Motion problem Final velocity of a dropped stone

In summary, the student was told to post homework problems and textbook problems in the Homework Help (HH) forums. The thread title should be descriptive so that other members of the forum can decide if they are interested in clicking into the thread.
  • #1
debjit625
40
0
Hi everybody

I got this from a book

A stone is dropped from a height of 50 m and it falls freely. Calculate the velocity of the stone when it reaches the ground.

Assuming g = - 9.8 ms2 ,negative as its going downwards .

Using v2 = u2 + 2a(x - x0)

v2 = 02 + 2(-9.8)(-50-0)

I got v = 31.30 ms1

But the book have an answer of 9.9 ms1

so what's wrong with my process or the answer of the book is wrong ?

Thanks
 
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  • #2
debjit625 said:
Hi everybody

I got this from a book

A stone is dropped from a height of 50 m and it falls freely. Calculate the velocity of the stone when it reaches the ground.

Assuming g = - 9.8 ms2 ,negative as its going downwards .

Using v2 = u2 + 2a(x - x0)

v2 = 02 + 2(-9.8)(-50-0)

I got v = 31.30 ms1

But the book have an answer of 9.9 ms1

so what's wrong with my process or the answer of the book is wrong ?

Thanks

Careful picking your directions.

g = 9.8 m/s2 [Down] = -9.8 m/s2 [Up]

Your answer is correct though, maybe the book had a typo.
 
  • #3
@Zondrina
I learned its a free choice i.e.. I can choose my positive and negative direction..
Yes it make sense that acceleration due to gravity is always downwards so that's its positive direction,but again mathematically I can choose it negative,if I see the motion in a Cartesian coordinate system.Am I wrong ?

Thanks

I found a notice,its not a homework so I didn't posted it in homework section.
 
  • #4
debjit625 said:
@Zondrina
I found a notice,its not a homework so I didn't posted it in homework section.

At the PF, all schoolwork-type questions need to go in the Homework Help (HH) forums. You should also use the HH Template when you post such questions (it is provided for you when you start a new HH thread) -- the Template helps to organize the way you present the question and your work on the solution.

Here is the text from your Notice:

Please post homework problems and textbook problems in the homework forums. Even if you're just self-studying and not taking a formal course, problems from textbooks always belong in the homework forums. I have moved it for you.

And here is a thread that should help you understand why the PF HH rules are the way they are:

https://www.physicsforums.com/showthread.php?t=373889

:smile:
 
  • #5
Oh, and we ask that you please use very descriptive thread titles, to make it easier for other folks on the forum to decide if they are interested in clicking into your thread. I have added some more descriptive words to your thread title. Cheers! :smile:
 

What is a motion problem?

A motion problem is a type of problem in physics that involves calculating the movement of an object based on given information such as distance, time, and velocity.

What is the difference between speed and velocity?

Speed is a measure of how fast an object is moving, while velocity is a measure of both the speed and direction of an object's motion.

How do you calculate the final velocity of a dropped stone?

The final velocity of a dropped stone can be calculated using the equation vf = vi + at, where vf is the final velocity, vi is the initial velocity (usually 0 for a dropped object), a is the acceleration due to gravity (9.8 m/s^2), and t is the time the object has been falling.

What factors can affect the final velocity of a dropped stone?

The final velocity of a dropped stone can be affected by air resistance, the height from which the stone is dropped, and the mass of the stone. Other factors such as wind and surface area may also play a role.

How does the final velocity of a dropped stone change if the stone is dropped from a greater height?

If the stone is dropped from a greater height, the final velocity will be greater because the stone has more time to accelerate due to gravity. This means it will be falling for a longer period of time, resulting in a higher final velocity.

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