Relative motion - disagreement at very low speeds

In summary, the conversation discusses the concepts of SR and Lorentz transformations and how they apply to a scenario involving a train car and a ball. The speaker wonders if an outside observer would measure the ball's average speed differently due to the train's movement, and questions the discrepancy at non-relativistic speeds. The response clarifies that the ball's speed is affected by the observer's state of motion, and not being a zero rest mass object, it does not move at the speed of light.
  • #1
csmcmillion
63
2
I understand the concepts behind SR and Lorentz transformations and was explaining the basics to my son by illustrating the classic laser-beam-in-a-train-car explanation. But then I started thinking... If I was in a boxcar w/ a clear wall and was throwing a ball straight up, measuring the time from toss to catch, and then computing the average speed... and an observer was standing outside and performed the same measurement s/he would see the ball take a much longer route than I would (assuming the train was moving at, say, 100 kph). That means we would have a significant disagreement about the average speed we compute for the ball.

Am I correct on this? It seems bizarre that two observers would see such different path lengths at everyday (non-relativistic) speeds. ?
 
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  • #2
The ball is not light; more precisely, it is not a zero rest mass object, so it doesn't move at the speed of light. For objects moving at less than the speed of light, their speed does vary with the state of motion of the observer.
 

1. What is relative motion?

Relative motion refers to the measurement of an object's position and velocity in relation to another object. It takes into account the movement of both objects and how they affect each other's position and velocity.

2. How is relative motion different from absolute motion?

Absolute motion refers to the measurement of an object's position and velocity in relation to a fixed point or reference frame, while relative motion considers the movement of multiple objects in relation to each other.

3. Can relative motion be observed at very low speeds?

Yes, relative motion can be observed at any speed, including very low speeds. Even when objects are moving at the same speed, their relative motion can still be observed if their directions of movement are different.

4. What causes disagreements in relative motion at very low speeds?

Disagreements in relative motion at very low speeds can be caused by factors such as friction, air resistance, and the shape and weight distribution of the objects. These factors can affect the objects' trajectories and relative positions, leading to discrepancies in their measured motion.

5. How do scientists account for disagreements in relative motion at very low speeds?

Scientists use mathematical equations and principles, such as Newton's laws of motion, to accurately calculate and predict the motion of objects at different speeds. They also take into account external factors, such as friction and air resistance, to minimize discrepancies between expected and observed relative motion.

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