Question

One Dimensional Kinematics (Module 4) Concept Question #6:
Isn't it asking for speed? In the video lecture, Lewin denoted speed as distance over time. If the ball is thrown upwards, doesn't it travel a larger distance? I agree that the velocities at the bottom are equal, but wouldn't the ball thrown downwards have a higher speed because it traveled less distance?

Answer 1

Speed is at all times the absolute value of the velocity (ignoring direction)

Answer 2

You are correct that speed is distance/time. The reason this works is because the ball that is thrown upwards will take longer to hit the ground than the ball that was thrown down so their speeds are equal.

Answer 3

OK, thanks.

Answer 4

By the way, I think the question asks about the speed at impact (which is the magnitude of velocity), so it's asking about the instantaneous rate. What you are referring to is "average speed", which is total displacement (a.k.a. distance) over total time. The only difference in the two equations would be that one starting velocity (v0) is negative and one is positive. The starting position is the same and acceleration is always 9.8 m/s2. I still haven't figured out what to do once you reason the signs. I have to get the textbook

Answer 5

This is correct, the signs are opposite to begin with, but this question asks you to think about what the velocity of the ball is when you return to the original height. The velocity of the ball when it returns of the original position will be equal, but opposite to its v0. So since the two balls started with the same velocity they will also have the same velocity when they hit the ground.

Answer 6

yeah that makes sense