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Do now! Can you talk with your partner about what we learned last lesson?

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Presentation on theme: "Do now! Can you talk with your partner about what we learned last lesson?"— Presentation transcript:

1 Do now! Can you talk with your partner about what we learned last lesson?

2 Last lesson  Stopping distances

3 Stopping distances The distance a car takes to stop is called the stopping distance.

4 Two parts The stopping distance can be thought of in two parts

5 Stopping distances Thinking distance is the distance traveled whilst the driver is thinking (related to the driver’s reaction time).

6 Thinking distance This is affected by the mental state of the driver (and the speed of the car)

7 Braking distance This is the distance traveled by the car once the brakes have been applied.

8 Braking distance This affected by the speed and mass of the car

9 Braking distance It is also affected by the road conditions

10 Braking distance And by the condition of the car’s tyres.

11 Today’s lesson  Momentum YouTube - Spectacular 100mph Train Crash Test

12 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?

13 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.

14 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.

15 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

16 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

17 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”).

18 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!

19 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?

20 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

21 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

22 Momentum is a vector  Momentum is a vector, so if velocities are in opposite directions we must take this into account in our calculations

23 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

24 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

25 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”)

26 That’s it! Now let’s try some questions!


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