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- one for the physicists
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 🙂

Posted 4 years ago0.8*1.8*9.81 = gpe = KE just as it hits the floor

Posted 4 years agoontor not sure where the 9.81 comes from?

Posted 4 years agoGravity init.

Posted 4 years agoAcceleration 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 🙂

Posted 4 years ago9.81 is m/s^2, acceleration due to gravity.

Posted 4 years agomolgrips she knows what g is which is more than I knew at 14 😯 every day is a school day

Posted 4 years agoAs it should be 🙂

Posted 4 years agoWhen I started typing I knew I was digging a huge hole for my tiny mind to cope with

Posted 4 years agoI’d have gone for B. If in doubt, always put B.

Posted 4 years agoIt’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.

Posted 4 years agoThe 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.

Posted 4 years agoThe 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 🙁

Posted 4 years agoeverything 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.

Posted 4 years agomaccruiskeen how do you know we have a cat 😯

Posted 4 years ago@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

Posted 4 years ago*really*do this)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?

Posted 4 years agothrows board rubber to back of class where Bregante is sitting

Posted 4 years agomaccruiskeen how do you know we have a cat

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

Posted 4 years agoWhat is the kinetic energy of the board rubber as it hits Bregante’s skull?

Posted 4 years agoActually you could work out how heavy the board rubber would have to be to kill him.

Posted 4 years agoThanks for all your help everyone my daughter thanks you as well.

Posted 4 years agoIs 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.

Posted 4 years agoWell 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.

Posted 4 years agoWho is putting up these shelves that stuff can just fall off? That’s the real issue here.

Posted 4 years ago

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.There might’ve been an earthquake.

Posted 4 years agomolgrips daughter tells me they round up to 10 ive told her to have a word with teach for being lazy.

Posted 4 years agoAre you assuming this is on earth?

Posted 4 years agoDaughters 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.

Posted 4 years agoIf 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?.

Posted 4 years agomolgrips daughter tells me they round up to 10

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

Posted 4 years agomolgrips 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.. 🙂

Posted 4 years agoDid the very upset cat initially cause the box to wobble and fall off the shelf?

Best just to put it back and say nowt.

Posted 4 years ago0.8kg would be a pretty small cat. Maybe a guinea pig.

Posted 4 years agoIt’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.

Posted 4 years agoWhat if the box falls out of the overhead luggage shelf of a plane on a treadmill?

Posted 4 years ago

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