Motion in different frames of reference

What does “at rest” really mean?Consider the questions on the right. These are not trivial questions. More than 2,000 years of thinking by many very smart people was necessary to come up with answers that are satisfactory. Read the text aloud
Observer watching another person on a moving train throw a dartTo illustrate the point, imagine a person on a train throwing a dart at a speed of 10 m/s. The train car the person is in is moving forward at 10 m/s. What is the speed of the dart? An observer on the train measures the speed to be 10 m/s. An observer on the ground, however, measures the speed to be 20 m/s. Who is correct? Read the text aloud
Both are correct because there is no absolute “zero” velocity! Whether an object is at rest or in motion depends entirely on your reference frame. Think of a reference frame as an imaginary “box” that you consider to be “at rest” for the purpose of making measurements. Velocities are measured with respect to your reference frame. The observer on the train might choose the inside of the train car to be the reference frame. In that reference frame the speed of the dart is 10 m/s. The observer outside might choose the ground to be the reference frame. In this reference frame the speed of the dart is 20 m/s. Read the text aloud
Velocity of the dart depends on the reference frame
The velocity of a moving object in one reference frame is equal to the velocity in a second reference frame plus the velocity of the second reference frame relative to the first. If this sounds confusing, consider the ground as the first reference frame. The moving train is the second. The velocity of the dart with respect to the ground (20 m/s) is the velocity of the dart with respect to the train (10 m/s) plus the velocity of the train with respect to the ground (another 10 m/s). Read the text aloud Show Is this always true?
An inertial reference frame is one in which the velocity is constant in both speed and direction. It was one of Einstein’s brilliant insights that no physical experiment can determine the velocity of an inertial reference frame without somehow looking “outside!” This means that, if the train car were on a smooth track moving at constant speed (with no windows), then nothing a physicist could do inside the car could determine its motion relative to any external reference frame. Read the text aloud

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