Reference Point, Frame of Reference and Position The most important idea when studying motion, is you have to know where you are. The word position describes your location (where you are). However, saying that you are here is meaningless, and you have to specify your position relative to a known reference point. For example, if you are 2 m from the doorway, inside your classroom then your reference point is the doorway. This defines your position inside the classroom. Notice that you need a reference point (the doorway) and a direction (inside) to define your location. 1/ Frames of Reference Definition: Frame of Reference A frame of reference is a reference point combined with a set of directions. A frame of reference is similar to the idea of a reference point. A frame of reference is defined as a reference point combined with a set of directions. For example, a boy is standing still inside a train as it pulls out of a station. You are standing on the platform watching the train move from left to right. To you it looks as if the boy is moving from left to right, because relative to where you are standing (the platform), he is moving. According to the boy, and his frame of reference (the train), he is not moving. A frame of reference must have an origin (where you are standing on the platform) and at least a positive direction. The train was moving from left to right, making to your right positive and to your left negative. If someone else was looking at the same boy, his frame of reference will be different. For example, if he was standing on the other side of the platform, the boy will be moving from right to left. For this chapter, we will only use frames of reference in the x-direction. Frames of reference will be covered in more detail in Grade 12. 2/ Position Definition: Position Position is a measurement of a location, with reference to an origin. A position is a measurement of a location, with reference to an origin. Positions can therefore be negative or positive. The symbol x is used to indicate position. x has units of length for example cm, m or km. Figure 3.2.2 shows the position of a school. Depending on what reference point we choose, we can say that the school is 300 m from Joan’s house (with Joan’s house as the reference point or origin) or 500 m from Joel’s house (with Joel’s house as the reference point or origin). The shop is also 300 m from Joan’s house, but in the opposite direction as the school. When we choose a reference point, we have a positive direction and a negative direction. If we choose Figure 3.3: The origin is at Joan’s house and the position of the school is +300 m. Positions towards the left are defined as positive and positions towards the right are defined as negative. the direction towards the school as positive, then the direction towards the shop is negative. A negative direction is always opposite to the direction chosen as positive. Exercise: PositionE - 1: Write down the positions for objects at A, B, D and E. Do not forget the units. Hướng dẫn x$_{A}$ = -3m; x$_{B}$ = -1m; x$_{D}$ = 1m; x$_{E}$ = 3m E - 2: Write down the positions for objects at F, G, H and J. Do not forget the units. Hướng dẫn x$_{F}$ = 3m; x$_{G }$= 1m; x$_{H }$= -1m; x$_{J}$ = -3m E - 3: There are 5 houses on Newton Street, A, B, C, D and E. For all cases, assume that positions to the right are positive. (a) Draw a frame of reference with house A as the origin and write down the positions of houses B, C, D and E. (b) You live in house C. What is your position relative to house E? (c) What are the positions of houses A, B and D, if house B is taken as the reference point? Hướng dẫn a/ b/ CE = x$_{E}$ - x$_{C}$ = 40m c/ x$_{A}$ = -20m; x$_{B}$ = 0; x$_{D}$ = 40m Xem thêm: Physics 10.II Motion in One Dimension High School Students Studying the Sciences Physics