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| Elastic and Inelastic Collisions in One and Two Dimensions |
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| A Karate expert strikes a board of mass 0.14 kg and breaks it, with his fist of mass 0.70 kg. He then does the same to a 3.2 kg concrete block. The spring constant 'k' for bending are, 4.1 x 104 N/m for the board and 2.6 x 106 N/m for the block. Breaking occurs at a deflection 'd' of 16 mm for the board and 1.1 mm for the block. How to calculate the energy stored in the object just before it breaks? What is the lowest fist speed required to break the object? |
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| One can solve this problem if one understands the laws in Physics, regarding collision. |
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| In everyday language, a collision occurs when objects crash into each other. For example, the collision between billiard balls and a hammer hitting a nail. Collisions range from the microscopic scale of subatomic particles to the astronomic scale of colliding stars and colliding galaxies. |
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| A collision is said to take place when one body coming forcibly in contact with another or when the path of one body is changed by the influence of the other body. |
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| As a result of collision, the momentum and kinetic energy of the interacting bodies change. The forces involved in a collision are action-reaction forces i.e., the internal forces of the system. So, the total momentum is conserved. The total energy is also conserved. |
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| A collision is said to be an elastic collision if both the kinetic energy and momentum are conserved in the collision. |
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| During collision, the bodies are deformed. However, they regain their original shape completely if the collision is elastic. The mechanical energy is not converted into any other form of energy. In an elastic collision, the forces of interaction are conservative in nature. |
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| Perfectly elastic collision is an extremely rare physical phenomenon. Some of the collisions between atomic or subatomic particles are almost perfectly elastic collisions. Collision between two ivory (or steel or quartz or glass) balls is also a nearly elastic collision. If a ball dropped from a certain height, rebounds to its original position, then the collision is elastic because there is no loss of kinetic energy. |
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| A collision is said to be an inelastic collision if the kinetic energy is not conserved in the collision. However the momentum is conserved. |
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| The kinetic energy lost in the collision appears in the form of heat energy, sound energy or light energy. The forces of interaction in an inelastic collision are non-conservative in nature. |
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| Most of the collisions between macroscopic bodies are inelastic collisions. If a ball is dropped from a certain height and it is unable to rise to its original height, it would mean that ball has lost some kinetic energy (which would appear as heat energy). This would mean that collision is an inelastic collision. |
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| If the two bodies stick together after the collision, the collision is said to be perfectly inelastic. |
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| In this type of collision, the loss of kinetic energy is maximum but not complete. |
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