Show Sometime in the third century B.C., Aristotle wrote that heavy objects fall toward Earth faster than lighter ones. His explanation for this behavior was profoundly influential, sophisticated for its time, and almost completely wrong. Almost 2,000 years later, Galileo proved that heavy objects and light objects fall to Earth at exactly the same rate. You can perform his experiment yourself.
At first glance, it's easy to side with Aristotle. If you drop a feather and a bowling ball from the same distance anywhere on Earth, they will fall at different rates. The feather will drift breezily to the ground while the bowling ball plunks downward immediately. But this explanation leaves an important factor out of the equation: air resistance. Since the feather is so light, air pressure acting on it from all directions is strong enough to counteract the force of gravity, which acts on it uniformly regardless of its weight.
Galileo proved Aristotle wrong with a simple stroke of genius — he used two cannonballs (bowling wasn't popular in 16th century Italy, but cannons were) and dropped them both off of the Leaning Tower of Pisa. If Aristotle was right, then the smaller cannonball should fall at a slower rate than the larger one. Instead, they both fell at the same exact speed: 9.8 m/s². Galileo's experiment became one of the most important pieces of the puzzle that Isaac Newton would later use to establish the modern theory of gravity.
You can recreate your own version of Galileo's experiment by tying a feather to a bowling ball and dropping them both at the same time. The feather-bowling ball duo doesn't fall at a slower rate because the feather is lighter than just the bowling ball alone — instead, they both fall at exactly the same rate. Similarly, if you pump all of the air out of a glass chamber to create a vacuum, you can drop both the feather and the bowling ball — no strings attached — and watch them hit the ground at the exact same time.
This experiment doesn't necessarily require a cumbersome vacuum chamber. Apollo 15 astronaut David Scott famously recreated this experiment on the moon in 1971 using a falcon feather and a hammer. Scott, an MIT-educated aeronautics engineer, knew his physics, so the fact that the feather and hammer hit the surface of the moon at the same time was no surprise. The moon has an atmosphere 10 quadrillion times less dense than the Earth's — so weak, in fact, that statically charged moon dust levitates around 10 centimeters above the surface of the planet. Despite the fact that modern audiences know what the outcome of Scott's experiment is, the live broadcast remains a sight to behold and a historical treasure.
This article first appeared on Curiosity.com. Gravity is a major player in the study of physical science. It is, of course, the force of gravity that causes objects to fall. One object always exerts a force of attraction on another object. This force of attraction is a pull, like the pull of gravity. The sun, the earth, and the moonThe larger an object is, the greater is the force of its attraction. Consider the fact that the sun, which is much, much larger than the earth, can, even at 90 million miles away, hold the earth and the other eight planets in orbit. The moon, on the other hand, is much smaller than the earth, and has only about one-sixth of the gravity of the earth. Astronauts who have walked on the moon feel light and weightless because there is very little gravity holding them down. On the other hand, if they were to go to Jupiter, which has much more gravity than the earth, they wouldn't even be able to lift a foot off the ground. Who was Galileo Galilei?It was Italian scientist Galileo Galilei who formulated the laws of accelerated motion and free-falling objects. He found that when an object is dropped and falls to the ground it has a falling rate of 9.8 meters per second, squared. So, why do some objects fall faster than others?You may wonder, then, why feathers float gently in the breeze instead of falling to the ground quickly, like a brick does. Well, it's because the air offers much greater resistance to the falling motion of the feather than it does to the brick. The air is actually an upward force of friction, acting against gravity and slowing down the rate at which the feather falls. The brick, on the other hand, can cut right through the air as if it didn't exist. Galileo discovered that objects that are more dense, or have more mass, fall at a faster rate than less dense objects, due to this air resistance. If a feather and a brick were dropped together in a vacuum-that is, an area from which all air has been removed-they would fall at the same rate, and hit the ground at the same time. Understanding these basic facts will help you to be able to answer the question of why some objects fall faster than others. Test this theory yourself!You can test the rate at which various objects fall, noting both the mass of each object, and how long it takes for it to fall. Be sure to drop all objects from the same height, and be careful to use only objects that can't break. Record all your information in a journal, and chart your results. Conduct three trials for each object so that you can calculate an average time.
