# Physics 1st year Kinematic world problems. IF you have the time, please help! Thanks!

Here is the problem that I am having trouble with (It probably is a no brainer, but i am a bit rusty- not a bit actually, a lot!).

Here is the question:
A student stands at the top of a building to perform a kinematics experiment. He throws a red ball directly upwards with an initial speed (Vo). At the same time, he throws a blue ball directly downwards with the same initial speed (Vo). Neglecting air resistance, which of the following statements are true regarding each of the balls, just before they reach the ground?

a) The speed of the blue ball is greater than that of the red ball.
b) The speed of the red ball is greater than that of the blue ball.
b) The two balls have the same velocity.
d) The blue ball has a greater acceleration than the red ball.

I think the answer is a) because since the blue ball is higher than the red ball when it is descending, the blue ball will be affected by the acceleration due to gravity for a longer period of time therefore right before it hits the ground it will have a higher speed. But what I am skeptical about is the last part of the question where it says just before they hit the ground, does that mean they both meet at a given time before they hit the ground or is it talking about each seperately? Because i am assuming that the red ball will hit the ground before the blue ball.

any ideas?

thanks in advance! - Mohammed.

## Answers and Replies

kreil
Gold Member

What will the velocity of the red ball be when it passes by it's initial position on the way back down? From this, what can you say about the velocity of the red ball in relation to the blue ball right before it hits the ground?

Note: the blue ball hits the ground before the red ball, but we are examining the velocity of each ball separately just before it hits

If there is no air resistance and you throw the ball up with a certain velocity, it stops turns around and comes back down, what will be the velocity as it passes by the spot you threw it from?

Thanks a lot guys! I had also thought that because they both end up hitting the ground, their velocity becomes zero. Thanks!!!