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# Calculating acceleration Watch

1. Basically I drop a ball bearing into sand. I know the impact velocity and the final velocity, as well as the displacement(depth of crater).

I want to find out the average rate of deceleration on the ball bearing by the sand.

I am only looking for an average approximation.. any ideas on how to do this?

Apologies if it's blindingly obvious ^^

EDIT: NEVERMIND im just retarded and used a wrong value... definitely been awake too long now :P
2. (Original post by xysm)
Basically I drop a ball bearing into sand. I know the impact velocity and the final velocity, as well as the displacement(depth of crater).

I want to find out the average rate of deceleration on the ball bearing by the sand.

I am only looking for an average approximation.. any ideas on how to do this?

Apologies if it's blindingly obvious ^^
well, i'm a bit confused by your question. your finding deceleration by the sand so the initial velocity (u in newton's equations) is the impact velocity, the final velocity (v) is probably zero and the displacement is s or x, depending on how you learned it.

s, u, v, a ==> v squared = u squared + 2as ryt?
this would giv you the value for a (which is negative as the ball is decelerating).

for the rate of something, you just divide by the length of time that the motion lasts for. so if the ball takes 3 seconds to stop (reach final velocity from impact velocity), divide the deceleration by 3. however, i'm not sure whether you need to do this or the question is worded badly. hmm..i think you probably don't need to do this.
3. (Original post by xysm)
Basically I drop a ball bearing into sand. I know the impact velocity and the final velocity, as well as the displacement(depth of crater).

I want to find out the average rate of deceleration on the ball bearing by the sand.

I am only looking for an average approximation.. any ideas on how to do this?

Apologies if it's blindingly obvious ^^

EDIT: NEVERMIND im just retarded and used a wrong value... definitely been awake too long now :P
I don't think you mean "average rate of deceleration", do you?
It's just the average deceleration you want.

The deceleration of the bearing in the sand will be irregular, not uniform, that is almost certainly true.
If you just apply the formula (as mentioned by ash in post #2)

(where is the distance the bearing sinks into the sand, = 0 and is the velocity with which the bearing strikes the sand*)
then the value of calculated will be an "average" value of deceleration.

*(u is found from the height you drop the bearing.
4. Thanks for both replies, they were very useful.

However in my tired state, the problem was that I was using displacement of the ball in stage 1 (the drop height) not the displacement in the sand.

And yes you are right, that was badly phrased, I did mean average deceleration. Naturally it is merely an approximation as the deceleration is not going to be uniform.

Anyway, thanks a lot
5. (Original post by xysm)
Thanks for both replies, they were very useful.

However in my tired state, the problem was that I was using displacement of the ball in stage 1 (the drop height) not the displacement in the sand.
And yes you are right, that was badly phrased, I did mean average deceleration. Naturally it is merely an approximation as the deceleration is not going to be uniform.

Anyway, thanks a lot
lol easy mistake to make. can we get rep (well me anyway)?
6. (Original post by ash92:))
lol easy mistake to make. can we get rep (well me anyway)?
what the ****! why'd i get neg rep for?!
7. (Original post by ash92:))
what the ****! why'd i get neg rep for?!
Probably because you're asking for rep
8. (Original post by soup)
Probably because you're asking for rep
okaaaay but wtf is anyone's problem with that? well that's the world for you. ask for someone to save your life and they'll laugh at you and stab you themselves.
hmm, actually come to think of it, i'm defo going too far - scrap that example

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