Gravational analogy

David

While I know it to be true that objects of different weights fall to earth at the same rate of acceleration I can't quite believe it. I look at Newton's numbers and explanations but then just as I'm starting to believe a picture of Wiley Coyote strapped to a boulder falling down the side of a mountain comes to mind and its all over. Aristotle had this problem too I believe. Can someone give me a layman's analogy please.

dion

not really an analogy but i think partly the problem is that if you imagine dropping a cannonball and it landing on your foot or head, it is going to hurt; however a tennis ball falling that far wouldn't hurt you at all. so your brain assumes that the cannon ball is going faster than the tennis ball; that weight and speed must be inextricably linked. quite logical really. i think the only way to disprove it is to do an experiment out of a window. please report back with results.

Chapter none

The other issue is terminal velocity. If you were to drop a cannon ball and a piece of paper they would have different drags and therefore different terminal velocities. Dropped from a mile up the cannon ball would be going faster when it hit the ground (supposing it had less drag).

A second experiment for both yourself and Dion is to take a rolled up sock and a very light receipt and drop them from the same height (I did about a meter). You will see the sock hits the ground much faster. This is because the drag is considerably less on the sock.

The rate of acceleration of gravity is 9.81m/s/s but drag must also be taken into account which will cause less aerodynamic objects to have lower terminal velocities and to reach them slower.

I believe the peregrine falcon has one of the fastest falls when it swoops to get it's prey, this is due to it being so aerodynamic.

Peter

If we ignore air resistance and assume the masses are falling in a vacuum then the force on each object is proportional to their mass. This means that if one object is twice as large as the other then the force on it is twice the size and hence both objects fall at the same speed.

But you probably know that already.

One way to think of it is to think of the larger object as being composed of two small objects stuck together. Both the 'small' objects will fall at the same speed and hence at the same speed as the 'large' object. Now think of separating the large object into two REAL small obejcts but leave them separated by only a tiny distance. Do you think they'd suddenly start falling more slowly? How far apart would they have to be in order for you to consider them as two objects?

Chapter none

I agree whole heartedly with Peter and he's absolutely right but the words "If we ignore air resistance" means that it doesn't apply to anywhere with an atmosphere (no where I have access to).

This now brings up the far more important questions:

Do Cartoon characters need to breath???

If not then no atmosphere makes everything fall at the same speed. But then we would lose all those big explosions...

andy

Peter's explanation is spot on. Acceleration is force/mass. In a vacuum, the only force is gravity. So everything accelerates at the same speed. Everything accelerates at the same speed (in a vacuum) because gravity is a force per kilo. The rate is a property of the gravitational field. But if you drop something out of a window, air resistance is also a force, which is a drag on acceleration. The level of air resistance is made up of the shape of the thing you're dropping, the thickness of the air and other stuff. When drag (the force slowing things down) = gravitational force (speeding things up) you reach [terminal velocity and turn into Charlie Sheen](http://www.imdb.com/title/tt0111400/). Without air resistance:  With air resistance: 

David

I love that most forums are prone to Godwins Law of probability that someone will be accused of being a Nazi, whereas ours is more around inevitable 80's brat pack star references.

Duncan

Chapter none:

Take tom and jerry and blindfold jerry, then get tom to chase jerry over a chasm. Jerry will happily run right over the chasm to the other side, because he cannot see the fall. Tom will follow jerry and then, after a few yards, he will see that he is running on thin air and plummet to another amusing catastrophe.

But you probably know that already.

A second experiment is to send Tom and Jerry into space and give a space suit to _Tom_. Both will be fine initially, Tom because he has a space suit and Jerry because he does not realise that he needs to breath. But as soon as Jerry pulls tom's face mask off, Tom will realise that he has no oxygen and choke dramatically as Jerry looks on nonchalantly.

Chapter none

Right I'm off to buy a cat and a mouse... Whilst I'm doing that I've left all the taps on and the freezer open; I'm sure my house mates will understand. All I need now is a fold down ironing board and the stage is set; let hilarity ensue...

underscore_cue

what you need is to turn your house into a vacuum chamber. paint the walls with a sticky air tight coating and seal up all the gaps and then set up some kind of pump to force all the air out. Then when you drop a feather and a hammer in the house they will fall at the same rate. Problem solved.

Philistine'sMonster

Peter's explanation has just made sense. I've struggled to understand that for nearly 40 years. Thanks Peter!

I wonder if you can explain another problem because I read a few years ago that there's some doubt on this theory; that heavier objects do, in fact, hit the ground first on scale that's measurable only at a sub-atomic level. OK, so it's a percentage that's almost impossible to measure with current technology, but can this be true, I wonder? I got confused when it then said that depending on whether the falling object was terrestrial or extra-terrestrial in origin, the lighter object could hit the ground first. Eh? What the fuck's that all about? And that some objects bend space around themselves or pull the earth towards them...? AAAAAGH! I think it was New Scientist wot told me.

Either that or I was drunk, watching Star Trek.

Anyway, whatever the case, blood started coming out of my ears at this point, but I'd be grateful for someone clever than me explaining it.

Chapter none

There will be a larger gravitation force between the larger object and the "floor" (hence 2 full super tankers can be pulled towards each other).

|f| = {G M_A M_B \over |r_{AB}|^2} (copied from Wiki)

Where G is the gravitational constant x Mass A x Mass B
Over
Distance between A and B squared

I'm not sure if that could account for it. I think this may only be effectual with large masses. And I'm not sure why that would only be on a sub atomic level.

I'm going to cower behind my chair as this is destroyed by someone with more physics understanding than I as I haven't studied it for years...