# Physics Energy - how fast ball is moving from certain height

astru025

## Homework Statement

You let a ball roll from rest from the top of a ramp sitting on a table. If the top of the ramp is 12 cm above the top of the table, how fast is it moving when it reaches the bottom? Use g=9.8m/s/s

## Homework Equations

Convert 12 cm to meters an you get .12. Then multiply that by 9.8 m/s.

## The Attempt at a Solution

My answer was 1.176 which was incorrect. I think my equation and solving is wrong but I'm not sure what to change. Any help would be appreciated!! Thanks!

## Answers and Replies

nasu
You forgot to show your equation. And why do you thing it applies here.

astru025
I don't know the mass of the ball so I took an educated guess and used the equation v= gh

nasu
What equation is this? Where does it come from? It is dimensionaly inconsistent, for one thing.

What would you do if you knew the mass? What equation will you apply then?

astru025
If I knew the mass I would do mgh as an equation

Panphobia
Is this your whole question? It does not even give you an angle of the ramp, or the length of the ramp with the height so you could at least calculate the angle. Is there any more information from the question that you left out? Also mgh = gravitational potential energy

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astru025
No this is all it gives me. Its an intro question to a lab that I am doing... I don't understand it but the lab is on energy.. In the lab it says that "you should be able to use conservation of energy to find speed of ball at the bottom of the incline."

Panphobia
Ebefore = Eafter so you need to find the equation that sums up all energy.

astru025
Conservation of energy= total energy of an isolated system does not change. So would It be 0?

Panphobia
just because it doesn't change doesn't mean there is 0 energy.

astru025
I don't know if this is relevant but kinetic energy= 1/2 mv^2

Panphobia
that is one half of it

astru025
How do I find the energy then?

astru025
Sorry I am just really struggling with this problem. I'm doing this class online through a university and the book I'm using is not very helpful. It's very heavy on concepts and light on math

Panphobia
If I give you the answer, then it would be like you cheating. But lets just say you had a quarter of the right idea at the beginning. Remember Etot = Etot' and Etot is the total energy in a system

astru025
Is this the correct equation? E total=( mv^ / 2) / mgh

astru025
Can you give me your equation? That's not cheating..

Panphobia
if i remember correctly from highschool it is Etot = mv^2/2 + mgh

astru025
How do I do this if I don't know the mass?! Do I cancel the masses?

Panphobia
I basically gave you the answer, from then on it is pretty simple. Just do the math.

astru025
Wow I feel really dumb right now. Why am I not seeing it...

Panphobia
You have all the numbers you need.

astru025
Okay so taking 1/2mv^2 + mgh I simplify to get v^2 = 2gh...

nasu
If I knew the mass I would do mgh as an equation

This is not an equation. It has no equal sign in it.
But this formula represents the potential energy of the body at height h. It's a good start.
What is the kinetic energy of the same body at the height h? What is his speed?

astru025
Okay so it's mass is 1.176 ? M=gh

nasu
You don't need the mass. And cannot calculate it from the given information.
You already wrote the answer above. Even though you got it by chance and not by solving corectly, what you wrote in post 23 is the right expression for velocity squared.

astru025
This problem is due by midnight tonight and even after all your help I am not even close to finding the right answer.... :(

astru025
Okay so I take 2 x 9.8 x .12 and then square that?

nasu
Your equation (you wrote it) is
v^2=2gh

So what do you get if you multiply 2 x 9.8 x 0.12?
Isn't v squared? How do you get v is v squared is given by the above multiplication?

astru025
2 x 9.8 x .12 is 2.352... That is the wrong answer though

nasu
This is v^2. You did not read the previous post?
How do you find v if v^2 is 2.352?

astru025
Yes so I take the square root of 2.532 to get what v is? Which would be 1.5

nasu
Yes, take the square root.

astru025
Ugh I entered this and it said it was incorrect. Is 1.5 m/s not the right answer?!?!?!

nasu
It may be a rounding error.
Or it may be a little more complicated. Depends on the level of your class.

Have you learned about moment of inertia and rotational kinetic energy?
They say that the ball is rolling so you may be supposed to consider the rotation of the ball too.
In this case the kinetic energy will have two terms,
KE=1/2mv^2 + 1/2 I ω^2
where I is the moment of inertia of the ball and ω is the angular speed. For rolling without slipping, ω is v/R where R is the radius of the ball.
Have you seen any of these?