Conservation of Momentum | A-Level Physics | Doodle Science

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A Level Physics

Doodle Science teaches you high school and College physics in a less boring way in almost no time!

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The idea of conservation of momentum can be used to work out what will happen when things collide. Conservation of momentum states that the total momentum of a closed system is constant. This means that the total momentum before a collision is equal to the total momentum after the collision.

For example, an ice hockey player of mass 85kg, charges into a practice dummy of mass 250kg, with a velocity of 10m/s. Since the dummy is covered in glue, the hockey player and the dummy both move off together. Using the fact that momentum before = momentum after we can calculate the velocity of the combination to be 2.54m/s.

There are two types of collision, elastic and inelastic. An elastic collision is a collision with no loss of kinetic energy. This only occurs on a microscopic level, where heat cannot be generated, such as the collision between two gas molecules. An inelastic collision is a collision with some loss of kinetic energy, this is usually the case with most collisions because the kinetic energy is transferred into other forms such has heat and sound.

For example, a particle of mass 5kg is moving at 4m/s when it strikes a second particle of mass 10kg moving at 2m/s towards the first particle. After the collision, the first particle has a velocity of 2.80m/s in the opposite direction to which it was first travelling. Using conservation of momentum here we can work out the velocity of the second particle to be 1.40m/s. Before the collision, the kinetic energy was 60J. After the collision the kinetic energy was only 29.4J. This means there was a loss of 30.6J of kinetic energy, so the collision was inelastic.

References:
1. CGP AS & A2 Physics for OCR A, ISBN: 9781847624192
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I have always struggled with the idea that even though energy may be lost, momentum stays constant. Since P=mv, and the mass can't change, then shouldn't a decrease in energy (given by KE=.5mv^2) cause a decrease in velocity and hence a decrease in momentum? Great video @DoodleScience

davidatri
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Would love to see you get into the details more on this subject.

iamjimgroth
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Velocity -2.8, how did you calculate?

KanR