newton feather coin experiment

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Galileo’s feather and coin

Intuitively we know that if we drop a feather and a coin from the same height, the coin will reach the ground first. However, this is due to the difference in air resistance between the feather and coin. When placing these items in a vacuum, removing the air, we see that the feather and coin do drop at the same rate regardless of their masses.

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Motion and Force

Gravitational Force, Newton’s Law of Gravitation, Gravity, Acceleration Due to Gravity, Coin and Feather Experiment, Height and Acceleration Due to Gravity, Mass and Weight, Freefall, Weightlessness, Equation of Motion for Freefall

Gravitational Force

The attraction force between two celestial bodies is called gravitational force. Its SI unit is Newton(N).

Newton’s Universal Law of Gravitation

Newton’s law of gravitation states that,.

 “The gravitational force between two celestial bodies is proportional to the product of their masses and inversely proportional to the square of their central distance.”

Let us consider the two bodies A and B having masses m 1 and m 2 respectively are at distance d. Then, if the gravitational force is F between them then, 

According to Newton’s law of gravitation,

Gravitational Constant (G)

• The force of attraction between two bodies having a mass of 1 kg each and separated by a distance of 1 m is called the gravitational constant. Its value is equal to 6.67 x 10 -11 Nm 2 /kg 2 . Henry Cavendish calculated its value by using a sensitive balance called the Torsion balance in 1978 AD.  

Variation In Gravitational Force with Mass & Distance  

Let us consider the two bodies A and B having masses m 1 and m 2 respectively are at distance d. Then, if the initial gravitational force is F between them then,   

According to Newton’s law of gravitation,  

1. Keeping the distance constant while varying the masses. 

Conclusion: If the mass of an object is increased by two times, the gravitational force increases by two times. Also, if the masses of both objects are increased by two times, the gravitational force increases by four times.

Keeping the masses constant while varying the distance. 

Conclusion: If the distance is decreased to half, the gravitational force increases by four times. Also, If the distance is increased by two times, the gravitational force decreases by four times. 

Consequences Of Gravitational Force

• The gravitational force exerted by the sun and moon gives rise to the tides experienced in seas and oceans.
• The sun acts as the center of attraction around which planets orbit.
• The presence of solar systems and galaxies is a direct consequence of gravitational force.
• Gravitational force governs the orbital motion of planets, moons, and satellites around larger celestial bodies.
• The gravitational force of the Earth enables rainfall and snowfall to occur. 

The force of attraction with which a planet, satellite or star pulls a body towards its center is called gravity. Its SI unit is Newton(N). All planets and heavenly bodies have their own gravity but the magnitude of force of gravity depends upon mass and radius.

Effects of Gravity

• Acceleration is produced on freely falling body.
• Every object has some weight.
• The earth is surrounded by the atmosphere.
• All objects fall towards the surface from a certain height.

Acceleration Due to Gravity

The acceleration produced in a freely falling body due to the force of gravity of a planet is called acceleration due to gravity. It is denoted by ‘g’. Its SI unit is N/m 2 .

Let the mass of any planet or satellite be M, its radius R and mass of an object m, then according to Newton’s universal law of gravitation,  

In above equation G is universal gravitational constant and M is the mass of the planet. Therefore, both are constant. So, we can write, 

Therefore, it makes clear that the mass of an object does not affect the acceleration due to gravity in falling objects. Thus, the acceleration due to gravity is inversely proportional to the square of the planet’s radius.

Variation In Acceleration Due to Gravity

The Earth is an oblate spheroid, meaning it is mostly spherical but slightly flattened at the poles and bulging at the equator. As a result, the acceleration due to gravity is slightly higher at the poles, measuring approximately 9.83 m/s², compared to around 9.87 m/s² at the equator. To simplify calculations, an average value of 9.8 m/s² is commonly used to represent the acceleration due to gravity on Earth. 

Coin And Feather Experiment

The feather and coin experiment demonstrates the impact of air resistance on falling objects. In a vacuum, both objects fall simultaneously and at the same rate, highlighting the absence of air resistance. In normal conditions, the feather falls slower due to air resistance, while the coin falls faster. This experiment emphasizes the influence of air resistance on falling speeds and the equal acceleration of objects in a vacuum under gravity.  

Conclusion: The acceleration produced due to gravity is same in all object if there is no air resistance. 

Height And Acceleration Due to Gravity

The acceleration due to gravity at height ‘h’ from the surface is calculated as,   

Mass And Weight

• The total quantity of matter contained in a body is called its mass. The SI unit of the mass is Kg. It is measured by a physical or a beam balance.  
• The force with which earth attracts any object on its surface towards its center is called weight. Its SI unit is Newton(N).It is measured by spring balance.  

The weight of an object is calculated as, W = m x g

Where, m = mass of the body, g = acceleration due to gravity 

An object falling without any resistance is called Freefall. The acceleration produced on an object during the freefall is equal to the acceleration due to gravity at that place. There is no perfect freefall of the object falling on the earth due to air resistance. The surface of the moon is not covered by the air. So, the falling of any object on moon is freefall.  

Weightlessness

The weight of an object falling freely under the effect of gravity appears to be zero. It is called Weightlessness.  

Conditions for weightless  

• When a body is falling freely under the action of gravity, it becomes weightless.  
• When a body is in the space or at null point, it becomes weightless.  
• When a body is in the center of the earth.  
• Inside an artificial Satellite revolving around the earth or heavenly body.  

Equation Of Motion For Free Fall

For equation of motion for free fall, we have to modify the equations of motion in straight line. For that we have to replace ‘S’ with ‘h’ and ‘a’ with ‘g’.  

