Questions pertaining to constant velocity + acceleration and relative motion

AI Thread Summary
The discussion focuses on understanding the signs of velocity and acceleration in various scenarios involving a ball's motion, with specific examples clarifying that upward motion is positive and downward motion is negative. The participants also explore relative motion, particularly how to calculate speed relative to other moving or stationary objects, emphasizing that direction matters in determining relative speed. Clarifications are sought on the reasoning behind the answers, especially regarding relative speeds when two cars are moving in opposite directions or when one is stationary. The conversation highlights the importance of understanding how relative distance changes over time to deduce relative speed. Overall, the thread seeks to clarify concepts of motion and relative velocity in physics.
Xelb
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1) In the following four scenarios, determine the sign of the ball’s velocity and acceleration.

Up is positive. The ball is thrown straight up. (Velocity is positive, Acceleration is negative)
Up is positive. The ball is dropped straight down. (Velocity is negative, Acceleration is negative)
Down is positive. The ball is thrown straight up. (Velocity is negative, Acceleration is positive)
Down is positive. The ball is dropped straight down (Velocity is positive, Acceleration is positive)

My answers are in parenthesis...but I'm not entirely 100% sure why these answers are what they are. A little clarification would be nice.

Next question pertains to relative motion:

You are in a car traveling south at 50 mph. What is your speed relative to:
A car traveling south at 50 mph? Zero (this one was obvious)
A car traveling south at 25 mph? 25...?
A car traveling north at 50 mph? 0?
A car traveling north at 25 mph? 75...?
A stationary car on the side of the road? I really not sure about this...
 
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Xelb said:
My answers are in parenthesis...but I'm not entirely 100% sure why these answers are what they are. A little clarification would be nice.

Your answers are correct. If we choose some direction in space, then anything going or acting in that direction is "positive", and anything in the opposite direction is "negative".

You are in a car traveling south at 50 mph. What is your speed relative to:
A car traveling south at 50 mph? Zero (this one was obvious)
A car traveling south at 25 mph? 25...?
A car traveling north at 50 mph? 0?

Imagine that these two cars meet (at the opposite sides of the road, so there is no collision) at some moment in time. If their relative speed is zero, then for any duration of time their relative distance must also be zero. Which is clearly not the case. How does their relative distance changes with time? Can you deduce their relative speed from how relative distance chages?
A car traveling north at 25 mph? 75...?

Answering previous question, you should be able to answer these, too.

A stationary car on the side of the road? I really not sure about this...

What is 50 mph of your car relative to?
 

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