Experiments with falling objects, something for us to learn about the behaviour of gravitation!




Experiment 1.  Misconceptions about falling objects , YouTube channel, Veritasium:


Dr Derek Muller takes to the streets with a 5kg medicine ball (exercise ball or fitness ball) and a 1 kg basketball and poses the question:

when dropped from the same height, which one will hit the ground first? You may know the correct answer, but can you explain why?



According of what Dr. Derek Muller’s explanation: the force on the medicine ball is greater than the force on the basketball, but it has more inertia and what’s really important is that the ratio of force to inertia is the same for all objects, so everything accelerates at the same rate and lands at the same time.


Let’s have a look at another experiment from Brian Cox:


Experiment 2.  Feathers and bowling ball:

In this experiment, a bowling ball and feathers are dropped in the air of the atmosphere, and then again in a vacuum. The result…might not surprise you since you learned what would happen in secondary school science class.




P.S.  :  the same experiment was done by Apollo 15 Hammer and Feather Drop (Aug., 1971):



So, why does the basket ball and the medicine ball take the same time to hit the ground?  they are in air, not in a vacuum chamber!  

How can you explain why the feather and the bowling ball in Brain Cox’s experiment do not arrive at the same time, when the experiment conducted in air?

The force on the bowling ball is greater than the force on the feather, but the bowling ball has more inertia, so that the ratio of force to inertia is the same for all objects, and everything accelerates at the same rate and lands at the same time.  But the fact is, in air, the feather falls much slower than the bowling ball due to the air resistance. The air resistance on the feather is much greater than the air resistance on the ball.


What is wrong on the Dr. Derek Muller’s experiment??   It seems everything he said is correct! 


Comparing these two videos, we find:


Dr. Muller’s experiment :  the medicine ball and the basket ball, their size and shape is exactly the same.  Professional Brian Cox’s experiment:  the feathers and the bowling ball, their size and shape is different, so they have a different air resistance! 

The reason why Dr. Muller is using the same size balls for his experiment is because they have the same air resistance , so he doesn’t need to do the experiment in a vacuum chamber to prove that gravity is a constant at a given place on earth.


Let us use now Newton’s second law to calculate the velocity in the Dr. Derek Muller’s experiment:





F = Force [N] Newtons,   m = mass [kg] kilo grams,  a = acceleration [m/s2] meter / second2


basket ball :

F = 1kg x 10 m/s2    ( actually, it should be  9.81 metres (32.2 ft) per second every second, for simplicity, we use 10 instead of 9.81) 

F = 10 Newtons.


medicine ball :

F = 5kg x 10 m/s2  

F = 50 Newtons.


because : a = F / m


basket ball:

a    = 10 Newtons / 1kg   =  10 kg * m / kg *  s= 10 m/s2    


a    = 50 Newtons / 5kg   =  50 kg * m / kg *  s= 10 m/s2    


That is the proof that the acceleration of earth is constant !  The gravitational constant.  (check this out :  http://en.wikipedia.org/wiki/Gravitational_constant  )



Experiment 3:  a 15 kg dumbbell and a  25 kg dumbbell :





And now, we are sure that:

A 15 kg dumbbell and a 25kg dumbbell will accelerates at the same rate and lands at the same time, when they are being dropped in a vacuum room.

But how about if we make a  dumbbell with 15kg that has exactly the same shape and size as the 25kg dumbbell would it also land at the same time if we drop them in air?

The answer is conditional, if we would manage to keep both dumbbells exactly aligned with each other during the fall then they would have the same air resistance and consequently land at the same time. But if the dumbbells are out of alignment with each other, during the fall, one could have more air resistance than the other and consequently they do not land at the same time.

This does not come into it in the experiments with the balls because the ball shape is homogeneous and thus the air resistance is not affected by the rotational alignment of the balls,  the same does not apply to the dumbbells!


Further more, it is worth to mention that the earth acceleration has the dimension [m/s²], it is only a force when we multiply it with a mass [kg], [m/s² * kg = N] N stands for Newton which is a force. So we can talk about all forces of the universe, which are measured in Newton. 

We have 4 forces in the Universe:

1. The electro magnetic force

2. The weak nuclear force

3. The strong nuclear force

    We know all about these three forces but not all about the fourth force.

4. The gravitational force : we know how big it is ( very very small) and we know all about its behaves, but we do not know what it is, as yet. 

    But there is some hope that with the discovery of the Higgs Boson (they call it the god particle) we will be able to explain gravity.


What is gravity? :


As you can see, most of the people will answer that it is a downward force that stops you from flying away from the earth.  Or a force that pulls all matter together.  The more matter, the greater the gravitational attracting force, so the things that have a lot of matter such as planets and moons and stars have a lot of attraction force. 

We all learned these answers in the secondary school, so we just need to remember it!  

But fact is that even science still cannot explain what the gravitational force is!

They know how it behaves and they know how big it is, but they still don’t know what it is! 


We know that every massive object falls to the ground and we know this is because of gravity.


But back in time of Aristotle the scientific opinion was that the speed at which an objects falls was directly related to their relative weights. Thus, a heavy object would fall faster than a lighter object.  After all, pick up a feather and a rock and drop the two of them, and Aristotle’s view will be prove,


the rock will indeed fall faster than feather.


As mentioned before, the gravitational force pulls all forces together.  In the examples of dropping balls, feathers and dumbbells onto the ground we have ignored the movement of the earth. But since the mass of the earth is so massive as compared with our dropping items used in the experiments, we can ignore the earths movement, it is immeasurable small. Nevertheless if the falling objects getting larger, like the size of the moon for example then the movement of the earth is detectable which is shown in the following video.




If an object with comparable mass to that of the Earth were to fall towards it, then the corresponding acceleration of the Earth would be observable.


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