Relative Motion Chapter 3 Section 4 Frames of Reference - If your in a car going down the road at 60mi/hr due north and a car passes you going 70mi/hr due north, it seems as if the car passing you is going 10mi/hr. - Someone standing on the side of the road would view the faster car going 70mi/hr, not the 10mi/hr which you see it as. Frames of Reference ► Ifyou are riding your bicycle along the road at 10mi/hr due east and another bicycle passes you going 15 mi/hr due west, it looks as if the faster moving bicycle is going 25mi/hr. ► To someone standing along the road, it looks as if the bicycle is only going 15 mi/hr. Frames of Reference ► The examples are good explanations of how velocities change when viewed in different reference frames. ► Frame of Reference: The motion of an object is determined by the reference point of the observer and can change from one point to the next. Relative Velocity ► There is no general equation used to determine the relative velocity of an object given different reference points, but vector addition is used to solve most problems dealing with relative velocity. Relative Velocity Example ►A boat heading north crosses a wide river with a velocity of 10.00 km/hr relative to the water. The river has a uniform velocity of 5.00 km/hr due east. Determine the velocity with respect to an observer on shore. ► Pg 108 sample problem 3F Practice Problems 1. A passenger at the rear of a train traveling at 15m/s relative to earth throws a baseball with a speed of 15m/s in the direction opposite the motion of the train. What is the velocity of the baseball relative to earth as it leaves the thrower’s hand? 2. A spy runs from the front to the back of an aircraft carrier as a speed of 3.5m/s. If the aircraft carrier is moving forward at 18m/s, how fast does the spy appear to be running when viewed by an observer on a near by submarine? 3. A ferry is crossing a river. If the ferry is heading due north with a speed 2.5m/s relative to the water and the river’s velocity is 3.0m/s to the east, what will the boat’s velocity be relative to Earth? 4. A pet-store supply truck moves at 25m/s north along a highway. Inside, a dog moves at 1.75m/s at an angle of 35º east of north. What is the velocity of the dog relative to the road?