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Published byDella O’Brien’ Modified over 9 years ago
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Do now! Can you talk with your partner about what we learned last lesson?
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Last lesson Stopping distances
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Stopping distances The distance a car takes to stop is called the stopping distance.
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Two parts The stopping distance can be thought of in two parts
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Stopping distances Thinking distance is the distance traveled whilst the driver is thinking (related to the driver’s reaction time).
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Thinking distance This is affected by the mental state of the driver (and the speed of the car)
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Braking distance This is the distance traveled by the car once the brakes have been applied.
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Braking distance This affected by the speed and mass of the car
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Braking distance It is also affected by the road conditions
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Braking distance And by the condition of the car’s tyres.
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Today’s lesson Momentum YouTube - Spectacular 100mph Train Crash Test
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Momentum What makes an object hard to stop? Is it harder to stop a bullet, or a truck travelling along the highway? Are they both as difficult to stop as each other?
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Momentum The bullet is hard to stop because it is travelling very fast, whereas the truck is hard to stop because it has a very large mass.
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Momentum It makes sense to assume that a bullet travelling twice as fast would be twice as hard to stop, and a truck twice the mass would also be twice as hard to stop.
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Momentum Momentum is a useful quantity to consider when thinking about "unstoppability". It is also useful when considering collisions and explosions. It is defined as Momentum (kgm/s) = Mass (kg) x Velocity (m/s) p = mv
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An easy example A lorry has a mass of 10 000 kg and a velocity of 3 m/s. What is its momentum? Momentum = Mass x velocity = 10 000 x 3 = 30 000 kgm/s
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Law of conservation of momentum The law of conservation of linear momentum says that “in an isolated system, momentum remains constant”. We can use this to calculate what happens after a collision (and in fact during an “explosion”).
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Conservation of momentum In a collision between two objects, momentum is conserved (total momentum stays the same). i.e. Total momentum before the collision = Total momentum after Momentum is not energy!
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A harder example! A car of mass 1000 kg travelling at 5 m/s hits a stationary truck of mass 2000 kg. After the collision they stick together. What is their joint velocity after the collision?
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A harder example! 5 m/s 1000kg 2000kg Before After V m/s Combined mass = 3000 kg Momentum before = 1000x5 + 2000x0 = 5000 kgm/s Momentum after = 3000v
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A harder example The law of conservation of momentum tells us that momentum before equals momentum after, so Momentum before = momentum after 5000 = 3000v V = 5000/3000 = 1.67 m/s
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Momentum is a vector Momentum is a vector, so if velocities are in opposite directions we must take this into account in our calculations
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An even harder example! Snoopy (mass 10kg) running at 4.5 m/s jumps onto a skateboard of mass 4 kg travelling in the opposite direction at 7 m/s. What is the velocity of Snoopy and skateboard after Snoopy has jumped on? I love physics
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An even harder example! 10kg 4kg-4.5 m/s 7 m/s Because they are in opposite directions, we make one velocity negative 14kg v m/s Momentum before = 10 x -4.5 + 4 x 7 = -45 + 28 = -17 Momentum after = 14v
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An even harder example! Momentum before = Momentum after -17 = 14v V = -17/14 = -1.21 m/s The negative sign tells us that the velocity is from left to right (we choose this as our “negative direction”)
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That’s it! Now let’s try some questions!
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