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1C20.10 Coin and Feather (Thor's Hammer)

Acceleration of gravity, drag

A 6-feet long Plexiglas tube containing a "coin" and "feather" is evacuated. The "coin" and "feather" fall at the same rate. When air is introduced into the tube, the "feather" falls at a slower rate.

The "coin" and "feather" objects in the tube are difficult to see from the back of a large lecture hall.

• [1] Plexiglas tube containing a packing peanut and wrapped washer
• [1] Vacuum pump
• [1] Power bar
• [1] Extension cord 

Classroom Assembly

The instructions assume the demo will show free-fall in air first, and then in vacuum second. The tube must also be pumped down before class if starting with free-fall in vacuum.

• Place the tube somewhere with lots of room around and above you.
• Open the valve on the tube.
• Plug in the pump.

Evacuating the tube takes a few minutes. Some people choose to evacuate the tube in advance and show the evacuated state before the air resistance state to eliminate pump-down time.

Important Notes

• This demonstration requires advanced practice. If the tube is not turned smoothly, the "feather" can grip the inside of the tube and fall more slowly in a vacuum.
• For best results, hold the tube horizontally and shake it gently so that the "coin" and the "feather" are side-by-side on the wall of the tube. Quickly and smoothly rotate the tube to the vertical.
• A few problems can occur. Very occasionally, the objects can become stuck at one end of the tube. Also very occasionally, the two objects interfere with each other while falling. The recommended gentle shake in the horizontal position makes these occurrences very unlikely.
• Pick up the tube and hold it vertically so the "coin" and "feather" fall to the bottom.
• Repeat as necessary. Point out that the feather falls at a slower rate than the "coin."
• Attach the pump to the tube. Make sure the valve on the tube is open.
• Turn on the pump and let it pump until the pressure in the tube has reached a stable low value.
• Close the valve on the tube. Turn off and disconnect the pump.
• Pick up the tube and hold it vertically so the "coin" and the "feather" fall to the bottom.
• Repeat as necessary. Point out that the "coin" and "feather" fall at the same rate.
• [Optional] Let some air back into the tube and repeat the experiment to show the effect of reduced pressure. This will demonstrate that a good vacuum is essential for the success of this experiment.

Additional Resources

• PIRA 1C20.10
• Video Encyclopedia 01-14
• Mechanical Universe Episode 1 Chapter 15, Frames 13994 to 14476: Astronaut David Scott on the moon dropping a feather and a hammer, narration by Scott. The picture quality of this clip is very poor.
• Mechanical Universe Episode 8 Chapter 18, Frames 17457 to 17769: very quick presentation of laboratory version, not worth showing separately. Part of Chapter 27, Frames 25196 to 25487: same footage as Episode 1, shortened, comments by the narrator.
• The experiment was performed on the Moon. A quote from the cover of Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions by William David Compton : "Apollo 15 commander David R. Scott confirms Galileo's hypothesis that in the absense of air resistance all objects fall with the same velocity. A geologic hammer in Scott's right hand and a falcon feather in his left hand reached the surface of the moon at the same time (see chapter 13). The demonstration was performed before the television camera on the lunar roving vehicle, and no photographs were made." 
• Sutton M-79; DaR M-088; Joseph ea p368.
• Conant, "Harvard Case Histories in Experimental Science", Case 1: Robert Boyle's Experiments in Pneumatics. Experiment 40 About the falling, in the exhausted receiver, of a light body, fitted to have its motion visibly varied by a small resistance of the air. [describes an experiment in which Boyle contrived to drop a small cross made of four feathers in an evacuated container. This is presumably a fore runner of the coin and feather demonstration.
• Millikan and Gale "A First Course in Physics" Revised Edition, 1913, page 89: That the air resistance is indeed the chief factor in the slowness of fall of feathers and other light objects can be shown by pumping the air out of a tube containing a feather and a coin (Fig.92). The more complete the exhaustion the more nearly do the feather and coin fall side by side when the tube is inverted. The air pump, however, was not invented until sixty years after Galileo's time.
• Don't attempt this at home!
• SFU is not affiliated with any external sites linked here and is not responsible for their content.

Last revised

• The tube was constructed in the SFU machine shop. It was made from a 6' x 4" plexiglass tube capped off and sealed on both ends by aluminum plugs with double O-rings. One end was fitted with a vacuum connection to allow evacuation of the tube. A large washer is used in place of a coin and a packing peanut is used in place of a feather. An Edwards single stage mechanical pump is used to evacuate the tube.
• The minimum pressure reading on the gauge is −28 mm Hg or −95 kPa.
• NASA posted video of a similar experiment done on the moon during the Apollo 15 mission.

Related demos

If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.

ucsc physics demo

Coin and Feather

This apparatus simultaneously drops  two objects of different masses such as a coin and a feather from the top an evacuated cylinder to demonstrate the independence of mass from gravitational acceleration. It is recommended for the instructor to drop the coin and feather in front of the audience before the experiment is performed to demonstrate how air resistance really affects the rate of acceleration at which an object falls to the earth.

• Vacuum pump connected to large plexiglass cylinder with two electromagnetic holders which will be found attached to the bottom of the cylinder cap.
•  Lift the cap off of the tube and place the feather and the coin into their respective holders as shown in Figure 2.
• Place coin and feather assembly on top of the tube.
• To evacuate the cylinder, close the vacuum valve(See Figure 3: C) and turn on the pump(See Figure 3: A)
• When the gauge shows about 27-28mm/Hg stop the pump. (See Figure 3: B)
• Use the push-button on the base of the tube to release the the coin and feather.
• Open the vacuum valve to let air in.
• Repeat if desired.

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