# Relative velocities of balls on reaching the ground.

• takando12
In summary, the conversation revolves around a physics problem involving three balls of different velocities being thrown and dropped from the same height. The question is to find the relation between their velocities when they hit the ground. Some possible solutions discussed include using energy conservation and common sense to determine that A and B will have the same final velocity and that C's final velocity will be 20m/s less than A and B's. The conversation also touches on the need to post in the appropriate forum and not to give answers for homework-style questions.
takando12
Here's a physics question that's been giving me trouble. At a height H ,a ball A is thrown upward with a velocity of 20m/s and another B one is thrown down with a velocity of 20m/s and a third ball C is just dropped. Note that it's the same height for all three.I need to find the relation between their velocities when they hit the ground.
attempt at solving:
A - The velocity on hitting the ground must be the same as the projected velocity. So I think it's 20 m/s
B - Since it starts with 20m/s it's definitely going to speed up, so it's reasonable to assume that it's final velocity will be greater than A's.
C-This is where I'm confused. Using the formula v=u+gt and u=0 . v=gt .So the final velocity of C is solely dependent on t. Taking the falling part of A's journey, u=0 and v= 20m/s( from first assumption) using v=u+gt. gt=20 . Is the time for the falling of A and C the same and so the answer is A=C<B ?How do i proceed? I'm stumped.

Are the sing of ##u## the same?

i can easily answer this question, but i can not understand that what kind of relation you want? can you please post the original question statement?

Sagar Singh said:
i can easily answer this question, but i can not understand that what kind of relation you want? can you please post the original question statement?
The last part of the question says " Find the relation between their velocities when they hit the ground"
there are options as well like A=B>C , A>B>C, A=C<B. Meaning of the course the velocity of A,B,C.

takando12 said:
How do i proceed?
Try energy conservation.

Brother this question not for solving it can easily be solved with just common sense , A is going upward with 20 m/s so after reaching H it will be 20m/s downward, so final velocity of A and B will be same, c is just dropped, and relative acceleration is 0, so difference between velocities V(a or b)- V(c)=20m/s

Sagar Singh said:
Brother this question not for solving it can easily be solved with just common sense , A is going upward with 20 m/s so after reaching H it will be 20m/s downward, so final velocity of A and B will be same, c is just dropped, and relative acceleration is 0, so difference between velocities V(a or b)- V(c)=20m/s
how can the final velocity of B be 20m/s? it's starting with 20m/s from the top and it will speed up right? it's accelerating under gravity so it should increase from 20m/s ? please correct me if i am wrong.

A.T. said:
Try energy conservation.

Write the energy conservation condition for the three cases.

Sagar Singh
takando12 said:
how can the final velocity of B be 20m/s? it's starting with 20m/s from the top and it will speed up right? it's accelerating under gravity so it should increase from 20m/s ? please correct me if i am wrong.
i said difference between velocities, not actual velocities, we don't know time, we don't know height, so we cannot find actual velocity.

Sagar Singh said:
i said difference between velocities, not actual velocities, we don't know time, we don't know height, so we cannot find actual velocity.
"so final velocity of A and B will be same"
you said the final velocity of A and B will be the same. A's final velocity is 20m/s, but B starts with 20m/s and speeds up right? so how can their final velocities be the same?
or does the velocity of B just be constant throughout the entire downward journey?

A will go upward, reach a certain height, calculating, h=u*u/2g= 20m
ar an height of 20+H it will stop and start falling, when it falls 20. its velocity will be =20m/s, so both A and B will fall rom 20m/s downward.

Takando12, this should have been posted in the introductory physics homework section, per the rules. If you still require help, please start a new thread there.

For the rest of you, please do not answer homework or homework-style questions that have been posted outside of the homework forums. Use the report function to bring them to the attention of the mentors.

Sagar Singh

## What is the concept of relative velocities?

The concept of relative velocities refers to the measurement of the speed and direction of an object in comparison to another object or reference point. It takes into account the motion of both objects and their relative position to one another.

## How do you calculate the relative velocity of balls on reaching the ground?

The relative velocity of balls on reaching the ground can be calculated by subtracting the velocity of the first ball from the velocity of the second ball. This will give the relative velocity between the two balls.

## What factors affect the relative velocities of balls on reaching the ground?

The relative velocities of balls on reaching the ground can be affected by factors such as the initial velocity of the balls, the angle at which they are thrown, air resistance, and gravitational force.

## Why is it important to understand relative velocities of balls on reaching the ground?

Understanding relative velocities of balls on reaching the ground is important in various fields such as physics, sports, and engineering. It helps in predicting the trajectory and motion of objects, calculating the impact of collisions, and designing efficient and safe structures.

## How can the concept of relative velocities be applied in real-life situations?

The concept of relative velocities is applied in various real-life situations, such as calculating the speed and direction of vehicles, predicting the path of projectiles, understanding the motion of celestial bodies, and designing efficient transportation systems.

Replies
9
Views
1K
Replies
46
Views
2K
Replies
2
Views
1K
Replies
15
Views
2K
Replies
3
Views
2K
Replies
11
Views
590
Replies
26
Views
2K
Replies
4
Views
779
Replies
4
Views
1K
Replies
4
Views
2K