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Momentum 18/07/2015 Any object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula: P Momentum = Mass x Velocity (in kgm/s) (in kg) (in m/s) M What is the momentum of the following? 1) A 1kg football travelling at 10m/s 2) A 1000kg Ford Capri travelling at 30m/s 3) A 20g pen being thrown across the room at 5m/s 4) A 70kg bungi-jumper falling at 40m/s V Conservation of Momentum 18/07/2015 In any collision or explosion momentum is conserved (provided that there are no external forces have an effect). Example question: Two cars are racing around Teville Gate. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Speed = 50m/s Mass = 1000kg Speed = 20m/s Mass = 800kg Mass = 1800kg Speed = ??m/s Momentum before = momentum after… …so 1000 x 50 + 800 x 20 = 1800 x V… …V = 36.7m/s More questions… 18/07/2015 1) A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have the same mass. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball? 2) A car of mass 1000kg heading up the M1 at 50m/s collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at? 3) A defender running away from a goalkeeper at 5m/s is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5.5m/s. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving? 4) A gun has a recoil speed of 2m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g what speed does the bullet come out at? (Hint: total momentum before is zero and the gun has a negative velocity afterwards) A past exam question… 18/07/2015 June 2000 Two lorries are travelling in the same direction along a motorway. Lorry A Lorry B Mass = 20,000kg Mass = 30,000kg Speed = 14m/s Speed = 20m/s 1) Calculate the momentum of Lorry A as it travels along the motorway. 2) Calculate the momentum of Lorry B as it travels along the motorway. (3 marks) 3) Lorry B collides with Lorry A and they stick together. Calculate the common speed of the lorries immediately after the collision. (3 marks) Energy loss in collisions 18/07/2015 In the “Forces” module we looked at how to calculate an object’s kinetic energy: Kinetic energy = ½ x mass x velocity squared in J in kg in m/s We’ve also said that in a collision momentum is conserved (unless an external force acts). The same cannot usually be said for kinetic energy… For example, consider the following collision. How much kinetic energy is lost? Before Speed = 50m/s Speed = 20m/s Mass = 1000kg Mass = 800kg After Mass = 1000kg Speed = 20m/s Mass = 800kg Speed = 30m/s Energy loss in collisions 18/07/2015 Consider a head-on collision where the cars stick together. How much kinetic energy is lost in this example? Where does all the energy go? Before Speed = 50m/s Speed = 30m/s After Speed = 10m/s In this example more kinetic energy was lost. We say it was a “less elastic collision”. An “elastic collision” is one where the kinetic energy is conserved.