Frames of Reference/ Relative Motion DO NOW!! Sitting at your desk, are you moving? P ull YES and NO! It all depends on what you are comparing it to

Frames of Reference/ Relative Motion An object is said to be in motion if it changes its position with respect to its surroundings. We know that a table in a room is at rest -- its position when compared to the walls of the room does not change.

Frames of Reference/ Relative Motion People who like watching the night sky would have observed that the position of stars and planets change while it seems we are not moving. But in reality the earth is also moving and so is everything on the surface of the earth. Thus, an object which appears to be at rest, may actually be in motion. Therefore, to describe the motion of an object we have to specify how its position changes with respect to a fixed point called the frame of reference. A frame of reference is another object or scene that an object's motion is compared to.

Frames of Reference/ Relative Motion Relative Motion of Trains Imagine you are travelling in a moving train. Do you observe any change in your position with respect to your co-passengers? Is there any change of scene you view through the window? The change of scene indicates that the train is moving.

Frames of Reference/ Relative Motion You might have even noticed relative motion while sitting at a red light…Have you ever been sitting at a red light with a bus stopped next to you? You’re kind of daydreaming, looking out the window at the side of the bus, when all of a sudden it feels like your car is rolling backwards! Then you realize that it was just the bus moving forwards. Your brain knows that the bus was just sitting there on the road… it became part of the frame of reference of the ground. When your brain saw the bus moving forwards, it had already “decided” that the bus won’t move. The only option remaining is that you must be moving backwards.

Example 1 Let’s say I am standing on the back of a pickup truck (that is motionless), and I am throwing apples forwards. I know that I can throw an apple at exactly 15m/s every time. If a person were standing on the sidewalk, how fast would she say the apples are moving? Since she will see them exactly the same way as me, she will say 15m/s.

Example 2 Now the truck starts to move forwards at 20m/s. I am still throwing apples forwards, exactly the same as I was throwing them before, at 15m/s.How fast does my friend on the sidewalk say the apple is moving? Well, even before I throw it, she’ll say that the apple is moving at 20m/s (the speed of everything on the truck). When I have thrown the apple forward, adding more velocity to it, she will say it is going at (20m/s + 15m/s) 35m/s!

Example 3 Now I turn around and start throwing the apples from the rear of the truck, backwards! I will still say that my apples are moving at 15m/s, because from my way of looking at it, that’s how fast the apple is moving. But My friend on the sidewalk will say that the apple is moving at (20m/s + -15m/s) 5m/s! In each of the above examples, we are really talking about two different people having two different frames of reference while measuring the relative velocity of one object.

Example 4 Study the picture below. There is a train robber and two officers shooting at him. Officer A is on the same train. Officer B is on a stationary tower. Which bullet will eventually strike the robber with more speed? And how fast?

Example 4 Understanding The Main Idea I dentify the frame of reference in each of the following situations: 1. The sun rises over the horizon. _______________________________________________________________________________ 2. A bus moves past people on the sidewalk. _______________________________________________________________________________ 3. A passenger on a train sees a ball roll down the aisle. _______________________________________________________________________________ 4. Two subway trains travelling at the same speed next to each other whiz past people on the platform. _______________________________________________________________________________ 5. A passenger on one of the subway trains looks out the window and sees another train standing still. _______________________________________________________________________________ 6. A passenger in the airplane looks down and sees the train moving backward. _______________________________________________________________________________

Reaching Out You are the tail gunner in a Hawker Hart two-seat bomber. Flying at a maximum speed of 272 Miles per hour you fire your gun in the exact opposite direction at a pursuing enemy fighter. Your gun has a muzzle velocity of 400 feet per second, which is exactly 272 miles per hour. How fast is the bullet going, and in what direction? Explain why. Pull The Bullet will stop and go nowhere. It will then fall