question with ez physics problem

[neonerd]

From the top of a cliff, a person uses a slingshot to fire a pebble straight downward with an initial speed of 9.0 m/s. After 0.50 s, how far beneath the cliff-top is the pebble?

Is the answer 3.28m?


if not, what is it? I'll work backword to find the solution...


this is the equation i used:

distance traveled = (initial velocity) x (time) - 1/2 (gravity) x (time)^2

 

[MikeMike]

that looks right from what i remember.

MIKE

 

[StageLeft]

9 m/s is gravity, so the pebble's speed will be constant. half a second at 9 m/s should be 4.5

 

[Garuda]

You should be adding the gravity component. It has an additive component to the downward velocity.

Never blindly use calculus derived formulas they give you. Always think about it to make sure it applies in the instance.

 

[neonerd]

Originally posted by: nourdmrolNMT1
that looks right from what i remember.

MIKE

ok, thx

 

[neonerd]

Originally posted by: Skoorb
9 m/s is gravity, so the pebble's speed will be constant. half a second at 9 m/s should be 4.5

gravity is 9.8m/s 😕

 

[StageLeft]

Originally posted by: neonerd

Originally posted by: Skoorb
9 m/s is gravity, so the pebble's speed will be constant. half a second at 9 m/s should be 4.5

gravity is 9.8m/s 😕

Oh right 😉 i can't see why it would be less than 4.5 even if gravity was as little as 9m/s

 

[RaynorWolfcastle]

Originally posted by: Skoorb
9 m/s is gravity, so the pebble's speed will be constant. half a second at 9 m/s should be 4.5

:shocked: I hope you're better at being a father than you are at being a physicist.

 

[mugs]

Originally posted by: neonerd

Originally posted by: Skoorb
9 m/s is gravity, so the pebble's speed will be constant. half a second at 9 m/s should be 4.5

gravity is 9.8m/s 😕

Gravity is 9.8 m/s^2

I'm fairly certain that your answer is wrong, because even without gravity it would have travelled 4.5 m in .5 seconds.

I haven't taken a physics class in a while, but I used your equation and got 5.725m. The intial velocity is negative, remember.

 

[StageLeft]

Originally posted by: RaynorWolfcastle

Originally posted by: Skoorb
9 m/s is gravity, so the pebble's speed will be constant. half a second at 9 m/s should be 4.5

:shocked: I hope you're better at being a father than you are at being a physicist.

I havne't done HS physics in a decade! Nonetheless, the pebble starts at 9 m/s, and it never decellerates, so half a second at that speed will be 4.5, not including the slighly accelerative effect of gravity's 9.8.

 

[neonerd]

Originally posted by: Skoorb

Originally posted by: neonerd

Originally posted by: Skoorb
9 m/s is gravity, so the pebble's speed will be constant. half a second at 9 m/s should be 4.5

gravity is 9.8m/s 😕

Oh right 😉 i can't see why it would be less than 4.5 even if gravity was as little as 9m/s

i can't either....cuz it's half a second....that's why i just used the formula, and wanted to double check on AT

 

[StageLeft]

OMG somebody kill me.

 

[neonerd]

Originally posted by: Skoorb
OMG somebody kill me.

did skoorbaby just ask you the same question? 😛

 

[mugs]

See my edit

 

[Garuda]

It's been 2 years since I did any physics.

I think it's more like 23/4 meters. That's a rough calculation using 10m^2/s for g, so it's should be around 5.5m.

 

[StageLeft]

5.725 is the answer 🙂

 

[daveshel]

I think that is a terminal velocity guestion.

 

[neonerd]

Originally posted by: Skoorb
5.725 is the answer 🙂

what'd u use to get it?

 

[MikeMike]

yup they are right. i didnt do any math numbers, but they are right you used the initial velocity as a positivie number while truly it is negative.

MIKE

 

[mugs]

Originally posted by: daveshel
I think that is a terminal velocity guestion.

He's like 15, I don't think they deal with that in high school physics, at least not this early in the year. (plus he'd need more information)

 

[neonerd]

Originally posted by: mugs

Originally posted by: daveshel
I think that is a terminal velocity guestion.

He's like 15, I don't think they deal with that in high school physics, at least not this early in the year.

i'm taking 11th grade physics in 10th grade, lol

 

[StageLeft]

Originally posted by: neonerd

Originally posted by: Skoorb
5.725 is the answer 🙂

what'd u use to get it?

9 m/s X .5 seconds = 4.5. That's the easy part. Next part is 9.8 over half a second gives you a final speed of 4.9. Divide that by 2 for an average speed of 2.45 That speed over half a second is 1.225. Add to the original and you've got 5.725. I was mixed up with beer and terminal velocity in the first post, but this is the correct answer.

 

[Mo0o]

Originally posted by: Skoorb
5.725 is the answer 🙂

oops nm, 5.725 is right

d= vt+(1/2)at^2

v=9
a=9.8
t=.5

 

[mugs]

Originally posted by: neonerd

Originally posted by: Skoorb
5.725 is the answer 🙂

what'd u use to get it?

(-9*.5)-(1/2)*(9.8*.25)

 

[neonerd]

Originally posted by: Skoorb
I was mixed up with beer and terminal velocity in the first post

hahaha