Heavy and Light - Both Fall the Same
Most recent answer: 10/22/2007
What you may be getting confused by is the fact that the force of gravity is stronger on heavier objects than lighter ones. Another way of thinking of this is to say that gravity has to pull harder on a heavy object than a light one in order to speed them both up by the same amount.
However, in the real world, we have things like air resistance, which is why sometimes heavy things do fall faster. For example, if you drop a feather and you drop a rock, the rock will land first since the feather is slowed down more by the air. If you did the same thing somewhere where there is no air, the feather and the rock would land at exactly the same time.
p.s. Although Galileo noticed that different things fall at the same rate, there was really no explanation of why until General Relativity was developed. If you would like us to try to say something about how that explanation works, we could make an attempt. mike w
(published on 10/22/2007)
Follow-Up #1: Earth falling toward you
- Erik E (age 40)
Monterey, CA, USA
(published on 07/26/2012)
Follow-Up #2: heavier falls faster?
- jon epperson (age 30)
kennewic wa benton
(published on 08/07/2012)
Follow-Up #3: Why fixed gravitational acceleration?
- Will (age 18)
Now we get to the trickier issue- why the gravitational acceleration depends only on the position of an object, not on its size or what it's made of. Although this was described by Galileo in about 1590, it wasn't explained until Einstein developed general relativity in 1916. Gravity is most accurately described not as a force but as a warping of the spacetime within which all things move. Each object at a particular place and time sees the same warped spacetime. If you try to describe the motion as if it were occurring in Newton's flat spacetime, as we like to do, you get the same acceleration for any slow-moving objects, because that acceleration really just is a measure of the same spacetime curvature.
(published on 01/03/2013)
Follow-Up #4: curving spacetime
- Smith (age 13)
The collision of you and the earth mainly comes from the big spacetime warping due to the earth, not the little warping due to you. However, you do a little warping yourself, and that does show up in the earth's trajectory. The feather does less. So, as you say, even ignoring air friction you and the earth would collide just barely sooner than a feather and the earth, if dropped from the same height.
(published on 01/30/2013)
Follow-Up #5: falling objects pulling on Earth
- Jayadev Vijayan (age 19)
Chennai, Tamil Nadu, India
(published on 03/23/2013)
Follow-Up #6: times for different falling objects
- Abe (age 49)
Yes, that all sounds right.
(published on 10/06/2013)
Follow-Up #7: Gravitatonal attraction of two unequal mass objects
- Tathagat Bhatia (age 15)
There is an equal and opposite force on each of the two objects: they will both move. Now since the acceleration of each object is inversely proportional to the mass, the lighter object will move a bit faster. If you do the arithmetic you will find that they will meet at their common center of mass. The lighter one will move a bit further than the heavier one.
(published on 10/31/2013)
Follow-Up #8: gravity on different weights
- Nathaniel Scherrer (age 34)
San Diego, California, USA
The acceleration is given by a=F/m. Since F=mg, you get a=g regardless of m.
So, in words, the key point is that the force is not "pretty much equal" on the different objects. It's proportional to their masses. So that means the accelerations are exactly equal.
(published on 07/10/2015)
Follow-Up #9: Newton vindicates Gallileo
- Father (age 55)
In the absence of air friction both heavy and light objects will reach the ground at the same time.
Galileo deduced this by devising clever experiments with balls rolling down inclined planes. Newton gave it his blessing by observing that a = F/M, i.e. the acceleration of an object is proportional to the force, F, on it divided by its mass, M. Furthermore the gravitational force on said object was proportional to its mass, F=Mg where g is the measurable acceration of a mass due to gravity on earth. Putting these two equations together you get a = Mg/M = g. The acceleration is independent of mass.
For more information see: https://van.physics.illinois.edu/qa/listing.php?id=164
(published on 02/21/2017)