- 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 ago
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.Posted 4 years agomaccruiskeenSubscriber
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.Posted 4 years ago
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.Posted 4 years ago
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.Posted 4 years agosomafunkSubscriber
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?.Posted 4 years agoaracerSubscriber
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.Posted 4 years ago
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