The Paper And The Weight

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The PaperAnd The Weight

Air resistance

T.H.

PDST Resources for Leaving Certificate Physics 2

Which will fall faster, the 1 kg weight or the sheet of paper?


Why is the 1 kg weight falling faster?


Aristotle taught that the speed of a falling body was proportional to its weight.


This view lasted for almost 2000 years until Galileo contradicted Aristotle.


Who was right?

If Galileo was right, why does the 1 kg weight fall faster than the sheet of paper?


Air Resistance

A falling body is acted on by a number of forces.

Its weight, due to the force of gravity, Fg, acts downwards.

Air resistance, or drag, Fd, acts upwards.


Terminal Velocity

As an object falls, its velocity increases.

This causes its air resistance, Fd, to increase.

When the air resistance, Fd, equals the weight, Fg, the object moves at a constant velocity, called its terminal velocity.


Skydiving

With arms out, a skydiver reaches a terminal velocity of about 55 m s–1.

With arms and legs folded up, skydivers have reached terminal velocities of about 90 m s–1.

Skydiving head down at high altitude, Joseph Kittinger has reached a terminal velocity of 274 m s–1.


Question

Convert Joseph Kittinger’s terminal velocity of 274 m s–1 to km hr–1.


Answer

1 1

1

1

1

274 m s 274 1000 km s

274 1000 60 km min

274 1000 60 60 km hr

986 4 km hr

Note: You might find it useful to learn to convert m s–1 to km hr–1 by multiplying by 3.6


Back to the 1 kg weight and the sheet of paper

Can we reduce the air resistance acting on the sheet of paper so that it falls as fast as the 1 kg weight?

How?


Roll it into a ball to reduce its surface area!


Now That Air Resistance Is Less Of A Factor, They Fall At the Same Rate.


Another Demonstration

Here is a coin and a piece of paper.

Which will fall faster?

Why?


Why Is The Paper Falling More Slowly Than The Coin?


Again, It’s Air Resistance

Place the paper on the coin.

Drop the coin.

The paper stays on the coin, i.e. they fall at the same rate when air resistance is not such a factor.


References

Air Resistance Terminal Velocity Joseph Kittinger Galileo Aristotle

The Paper And The Weight

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Transcript of The Paper And The Weight