Air and its inertia

Mateusz JuraI LO Tadeusz Kosciuszko in Wielun

POLAND

PLAN OF THE PRESENTATION1.1. Determination of atmospheric pressure.Determination of atmospheric pressure.2.2. What is the force exerted on the newspaper?What is the force exerted on the newspaper?3.3. Air has mass and so it also has inertia. Air has mass and so it also has inertia.

a.a. Experiment 1 – a newspaper, a flat piece of Experiment 1 – a newspaper, a flat piece of woodwood

b.b. Experiment 2 – a plastic tile, a newspaper, Experiment 2 – a plastic tile, a newspaper, a piece of stringa piece of string

4. Findings4. Findings

Determination of Determination of atmospheric pressureatmospheric pressure

Procedure: We push in the plunger of the syringe as

far as it will go and with the aid of the piece of thread and the spring scale we read what force is required to move the plunger.

We push in the plunger of the syringe as far as it will go and close the opening.

Keeping the syringe horizontal we slowly pull back the spring scale and read the reading.

Materials required:• A spring scale, a syringe,

callipers, a piece of thread

F1- the force required to overcome frictionF2- the force required to ovecome friction and the weight of any air2r= (2.00 ± 0.01)cm – the diameter of the plunger measured with the

use of callipers S- the surface area of the plungerF= F1-F2=p*S – the force of the weight of the air acting on the plunger

of the syringe when its opening is closed S= r² p = F/S

Calculations

Series of measurements

F1(N)

F2(N)

F(N)

S(m^2)

p(hPa)

1 4.9 36.1 31.2 0.000314 993.63

2 4.6 36.1 31.5 0.000314 1003.18

3 5.4 36.5 31.1 0.000314 990.44

4 5 36.3 31.3 0.000314 996.81

5 5.2 36.8 31.6 0.000314 1006.36

6 4.8 36 31.2 0.000314 993.63

7 5.6 37.4 31.8 0.000314 1012.73

8 5.5 36.5 31 0.000314 987.26

9 4.4 36.2 31.8 0.000314 1012.73

10 5 35.8 30.8 0.000314 980.89

Calculation: the standard deviation of the mean

(average) value for atmospheric pressure is -

pav ≈ 998hPa p = (998 ± 4)hPa

2

2

10

2

1 41

...hPa

nn

ppppavav

What is the force exerted on the newspaper?

Pressure is the ratio of a force to the surface area over which it is distributed:

p= F/S F= S*p Dimensions of the newspaper-35cm x 58cm p= 101325Pa (normal pressure) F= 20570N Thus the air exerts a force of 20570N on the

newspaper. There is a correspondence between the

mass of the air and the force pressing down:

F= m*g g= 10m/s^2 m= F/g m= 2057kg Thus the mass of the air pressing down on

the newspaper is 2057kg. The greater the mass, the greater the inertia of the body.

Air has mass and so it also has inertia

Air has a certain mass, so it has inertia. We will prove this with two experiments.

Experiment 1

Materials required:• a small, thin, flat piece of wood, • a hammer, • a newspaper

The course of the experiment: We put the small, thin, flat

piece of wood on a table in such a way that part of it is hanging over the table’s edge. Then we cover the part on the table with the newspaper. When we press down slowly on the flat piece of wood, we notice strong resistance. Next we hit the protruding end of the flat piece of wood powerfully with the hammer (N.B. the blow must be hard and precise). The flat piece of wood breaks, but the newspaper remains undamaged and unmoved.

Explanation:• the force of the hammer’s impact is probably about 2500N (as

estimated by Warsaw Technical University), if:o the mass of the hammer is 0.5kgo the end velocity of the blow is 10m/s http://www.if.pw.edu.pl/~wosinska/am2/w2/segment2/main.htm

• the force exerted on the newspaper is 20570N • when we press down on the flat piece of wood, we notice

resistance resulting more from atmospheric pressure than from inertia

• By contrast when we hit powerfully, there is a large force of inertia from the air with a mass of about 2060kg (the result of the previous calculation)

• the force of the hammer’s impact is 8 times lessRESULT – THE FLAT PIECE OF WOOD BREAKS!

Experiment 2

Materials required: A plastic tile with a handle fitting, a

piece of string, a newspaper

Course of the experiment: We put the plastic tile on a flat

piece of floor and tie the string to the handle fitting. We spread a layer of newspaper widely over the tile. The plastic tile can be raised with a calm, steady lift despite the experience of some resistance. By contrast, if an abrupt, powerful lift is attempted, the string snaps.

Findings

• the atmospheric pressure determined with the aid of a spring scale on the day of the experiment was (998±4) hPa

• The force exerted on the newspaper was 20570N

• air has inertia

BibliographyBacke Hans, Z fizyką za pan brat, Warszawa, Państwowe

Wydawnictwo „Iskry”,1974http://www.if.pw.edu.pl/~wosinska/am2/w2/segment2/main.htmMaria Fiałkowska, Barbara Saganowska, Jadwiga Salach,

„Z fizyką w przyszłość”, Kraków, ZamKor, 2013