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Noadswood Science, 2012

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To understand how a force can affect momentum Monday, May 04, 2015

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Imagine an egg was being thrown around – would it be possible to catch it, without the egg breaking? What technique is needed to catch it?

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Egg Catching For a given change in momentum, the longer the time taken, the smaller the force needed So if you can increase the time taken for the egg to lose its momentum, then you are more likely to catch it than break it This is the idea behind many car safety features and it also explains why it takes a long time to stop a super-tanker at sea, or to change its direction

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Crumple Zones Crumple zones at the front and rear ends of cars are designed to lessen the force of an impact – this force changes the momentum of the car In a front-end impact the momentum of the car is reduced In a rear-end impact, the momentum of the car is increased Why is rubber matting under a child’s swing a good idea? If a child falls off the swing the rubber matt reduces the impact force by increasing the impact time when the child hits the ground

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Practical Design a practical to investigate the affect of force on momentum – how can we test the impact? Using a trolley with different amounts / shapes of Plasticine on the front, impact them into a stationary wall and observe their momentum change Note your findings…

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Practical The Plasticine flattens on impact, increasing the impact time = force reduction The more time an impact takes, the less force is exerted

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Force & Momentum Force (N) = change in momentum time taken Making the time longer makes the force smaller (crumple zones in cars are designed to make impact times longer, so impact forces are reduced) When a resultant force acts on a moving object, a change of momentum takes place

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Questions Force (N) = change in momentum time taken 1. A 25kg bike is travelling at 12m/s – what force is needed to bring it to a stop in 5 seconds? 2. A bullet of mass 0.004kg is moving at a speed of 90m/s when it is stopped by a bullet proof vest in seconds – what is the impact of the force?

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Questions Force (N) = change in momentum time taken 1. A 25kg bike is travelling at 12m/s – what force is needed to bring it to a stop in 5 seconds? Momentum = mass x velocity Momentum at start = 25kg x 12m/s = 300kg m/s Momentum at end = 25kg x 0m/s = 0kg m/s Change in momentum = 300kg m/s Time taken for change = 5 seconds Force (N) = 300kg m/s ÷ 5 s = 60N

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Questions Force (N) = change in momentum time taken 2. A bullet of mass 0.004kg is travelling at 90m/s when it is stopped by a bullet proof vest in seconds – what is the impact of the force? Momentum = mass x velocity Momentum at start = 0.004kg x 90m/s = 0.36kg m/s Momentum at end = 0.004kg x 0m/s = 0kg m/s Change in momentum = 0.36kg m/s Time taken for change = seconds Force (N) = 0.36kg m/s ÷ s = 1200N

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Summary Complete the sentences below using the key words: decreases; increases; is zero; and stays the same 1. The momentum of a moving object… if no forces act upon it 2. In an impact in which a moving object is speeded up, its momentum… 3. When an object is thrown into the air, the force of gravity on it… its momentum as it goes up 4. Calculate the initial momentum of an 800kg car travelling at 30m/s 5. What force is required to stop the car in: - a) 12 seconds b) 30 seconds

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Summary 1. The momentum of a moving object stays the same if no forces act upon it 2. In an impact in which a moving object is speeded up, its momentum increases 3. When an object is thrown into the air, the force of gravity on it decreases its momentum as it goes up 4. Calculate the initial momentum of an 800kg car travelling at 30m/s 24’000kg m/s (mass x velocity) 1. What force is required to stop the car in: - a) 12 seconds b) 30 seconds

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Summary 5. What force is required to stop the car in: - a) 12 seconds Initial momentum of car = 24’000kg m/s (mass x velocity) Final momentum = 0kg m/s Time taken for stopping = 12 seconds Force = 24’000 ÷ 12 = 2000N

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Summary 5. What force is required to stop the car in: - b) 30 seconds Initial momentum of car = 24’000kg m/s (mass x velocity) Final momentum = 0kg m/s Time taken for stopping = 30 seconds Force = 24’000 ÷ 30 = 800N

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