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• one for the physicists
• molgrips
Subscriber

When you put it on the shelf, you gave it potential energy. When you knock it off the shelf, that potential energy gets converted into kinetic energy.

So if you have the potential energy when it’s on the shelf, there’s your answer.

1/2 x mass x speed squared is the formula she has

That’s the wrong formula anyway, that applies to constant speed of course so that you have something to put in the forumla.

If you want to use the equations of motion, you want

v^2 = u^2 + 2ax

to calculate its final speed, where x is the distance, a is the acceleration which is g, and u is the initial velocity which is 0.

So v = 5.94m/s

Then E = 1/2 mv^2 which is 14.11J.

Calculating it the easy way, E = mgh (as below) = 14.12.

Thank **** for that, I’d have looked pretty silly 🙂

ontor
Member

0.8*1.8*9.81 = gpe = KE just as it hits the floor

sturmey
Subscriber

ontor not sure where the 9.81 comes from?

jam bo
Subscriber

Gravity init.

molgrips
Subscriber

Acceleration due to gravity is 9.81m/s/s and is written as a lowercase g.

Are you sure you’re qualified to be doing your daughter’s homework? She ought to know what g is, ask her 🙂

richmars
Subscriber

9.81 is m/s^2, acceleration due to gravity.

sturmey
Subscriber

molgrips she knows what g is which is more than I knew at 14 😯 every day is a school day

molgrips
Subscriber

As it should be 🙂

sturmey
Subscriber

When I started typing I knew I was digging a huge hole for my tiny mind to cope with

Bregante
Subscriber

I’d have gone for B. If in doubt, always put B.

molgrips
Subscriber

It’s not as difficult as you think.

There are a set of formulae called equations of motion. These are given to you. You just find the one that uses all the values you have, and doesn’t use the values you don’t have except for the thing you are interested in.

You don’t know the time taken for the box to fall, and the formula I posted is the only one that doesn’t ask for it. Simples.

maccruiskeen
Subscriber

The more pressing question is – is the the kinetic energy at the moment of interest sufficient to kill the cat inside the box? You need an equation for this as if its not dead its going to be fricking angry in which case I don’t want to be the one who opens the box.

scaredypants
Subscriber

The more pressing question is – is the the kinetic energy at the moment of interest sufficient to kill the cat inside the box?

We’ll never know 🙁

ontor
Member

everything close enough to Earth to be considered in a uniform gravitational field accelerates at 9.81 m/s^2 toward the Earth. Crudely, Force=mass times acceleration so the force exerted by an object as it’s weight is it’s mass times the acceleration due to gravity.

GPE is a special case of the work done by a force, so the work done against/by gravity is the force (weight) times the distance moved by the force (the height). in this case the weight of the box is 0.8*9.81 and the distance over which gravity does work on it is 1.8m.

sturmey
Subscriber

maccruiskeen how do you know we have a cat 😯

ontor
Member

@maccruiskeen
No it’s not, I recon you could work it out, but as it’s a cat why not try a practical?

(even though I’m a dog person, please don’t really do this)

Bregante
Subscriber

The more pressing question is – is the the kinetic energy at the moment of interest sufficient to kill the cat inside the box?

And if there was nobody in the room at the time, would it have made a sound when it hit the floor?

sturmey
Subscriber

throws board rubber to back of class where Bregante is sitting

maccruiskeen
Subscriber

maccruiskeen how do you know we have a cat

How do you know if you’ve still got one – dare you open the box?

molgrips
Subscriber

What is the kinetic energy of the board rubber as it hits Bregante’s skull?

Bregante
Subscriber
molgrips
Subscriber

Actually you could work out how heavy the board rubber would have to be to kill him.

sturmey
Subscriber

Thanks for all your help everyone my daughter thanks you as well.

Subscriber

Is it not just 14.4 as all the potential energy has been converted into kinetic energy by the point it reaches the floor? Not considering loss through friction in the air, etc.

ampthill
Subscriber

Good going for 14

molgrips
Subscriber

Well yes honeybadger, but the potential energy doens’t work out to be quite 14.4 unless that’s a typo.

Actually, just checked, if you assume g = 10 then it comes to 14.4, I’ve seen that used as a convenient shortcut.

kayak23
Subscriber

Who is putting up these shelves that stuff can just fall off? That’s the real issue here.
I’d suggest getting a professional in to check your other shelves, and in the mean time, please don’t put any 0.8kg items on any shelf.

molgrips
Subscriber

There might’ve been an earthquake.

sturmey
Subscriber

molgrips daughter tells me they round up to 10 ive told her to have a word with teach for being lazy.

Are you assuming this is on earth?

sturmey
Subscriber

Daughters homework 0.8kg box falls off a shelf 1.8m high state the kinetic energy of the box just before it hits the floor? 1/2 x mass x speed squared is the formula she has but we dont have the speed we do have the gravitational potential energy 14.4 if that helps.

somafunk
Subscriber

If the cat is heated in an enclosed pressurised box filled with an inert gas dropped from the shelf i could use my knowledge of thermodynamics/Boyles law to work out how much trouble we are in from the RSPCA/SSPCA and whether or not the cat will be reduced to superheated steam by the time it hits the floor?, i dunno if that’d help with your daughters homework though?.

scaredypants
Subscriber

molgrips daughter tells me they round up to 10

molgrips’ daughter told you that ? He calls himself a physicist an’ all 🙄

molgrips
Subscriber

molgrips daughter tells me they round up to 10 ive told her to have a word with teach for being lazy.

It’s not as if it makes much difference these days with calculators and all.. 🙂

slowoldgit
Member

Did the very upset cat initially cause the box to wobble and fall off the shelf?

Best just to put it back and say nowt.

molgrips
Subscriber

0.8kg would be a pretty small cat. Maybe a guinea pig.

aracer
Subscriber

It’s not as if it makes much difference these days with calculators and all..

Except you’re still expected to be able to estimate the answer to your question without using a calculator, to ensure a misunderstanding doesn’t result in your Mars Orbiter disintegrating. At least that appears to be the case in Y6 at our local school when I’ve heard the teacher mention estimating the answer several times when I’ve been in there. In which case using 10 for g is reasonable – I’ve certainly used that before now when doing a quick check in my head where I only care about the order of magnitude.

portlyone
Subscriber

What if the box falls out of the overhead luggage shelf of a plane on a treadmill?

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