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

1. Jul 17, 2007

### 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.

Last edited: Jul 17, 2007
2. Jul 17, 2007

### 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!)

3. Jul 17, 2007

### 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.

4. Jul 17, 2007

### 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.

Last edited: Jul 17, 2007
5. Jul 17, 2007

### christianjb

Last edited by a moderator: Sep 25, 2014
6. Jul 17, 2007

### 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.

7. Jul 17, 2007

### MIA6

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

8. Jul 17, 2007

### Gza

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

9. Jul 17, 2007

### MIA6

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?

10. Jul 17, 2007

### Gza

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

11. Jul 17, 2007

### 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?

12. Jul 17, 2007

### ranger

13. Jul 17, 2007

### 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.

14. Jul 17, 2007

### MIA6

15. Jul 17, 2007

### ranger

If their acceleration is the same then that would imply that both the light object and the heavy object would have the same acceleration, wouldnt it? But if one hits the ground first then that means they have different accelerations, right?
The difference in acceleration arises becuase 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.

16. Jul 17, 2007

### ranger

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

17. Jul 17, 2007

### ice109

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 :tongue2:!

18. Jul 17, 2007

### mgb_phys

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

Last edited: Jul 17, 2007
19. Jul 17, 2007

### ice109

theres no article there

Last edited by a moderator: May 3, 2017
20. Jul 17, 2007

### 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!