# Find the Height of a Building by dropping a ball from the top

• Tesla In Person
In summary, the conversation discusses a kinematics question involving a ball being dropped from a building on Earth and on a planet with no atmosphere. The problem is that there is not enough information given to solve the question, as it is unclear if air resistance should be taken into account. The teacher who made the question is criticized for being sloppy and not providing enough information. The conversation ends with a question about which answer would be chosen if the first two answers were eliminated.

#### Tesla In Person

Homework Statement
Find height of a building
Relevant Equations
s= ut + 1/2 at^2
v=u +at
Hi, I have this kinematics question I am struggling with. There is a building from which a ball is dropped and it takes 5 second to reach the ground. Then they say that the same building is on a planet w/o atmosphere where g= 6 m/s^2 . What is the height of the building ?

I approached this question like I approach every other projectile motion questions but the problem here is that I don't have enough information to solve this problem. For the ball being dropped on earth, we only have the time it takes to hit the ground , that's it. There is air resistance acting on the ball so acceleration won't be 10m/s^2. I get this equation for the first using the equation s= ut + 1/2 at^2 => s= 25/2 a . I am assuming the ball is dropped from rest in both cases.

For the second case, where the ball is dropped from the same building but in a planet w/o atmosphere I get using the same equation , s= 3t^2. Here we are given a=6 and because it has no atmosphere, there is no air resistance.

s is the same so I can get = > 25/2a = 3t^2 . Now it's cleat that we need more information, there are 2 variables in the equation and I can't think of any other equation.

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Tesla In Person said:
Homework Statement:: Find height of a building
Relevant Equations:: s= ut + 1/2 at^2
v=u +at

There is air resistance acting on the ball so acceleration won't be 10m/s^2

You do not need more information, you only need to assume ball is dropped at rest (u=0 m/s) and air resistance is negligible (as is mentioned in the problem)
s= ut + 1/2 at^2 becomes
s = at^2/2

The teacher who made the question is sloppy. You can just from the numbers given eliminate the first two answers because they are with two significant digits.

Tesla In Person said:
I approached this question like I approach every other projectile motion questions but the problem here is that I don't have enough information to solve this problem.

How many projectile motion problems have you solved yet which involves taking air resistance into account?

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Tesla In Person said:
For the ball being dropped on earth
There is no ball being dropped on Earth in the problem. There is just a ball being dropped on a planet with g = 6 m/s^2 and no atmosphere.

DrJohn, Tesla In Person and Delta2
We have to say the problem is not perfectly stated and it induces misinterpretation.

Lnewqban, Tesla In Person and malawi_glenn
Delta2 said:
We have to say the problem is not perfectly stated and it induces misinterpretation.

The properties of the buildings environment should be given in the first sentance.

Delta2
Delta2 said:
We have to say the problem is not perfectly stated and it induces misinterpretation.
Not the first such problem and it won't be the last ...

Delta2, Tesla In Person and jbriggs444
Thank you all for helping me out, I've wasted so much time on this question ...

Delta2 and Lnewqban
malawi_glenn said:
The teacher who made the question is sloppy. You can just from the numbers given eliminate the first two answers because they are with two significant digits.
If you do that then which of the remaining answers would you choose?