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#### Azrael

##### Guest
I got trouble in a Physics question.

An Olympic runner leaps over a hurdle. If the runner's initial vertical speed is 2.2 m/s, how much will the runner's center of mass be raised during the jump?

#### The Fishman

I got trouble in a Physics question.

An Olympic runner leaps over a hurdle. If the runner's initial vertical speed is 2.2 m/s, how much will the runner's center of mass be raised during the jump?

Ki + Pi = Kf + Pf ---> it's a conservation of energy problem.
you know the initial potential energy is 0 and final kinetic energy is 0, correct?
So, Ki = Pf

so, substitute the formulas

(1/2)mv^2 = mgh ---> remember mass is irrelevant in this problem since it's a conservation of energy problem, also because you can cancel mass out in this case but remember you can't do that in all cases.

(1/2)v^2 = gh

substitute known values for variables and solve for h, which should be about .25 m.

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#### Azrael

##### Guest
Ki + Pi = Kf + Pf ---> it's a conservation of energy problem.
you know the initial potential energy is 0 and final kinetic energy is 0, correct?
So, Ki = Pf

so, substitute the formulas

(1/2)mv^2 = mgh ---> remember mass is irrelevant in this problem since it's a conservation of energy problem, also because you can cancel mass out in this case but remember you can't do that in all cases.

(1/2)v^2 = gh

substitute known values for variables and solve for h, which should be about .25 m.

Thank you my man. I know it but I didn't know too much about it. Teacher never taught me nothing on it.

I have some more problems.

#### The Fishman

Thank you my man. I know it but I didn't know too much about it. Teacher never taught me nothing on it.

I have some more problems.

Eh, physics is somewhat confusing and thought-provoking in the first place, so a teacher that doesn't know how to teach it makes it worse.

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#### Azrael

##### Guest
Eh, physics is somewhat confusing and thought-provoking in the first place, so a teacher that doesn't know how to teach it makes it worse.

It's not that. She just never taught us it.

A 755 N diver drops from a board 10.0 m above the water's surface.

a) Find the diver's speed 5.00 m above the water's surface.

b) Find the diver's speed just before striking the water.

c) If the diver leaves the board with an initial upward speed of 2.00 m/2, find the diver's speed when striking the water.

I need to do good on this take home to boost my average. It's a 85.

#### The Fishman

oh ok, well so far, for the first one, it's a conservation of energy problem so do what I showed you above with the formula and solve for velocity.

(1/2)v^2 = gh
v = sqrt(2gh) = about 9.9 m/s

Edit: For the second one, same thing except h will be 10 instead of 5, and you should get 14 m/s

2nd edit: for c, is it 2 m/s^2 or 2 m/s?

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#### Azrael

##### Guest
oh ok, well so far, for the first one, it's a conservation of energy problem so do what I showed you above with the formula and solve for velocity.

(1/2)v^2 = gh
v = sqrt(2gh) = about 9.9 m/s

Edit: For the second one, same thing except h will be 10 instead of 5, and you should get 14 m/s

I used a completely different formula to get an answer for that. But thank you. You should be a physics teacher.

edit: It said an initial upward speed of 2.00 m/s^2.

#### The Fishman

I used a completely different formula to get an answer for that. But thank you. You should be a physics teacher.

Oh shit, actually there is another way to do that problem, I'm so sorry. You could use another formula, I'm not sure what it is instead of the conservation of energy. I just found it easier to find the answers using that, but if you got the answers your way, use that. I wondered why you didn't know about it, I guess you didn't learn about it yet?

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#### Azrael

##### Guest
Oh shit, actually there is another way to do these problems, I'm so sorry. You could use another formula, I'm not sure what it is instead of the conservation of energy. I just found it easier to find the answers using that, but if you got the answers your way, use that. I wondered why you didn't know about it, I guess you didn't learn about it yet.

I know. You had to use another formula to get the joules and plug it in using a manipulated version of the kinetic formula but to be used to get velocity.

I never learned. Can you help me with another problem

A bird is flying at 18.0 m/2 over water when it drops a 2.00 kg fish. If the altitude is 5.40 m and friction is disregarded, what is the speed of the fish when it hits water?

I got 7.28 m/s.

#### The Fishman

A bird is flying at 18.0 m/2 over water when it drops a 2.00 kg fish. If the altitude is 5.40 m and friction is disregarded, what is the speed of the fish when it hits water?

I got 7.28 m/s.

No, this is a tough one. First you need to find the initial kinetic energy by (1/2)mv^2 = (1/2)(2)(18^2) = 324 J. The way I think of it is that since it's flying horizontally, the bird and fish technically have an initial kinetic energy. You probably thought that the initial kinetic energy is 0, but that's not right. Then you find the initial potential energy of the fish by mgh = 9.8(2)(5.4) = about 106 J.

