Two objects of different mass fall with gravity - which falls faster?

[MIA6]

1. IF there are two objects, one is heavier, the other is lighter. Now someone drops them both from the top of a building, which object accelerates faster? do they both have the same acceleration that comes from the earth? 9.81m/s^2? or we use the formula net force=mass* acceleration which the answer may not equal to 9.81. i know the heavy one will land first. but how about acceleration?
2. btw, in our daily life, when we ask people's weight, people may answer like 50kg, are they actually talking about their mass not weight? because our teacher today asked a student's weight, and she answered 50kg, and then my teacher wrote W(weight)=mg=50kg*9.81m/s^2? so my teachcer had actually asked the student's mass, and then calculated weight?
thank you.

 

[christianjb]

Why don't you do the experiment to see? Report back to us with your findings.

I'd be particularly interested seeing how much faster the heavy object is than the light object when you drop two objects from a height of a few meters.

(Everyone else... Shhhh!)

 

[MIA6]

i think i got the answer already by myself. but anyway, i want to ask you whether the acceleration is 9.81 or we use formula to find the acceleration. i was confused about these two.

 

[Schrodinger's Dog]

Did you ever watch the experiment on the Apollo 15 mission where David Scott dropped a feather and a hammer on the moon?

Since you know the answer, here it is.

http://nssdc.gsfc.nasa.gov/planetary/image/featherdrop_sound.mov

In an atmosphere we have to account for air resistance, but in the vacuum on the moon we could just conveniently use an arbitrary and equal value for both objects.

 

[christianjb]

Did you ever watch the experiment on the Apollo 15 mission where David Scott dropped a feather and a hammer on the moon?

Since oyu know the answer, here it is.

http://nssdc.gsfc.nasa.gov/planetary/image/featherdrop_sound.mov

In an atmosphere we have to account for air resistance, but in the Vacuum on the moon we could just conveniently us 9.8ms^-2

Pretty cool. Here's the link:


Oh- ok, you already had a link. Anyway- two links are better than one.

 

[ranger]

MIA6, this type of question has been asked and answered many times. Do a search on the forums and I'm sure you'll find something interesting.

 

[MIA6]

the accelerations of two objects were different, so their accelerations were not equal to 9.81?

 

[Gza]

the accelerations of two objects were different, so their accelerations were not equal to 9.81?

The acceleration of an object within a uniform gravitational field is independent of mass, and for Earth is ~9.81 m/s^2

 

[MIA6]

The acceleration of an object within a uniform gravitational field is independent of mass, and for Earth is ~9.81 m/s^2

I sort of get what you said, but can u be more specific? so the formula a=net force/mass is used in what situation?

 

[Gza]

I sort of get what you said, but can u be more specific? so the formula a=net force/mass is used in what situation?

Start with the basics: F=ma

F in this case is the force of gravity which is causing m's acceleration

We can determine the magnitude of the gravitational force from: F_gravity = mg, where g is ~9.8m/s^2

Plug this into the left side of F=ma to get:

mg = ma

The m's cancel (algebraically, but there is also something deep happening by allowing this cancellation, don't worry about it for now)

we get:

a = g

No mass dependence

 

[MIA6]

ok, i think the lighter object and heavier object have different accelerations, but why they are not both equal to 9.81 from earth?

 

[ranger]

ok, i think the lighter object and heavier object have different accelerations, but why they are not both equal to 9.81 from earth?

https://www.physicsforums.com/showpost.php?p=1215151&postcount=3

 

[MIA6]

ah, so their accelerations are both 9.81? and the reason lighter one will land first because of the air resistance and other things that my teacher said in the class.

 

[MIA6]

who can answer my 2nd question about 'weight'?

 

[ranger]

ah, so their accelerations are both 9.81? and the reason lighter one will land first because of the air resistance and other things that my teacher said in the class.

If their acceleration is the same then that would imply that both the light object and the heavy object would have the same acceleration, wouldn't it? But if one hits the ground first then that means they have different accelerations, right?
The difference in acceleration arises because of air resistance as i indicated in my quoted post. If you consider a sheet of paper and a bowling ball (on earth), the ball would obviously hit the ground first and therefore have different acceleration when compared to the sheet of paper. Upon dropping the sheet of paper, it moves air aside as it falls. But so does the bowling ball. The amount of air displaced is dependent on the frontal cross-section area. It should be obvious from this to figure out which object would be more affected by the air resistance.

 

[ranger]

who can answer my 2nd question about 'weight'?

Are you familiar with apparent weight and actual weight? Which one a scale measures? Can you do a little research first before saying, No?

 

[ice109]

who can answer my 2nd question about 'weight'?

there are differences between formal language, the language of science, and colloquial language, common language. colloquially weight means mass, or at least people think weight means how much of you there is hence "lose weight". formally weight means the f in f=ma. an additional point of confusion is that in the metric system kg is a measure of mass and Newtons is a measure of weight though of course most people don't know about Newtons and use kg as weight. in the imperial system pounds is a measure of weight and slugs is a measure of mass but of course no one knows about slugs so out of ignorance and by default they use pounds correctly, but incorrectly.

if your mom asks you how much you weight and you live in britain she expects kg but the correct answer would be in Newtons.

if a physicist asks how much you weigh and you live in britain he/she expects Newtons

if your mom asks you how much you weigh and you live in the us she expects pounds and the correct answer is in pounds, but not because she knows any better.

if a physicist asks you how much you weigh and you live in the us he/she expects Newtons !

 

[mgb_phys]

if your mom asks you how much you weight and you live in britain she expects kg but the correct answer would be in Newtons.

