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1.5 Frames of reference

1.5 Frames of reference. Imagine…. Your friend and yourself have set up a small experiment on your spare time, because you have nothing better to do Your friend stands in front of a forest – and in front of the forest is a railway track upon which is a special railcar

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1.5 Frames of reference

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  1. 1.5 Frames of reference

  2. Imagine… • Your friend and yourself have set up a small experiment on your spare time, because you have nothing better to do • Your friend stands in front of a forest – and in front of the forest is a railway track upon which is a special railcar • This railcar contains a large window and through it you can see your friend

  3. Imagine…

  4. Movement • Now, imagine that the rail car is moving at 10 km/h • How will the scene look from your perspective (on the ground) vs. your friend’s perspective?

  5. From the ground…

  6. From the car…

  7. Various views • If you notice – depending on the viewpoint of the person involved, the velocity of the various objects in the scene will differ • For example – take two objects in the scene – the trees, and the car itself

  8. Are they the same? • How would you describe the speed of the trees and the car if you were: • Standing on the ground? • Standing in the car? • Take the right as positive and the left as negative

  9. What are the values? • If you were standing on the ground: • tVg = 0 km/h • cVg = 10 km/h • If you were standing on the railway car: • tVg = -10 km/h • cVg = 0 km/h

  10. Frame of reference(FOR) • Notice that the velocity of the objects were different depending on where you are observing the event • A frame of reference refers to the point of view that you are choosing to analyze an event

  11. Important things to consider regarding a frame of reference • In a given FOR, the type that we will be focusing on are ones in which no acceleration takes place • This is known as an INERTIAL FOR • In an inertial FOR, all the laws of physics will apply

  12. For example… • Imagine you find yourself in a room – you are sitting in apparatus that cushions you from all movement • There is a ball sitting still on a table • The ball doesn’t move • And you notice that when you drop a ball from where you sit, it falls to the ground

  13. But what if… • In the same room, the ball on the table suddenly flies forwards? • Or if when you try to drop a ball on the ground, it doesn’t take a straight path to the floor? • HOW WOULD YOU EXPLAIN THIS?

  14. What if the car was moving? • In the first situation, the room that you were sitting in could be either completely still or moving at a constant speed – both would produce those results • Think about sitting in a car that is moving smoothly at a constant speed – and throwing a ball up and down or placing it on the seat

  15. What if the car were to stop? • Now imagine sitting in that same car, and suddenly, the driver slams on the brakes • What would happen if you were trying to catch a ball that you just threw upwards, or a ball that was placed on the edge of the seat?

  16. Non-inertial FOR • In non-inertial FOR, the FOR experiences a change in motion • Since the laws of physics don’t apply in non-inertial FOR (think – objects don’t suddenly slide off tables without a visible force applied) it becomes hard to compare it with an inertial FOR • For many of the relative velocity questions we deal with, we assume that all FOR’s are inertial

  17. Relative velocity • Therefore, velocity of an object can change depending on the FOR that you choose to analyze it from

  18. Moving within a FOR • There are 2 basic types of relative velocity that we are going to look at – and therefore, 3 basic types of relative velocity questions that you are going to come across • One type is relative velocity in ONE DIMENSION – that can be thought about by looking at the railway track example again

  19. Imagine… • What would happen if your friend began walking towards the back of the railway car at 6 km/h as it passed you at 10 km/h? • Would your friend’s speed look the same as from your point of view as it did before?

  20. One dimensional FOR

  21. Notice… • That your friend’s velocity relative to the ground DECREASED • Because they were moving towards the back of the car, their backwards velocity is subtracted from the forward velocity of the car • How would your friend’s velocity appear to you if your friend was moving towards the front of the car?

  22. One dimensional FOR

  23. Two-dimensional FOR • In a 2D FOR, motion can occur on a plane • An outside “force” pushes the object, and contributes a velocity vector that provides a perpindicular direction to the object’s motion

  24. If you are the swimmer – what is your direction relative to the water?What is the water’s direction relative to the ground? If you are standing on the shore and watching, what will happen to a swimmer that tries to travel across the water? wVg sVw sVg

  25. Notice… • In 2D vector questions, there are 3 vectors that are described • fVg: The force’s (water, wind, another moving FOR, etc.) velocity relative to the ground • oVf: The object’s (ship, boat, swimmer, etc.) velocity relative to the force • oVg: The object’s velocity relative to the ground

  26. Limited situations… • In general, most of the questions you will encounter are limited in how these vectors can be oriented relative to each other • Ofcourse, like all basic vector questions, 3 vectors means that you can create either a right angled or non-right angled triangle

  27. And in real life… • There is only a few ways that a force and an object can interact to create the various types of vector questions that you’re going to see • Therefore, there are 2 basic situations that you will come across when solving relative velocity questions

  28. “Push” • In these types of questions, the force pushes the object away from the original path that the object wanted to take • This is the example that we have seen earlier

  29. Notice how the swimmer’s body is oriented – they face the shore and swim – so although the body faces towards the shore, the body takes a diagonal path across the water fVg oVf oVg

  30. “Fight” • In these types of questions, the object is trying to oppose the force to follow a particular path

  31. Notice how the swimmer’s body is oriented – they turn their body so that they swim diagonally relative to the water – but the water pushes the swimmer back so that their velocity relative to the ground is directly across the river fVg oVg oVf

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