fairs worse against the steel ball because it's heavier, which increases the force. As for the impulse, from my understanding, the steel ball would have a high impulse while the clay ball might have a lower impulse.berkeman said:
I was assuming clay and steel balls of equal mass, since your question seems to be about the deformability of the falling mass.Decaff said:
Yes. When one or both of the masses in a collision can deform, that spreads the impact time over a wider time interval, which lessens the impulse and the severity of the collision. That's why cars are designed with "crush zones", to lower the peak forces felt by occupants of the vehicle during collisions. Lower peak forces translate directly into less severe injuries for vehicle occupants, and similarly, into less severe crush forces on the knife in your falling clay ball scenario.Decaff said:
If the balls are equal mass, then the steel ball would deform less, causing more force in the collision due to the objects not giving any "leeway," similar to the "Crush Zones" of cars. But the Clay ball being softer, gives way to the knife, which means less force is applied?berkeman said:
The ##F## in ##F=ma## is not a single force. It is the net force. The net force on the clay will almost always be different from the force on the tip of the knife.Decaff said:
You have the wrong idea about force. Force is not something that an object has. Actually there is no force if you have only one object. The force is describing the interaction between two objects. The falling ball, before touching the knife, only interacts with the Earth so there is a force describing this, the weight of the ball. And an equal and opposite force acting in the Earth. If we neglect the interaction with te air, this weight is the net force and will determine the ball's acceleration.Decaff said: