Elastic and inelastic collisions

The total kinetic energy before the collision is 1/2mv^2 + 1/2mv^2 = mv^2, while after the collision it is 0. However, the final velocity is 0, so the momentum is also 0, satisfying the law of conservation of momentum. In summary, in a closed system, an inelastic collision conserves momentum and total energy, but not total kinetic energy. This is possible because while the kinetic energy may change, the momentum can still remain constant.
  • #1
MBBphys
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Homework Statement


In a closed system, in an inelastic collision, momentum and total energy is conserved, but total kinetic energy is not.
But how is this possible? If we have a change in total kinetic energy, we have a change in velocity, and so we must have a change in momentum as:
momentum=mass*velocity
So I am really confused!
Thanks :)

Homework Equations


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The Attempt at a Solution


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  • #2
Keep in mind that kinetic energy is a scalar quantity and momentum is a vector quantity.

Think about the example of two equal masses moving toward each other with equal speeds and the two masses stick together and come to rest after they collide.
 

What is the difference between elastic and inelastic collisions?

Elastic collisions are those in which the total kinetic energy of the system is conserved, meaning that the objects involved bounce off of each other without losing any energy. Inelastic collisions, on the other hand, involve a transfer of kinetic energy between the objects, resulting in a change in their velocities.

How do you calculate the final velocities of two objects in an elastic collision?

In an elastic collision, the final velocities can be calculated using the conservation of momentum and conservation of kinetic energy equations. The equations are as follows:
Momentum: m1v1i + m2v2i = m1v1f + m2v2f
Kinetic Energy: 1/2m1v1i2 + 1/2m2v2i2 = 1/2m1v1f2 + 1/2m2v2f2
Where m is the mass and v is the velocity of the objects, and the subscripts i and f represent initial and final values.

What factors affect the degree of elasticity in a collision?

The degree of elasticity in a collision is affected by the materials and surface properties of the objects involved. Objects made of more elastic materials, such as rubber, will have a higher degree of elasticity in their collisions compared to objects made of less elastic materials, such as clay. The roughness of the surfaces also affects the degree of elasticity, as rougher surfaces will result in less energy being conserved during a collision.

Are there any real-world examples of elastic and inelastic collisions?

Yes, there are many real-world examples of both elastic and inelastic collisions. Some common examples of elastic collisions include billiard balls colliding on a pool table, a rubber ball bouncing off of a hard surface, and a racket hitting a tennis ball. Inelastic collisions can be seen in car accidents, where the kinetic energy of the moving car is transferred to the surrounding objects, resulting in damage and deformation.

How can the conservation of momentum and energy be used to analyze collisions?

The conservation of momentum and energy principles can be used to analyze collisions by allowing us to calculate the final velocities of the objects involved. These principles also help us understand the transfer of energy between objects during a collision and how it affects the final outcome. Additionally, they can be used to predict the motion of objects after a collision, making them valuable tools in studying and understanding the behavior of matter in motion.

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