Use the law of conservation of energy(I can't think of any other way of doing it)

Ki + Pi = Kf + Pf ---> since the fish is dropping into the water, the Pf is zero, correct?

Fill in values for variables: 106 + 324 = (1/2)m(vf^2) + 0 ---> vf = final velocity

and you should get vf is about 20.7 m/s

it might sound confusing, but hope this helps.

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#### Azrael

##### Guest
Thanks. It's pretty complicated. lol

#### The Fishman

Thanks. It's pretty complicated. lol

actually, I'm not so sure, but the formula I thought about using was vf^2 = vi^2 + 2ad where vf is the final velocity, vi is the initial velocity, a is the acceleration, and d is the distance. The acceleration is given, distance is 10. I'm not sure what the initial velocity would be, which is why I'm confused, unless there's another formula I'm not thinking of that will help

Edit: Are you sure it's 2 m/s^2 and not 2m/s, it would be a lot easier if it was 2 m/s, because then the answer would be 14.14 m/s

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#### Nostalgia

##### livin' in the past
This question doesn't necessarily refer to homework, but it pertains to math. I've never learned percentiles, and I'd like to assume that they work similarly to percentages, but I don't know for sure.

Anyway, under the assumption that my class rank is 43 out 214, I subtracted it from the total to get 171. Then, I set up the proportion, divided 214 by 17,100 and came up with 79.91 approximately. Is that my percentile?

##### Iron Tomato
It's not that. She just never taught us it.

A 755 N diver drops from a board 10.0 m above the water's surface.

a) Find the diver's speed 5.00 m above the water's surface.

b) Find the diver's speed just before striking the water.

c) If the diver leaves the board with an initial upward speed of 2.00 m/2, find the diver's speed when striking the water.

I need to do good on this take home to boost my average. It's a 85.

To answer part c) - I'm assuming you meant to type "...upward speed of 2.00m/s..."

We're given: vi = 2.00m/s, a=acceleration due to gravity (denoted g) = -9.8 m/s^2 and d = -10.0m
[Note: g and d are negative because they are "downward" and I'm using "upward" as postive]

Let's find out high the diver jumps.
So, set vf^2 = 0 and solve for h (when he's at his maximum height, he has no vertical speed).
0 = vi^2 +2gh
h = -(vi^2) / 2g
= -(4.00m^2/s^2) / 2*(-9.8m/s^2)

Here, check your units to make sure your equation was manipulated properly. We have m^2/s^2 on top and m/s^2 on bottom. One of the m on the bottom will cancel on on the top, and the s^2 on the bottom cancels the one on top, leaving just m. Yay!

h = 0.204m

So we can add this to the initial 10.0m and get 10.204m. Why do this? Because he now has no initial vertical speed, and only acceleration due to gravity.

vf^2 = vi^2 +2gd
vf^2 = 0 + 2 (-9.8m/s^2) (-10.204m)
vf^2 = 2.0x10^2 m^2
vf = 14.1m

This question doesn't necessarily refer to homework, but it pertains to math. I've never learned percentiles, and I'd like to assume that they work similarly to percentages, but I don't know for sure.

Anyway, under the assumption that my class rank is 43 out 214, I subtracted it from the total to get 171. Then, I set up the proportion, divided 214 by 17,100 and came up with 79.91 approximately. Is that my percentile?
43/214 x 100% = 20.1%. That means you're in the top 20.1% of your class. A percentile refers to (in this case) the number of people below you. Which, as you calculated, is 79.9%.

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#### Azrael

##### Guest
A ferryboat is 4.0 m wide and 6.0 m long. When a truck pulls onto it, the boat sinks 4.00 cm in the water. What is the combined weight of the truck and ferry?

#### ▽

##### very differentiable
M displaced is equal to the M that's floating, we know the area of the boat, L*W and we know the difference in height, H, which, when multiplied with the area of the boat, gives us the volume of the displaced water, V=L*W*H. I assume you're allowed the density of water most often used, 1 kg in every cubic dm. I'm not going to do so, but just plug in the numbers, with centometres being 10^-2 metres, and going from cubic metres to cubic decimetres and vice versa is done with a multiplication factor of a thousand.

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#### Azrael

##### Guest
Thanks bro though I managed to do it myself.

#### The Fishman

Anyone know how to do genetics problems?

#### State

##### New member
Anyone know how to do genetics problems?

Are you talking about something like this?:
In humans, brown eyes (B) are dominant over blue (b)*. A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children?