Actually she expects stones but that is just getting too complicated!
http://en.wikipedia.org/wiki/Stone_(weight)

 

[ice109]

Actually she expects stones but that is just getting too complicated!
http://en.wikipedia.org/wiki/Stone_(weight )

there's no article there

 

[mgb_phys]

Sorry, forum software stripped the closing ")"

You have to love the UK. Forefront of science and engineering and manages to deal with both metric and 13Century units on a daily basis!

 

[Schrodinger's Dog]

In the UK we are backward, we still work in stones and pounds, in fact if you told us you were 200 pounds we'd divide by 14. An ancient land with a metric system we are loathe to take on. 200 pounds is what now in kilos? NAh you've lost me...

Back to the questions, I think your best bet is to really Google the subject and get a firm idea of how things work, then come back to us.

 

[HallsofIvy]

btw, in our daily life, when we ask people's weight, people may answer like 50kg, are they actually talking about their mass not weight? because our teacher today asked a student's weight, and she answered 50kg, and then my teacher wrote W(weight)=mg=50kg*9.81m/s^2? so my teachcer had actually asked the student's mass, and then calculated weight?
thank you.

Well, you say that the teacher asked for the student's weight- but the student answered with mass instead. The teacher then calculated the student's weight (in Newton's). Mass is often used interchangably with weight since, as long as you stay on the surface of the earth, they are proportional. If you were standing on the moon you would have the same mass but much lower weight than on the earth.

 

[mgb_phys]

Well, you say that the teacher asked for the student's weight- but the student answered with mass instead.

This is one of those questions that is supposed to make the student think about the deeper meaning of physics but I think just comes off making the teacher look smug and the student look stupid.
This sort of thing puts a lot of people off physics - they think it is just a series of trick questions and slightly smug in-jokes. At least my wife (a chemist) thinks so!

 

[Schrodinger's Dog]

This is one of those questions that is supposed to make the student think about the deeper meaning of physics but I think just comes off making the teacher look smug and the student look stupid.
This sort of thing puts a lot of people off physics - they think it is just a series of trick questions and slightly smug in-jokes. At least my wife (a chemist) thinks so!

Believe me studying physics at any level above High school soon wipes the smug expression off their faces. I disagree, I think some people in physics tend to take there subject as a mark of distinction and can be quite arrogant, but most people on this site, if not all are just keen to help or to discuss something they get a great deal of enjoyment out of. It's like any subject though, when you have deep underlying understanding of something it's going to make you feel a little smug, let's not hope too smug though. People who study physics or any science tend to be about the same for overall smugness, I've found

What we're talking about here, is common knowledge to anyone who has studied physics, mass and weight are not equivalent. But it's hardly a trick, and to frank I don't think their are that many tricks, well perhaps imaginary numbers, but most physics is pretty mathematical and so pretty formalised.

Mass=volumexdensity

weight=gravitational force acting on an object of x mass.

In space does a volume of say a piece of metal increase, does its density decrease? No so it's mass is always the same assuming it's structure isn't damged or changed.

In space is the gravitational force weaker? Yes.

Pretty simple and their is no trick, but I think you'll agree in terms of physics it's important to make the distinction between weight and mass.

 

[mgb_phys]

I agree the point of physics is to make people take a deeper look at how things really work and how their assumptions are wrong. And the more you study physics the more you want to share that deeper insight.

I just think that in a (presumably introductory) class asking someone their 'weight' and saying "wrong" when they give units of mass is counter productive and gives the wrong impression.

It's a bit like if someone asks you do you want tea or coffee - should you reply 'true' - ie. in a boolean sense you want (tea|coffee)

A student of mine made the reasonable complaint that most exam questions are written in 'trick' language - you have to know the code that 'a light spring' means treat it as SHM and 'smooth plane' means ignore friction. Instead of the question just stating that you should ignore friction.

 

[Schrodinger's Dog]

I think this is done with the idea that in order to answer questions they want you to know about underlying facts, the jargon should be part of your understanding, if it is not then you have perhaps not studied the course sufficiently or as in your case you need to know more.

I know what your saying and to some extent you're right, but after all is that not why you can raise your hand in class; I'm sure people who give lessons, want to know that their students are on the same page, and if not want them to feel free to ask questions, in the same way a lecturer might use very obscure language and expect his students to be up to speed. If it's done for no reason then it becomes obscure for obscures sake, but if it prompts someone to learn more about the subject to unravel the question, then that is the idea no?

That said if the student had never had any reason to encounter such jargon, then he has every right to complain. That is the exams fault surely and the syllabus should specify people learn different terminology so that exams only cover what they have studied, or could have been expected to have studied- not impenetrable code.

 

[mgb_phys]

I agree that the most important (and hardest) thing in teaching introductory physics is to make it clear that terms like mass, weight, acceleration, velocity have specific meanings which aren't the same as everyday language and that these differences are improtannt for understanding physics.

I remember being marked down for writing that something was 'weighed' in an experiment instead of writing 'it's mass was determined' I pointed out that I was using a spring balance so I was actually 'weighing' it. I must have been an annoyingly smug schoolkid!

ps. Talking to people still in teaching, the old 'code word' exam questions are now being changed to explicitly state what you should assume and what technique you should use - probably part of the dumbing down of physics.

 

[daniel_i_l]

This has been mentioned in pretty much every thread on the subject but I'll say it again because I think it's interesting:
If you drop the heavy ball and the light ball one after the other or at the same time but held apart the heavy one will accelerate slighty more because the force of gravity depends on the mass of both the Earth and the ball.