The Paper And The Weight Air resistance T.H. The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan

Which will fall faster, the 1 kg weight or the sheet of paper? PDST Resources for Leaving Certificate Physics 2

Why is the 1 kg weight falling faster? PDST Resources for Leaving Certificate Physics 3

Aristotle taught that the speed of a falling body was proportional to its weight. PDST Resources for Leaving Certificate Physics 4

This view lasted for almost 2000 years until Galileo contradicted Aristotle. PDST Resources for Leaving Certificate Physics 5

Who was right? If Galileo was right, why does the 1 kg weight fall faster than the sheet of paper? PDST Resources for Leaving Certificate Physics 6

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. PDST Resources for Leaving Certificate Physics 7

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. PDST Resources for Leaving Certificate Physics 8

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. PDST Resources for Leaving Certificate Physics 9

Convert Joseph Kittinger’s terminal velocity of 274 m s–1 to km hr–1. Question Convert Joseph Kittinger’s terminal velocity of 274 m s–1 to km hr–1. PDST Resources for Leaving Certificate Physics 10

Answer Note: You might find it useful to learn to convert m s–1 to km hr–1 by multiplying by 3.6 PDST Resources for Leaving Certificate Physics 11

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? PDST Resources for Leaving Certificate Physics 12

Roll it into a ball to reduce its surface area! PDST Resources for Leaving Certificate Physics 13

Now That Air Resistance Is Less Of A Factor, They Fall At the Same Rate. PDST Resources for Leaving Certificate Physics 14

Another Demonstration Here is a coin and a piece of paper. Which will fall faster? Why? PDST Resources for Leaving Certificate Physics 15

Why Is The Paper Falling More Slowly Than The Coin? PDST Resources for Leaving Certificate Physics 16

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. PDST Resources for Leaving Certificate Physics 17

References Air Resistance Terminal Velocity Joseph Kittinger Galileo Aristotle PDST Resources for Leaving Certificate Physics 18