Most recent answer: 10/22/2007 which ball falls fastest, a basketball, or tennis ball??? heavy ball vs. lighter ball- alan (age 9) citrus heights Alan, Great question. As it turns out, the force of gravity tries to make everything accelerate downward at EXACTLY the same rate, no matter how light or heavy it is. This means that if you dropped a basketball and a tennis ball at the same time (from the same height) they will hit the ground at the same time (try it!). Is this true for all objects? If you drop a feather and a rock at the same time, you know the rock will hit the ground first and the feather will fall much slower. This does NOT mean that what I said above is wrong. The problem with dropping stuff near the surface of the earth is that gravity is not the only force acting…there is also air resistance. Air tends to make stuff fall slower, especially light objects, which is why the feather hits the ground last. Since tennis balls and basketballs are quite heavy, at least compared to feathers, we don’t easily notice the effect of air resistance on these (although it’s still there). If there were no air, all objects would fall at exactly the same rate. In other words, if you went to the moon (where there is no atmosphere) and dropped a feather, a rock, a tennis ball and a basketball, they would all hit the ground at the same time. Hope this helps. MS (published on 10/22/2007) Follow-up on this answer
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QUESTION #6 Asked by: Terri If no air resistance is present, the rate of descent depends only on how far the object has fallen, no matter how heavy the object is. This means that two objects will reach the ground at the same time if they are dropped simultaneously from the same height. This statement follows from the law of conservation of energy and has been demonstrated experimentally by dropping a feather and a lead ball in an airless tube. When air resistance plays a role, the shape of the object becomes important. In air, a feather and a ball do not fall at the same rate. In the case of a pen and a bowling ball air resistance is small compared to the force a gravity that pulls them to the ground. Therefore, if you drop a pen and a bowling ball you could probably not tell which of the two reached the ground first unless you dropped them from a very very high tower. Answered by: Dr. Michael Ewart, Researcher at the University of Southern California The above answer is perfectly correct, but, this is a question that confuses many people, and they are hardly satisfied by us self-assured physcists' answers. There is one good explanation which makes everybody content -- which does not belong to me, but to some famous scientist but I can't remember whom (Galileo?); and I think it would be good to have it up here. (The argument has nothing to do with air resistance, it is assumed to be absent. The answer by Dr. Michael Ewart answers that part already.) The argument goes as follows: Assume we have a 10kg ball and a 1kg ball. Let us assume the 10kg ball falls faster than the 1kg ball, since it is heavier. Now, lets tie the two balls together. What will happen then? Will the combined object fall slower, since the 1kg ball will hold back the 10kg ball? Or will the combination fall faster, since it is now an 11kg object? Since both can't happen, the only possibility is that they were falling at the same rate in the first place. Sounds extremely convincing. But, I think there is a slight fallacy in the argument. It mentions nothing about the nature of the force involved, so it looks like it should work with any kind of force! However, it is not quite true. If we lived on a world where the 'falling' was due to electrical forces, and objects had masses and permanent charges, things would be different. Things with zero charge would not fall no matter what their mass is. In fact, the falling rate would be proportional to q/m, where q is the charge and m is the mass. When you tie two objects, 1 and 2, with charges q1, q2, and m1, m2, the combined object will fall at a rate (q1+q2)/(m1+m2). Assuming q1/m1 < q2/m2, or object 2 falls faster than object one, the combined object will fall at an intermediate rate (this can be shown easily). But, there is another point. The 'weight' of an object is the force acting on it. That is just proportional to q, the charge. Since what matters for the falling rate is q/m, the weight will have no definite relation to rate of fall. In fact, you could have a zero-mass object with charge q, which will fall infinitely fast, or an infinite-mass object with charge q, which will not fall at all, but they will 'weigh' the same! So, in fact, the original argument should be reduced to the following statement, which is more accurate: If all objects which have equal weight fall at the same rate, then _all_ objects will fall at the same rate, regardless of their weight.In mathematical terms, this is equivalent to saying that if q1=q2 then m1=m2 or, q/m is the same for all objects, they will all fall at the same rate! All in all, this is pretty hollow an argument. Going back to the case of gravity.. The gravitational force is
( G is a constant, called constant of gravitation, M is the mass of the attracting body (here, earth), and m1 is the 'gravitational mass' of the object.) And newton's law of motion is
where m2 is the 'inertial mass' of the object, and a is the acceleration. Now, solving for acceleration, we find:
Which is proportional to m1/m2, i.e. the gravitational mass divided by the inertial mass. This is our old 'q/m' from the electrical case! Now, if and only if m1/m2 is a constant for all objects, (this constant can be absorbed into G, so the question can be reduced to m1=m2 for all objects) they will all fall at the same rate. If this ratio varies, then we will have no definite relation between rate of fall, and weight. So, all in all, we are back to square one. Which is just canceling the masses in the equations, thus showing that they must fall at the same rate. The equality of the two masses is a necessity for general relativity, and enters it naturally. Also, the two masses have been found to be equal to extremely good precision experimentally. The correct answer to the question 'why objects with different masses fall at the same rate?' is, 'beacuse the gravitational and inertial masses are equal for all objects.' Then, why does the argument sound so convincing? Since our daily experience and intuition dictates that things which weigh the same, fall at the same rate. Once we assume that, we have implicitly already assumed that the gravitational mass is equal to the inertial mass. (Wow, what things we do without noticing!). The rest of the argument follows easily and naturally...Answered by: Yasar Safkan, Physics Ph.D. Candidate, M.I.T. |