EDUF 1017 Physical Phenomena - L5
Gravity again
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•
•
Weight - Gravitational Force
m
The gravitational force is given by Fgravity = G M earth
2
•
r
The force of gravity between any two objects is
• along the line joining them
• proportional to their masses
• inversely proportional to the square of the distance between
them
The distance required is the distance to the centre of the Earth i.e. 6378 km at the equator
August 10, 2010
EDUF 1017 Physical Phenomena L5
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Fgravity = G
A more massive object has a bigger gravitational
force on it than a smaller mass - “it weighs more”
more”
•
But…
But… it needs a bigger force to make it accelerate
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1
We might say a person’
person’s “weight”
weight” is 50 kg. We should say her
mass is 50 kg and her weight is 50 kg x 9.8 m/s2 = 490 N
August 10, 2010
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resistance)
t(s) v (m/s
(m/s))
0
0
1
9.8
2
19.6
3
29.4
…
…
aMoon = G
EDUF 1017 Physical Phenomena L5
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August 10, 2010
M Earth
M Earth
=G
2
rEarth
(1.08 rEarth )2
•
! 1 $ ! M Earth $
2
aMoon = #
2 & #G
2
& = 8.4 m / s
" (1.08 ) % " rEarth %
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August 10, 2010
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only ~14% less
EDUF 1017 Physical Phenomena L5
EDUF 1017 Physical Phenomena L5
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Illustrating gravity
On the ground r ~ 6378 km
In orbit (say 500 km) r ~ 6878 km (6378+ 500)
aMoon = G
9.8 m/s2
So, if her mass is 50 kg, her weight on the Moon is
50 kg x 1.6 m/s2 = 80 N
(~1/6 weight on Earth)
•
Is there Gravity in space?
•
M Moon
( 0.0123) M Earth
=G
2
rMoon
( 0.273 rEarth )2
! 0.0123 $ ! M Earth $
2
aMoon = #
2 & #G
2
& = 1.6 m / s
" ( 0.273) % " rEarth %
(see http://www.
http://www.aerospaceweb
aerospaceweb.org/question/dynamics/gravity/apollo15.
.org/question/dynamics/gravity/apollo15.avi
avi))
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Moon
• mass = 0.0123 x mass of Earth (~ 1/81)
• radius = 0.273 x radius of Earth
•
F = ma
So the acceleration that results when you drop
them is the same - i.e. for any object, a = g = 9.8
m/s2 (on Earth)
Different masses all fall at the same rate despite
having different weights. (if we ignore friction, air
EDUF 1017 Physical Phenomena L5
Weight - on the Moon
Fgravity = mg
•
M Earth m
! M
$
= m # G Earth
& = mg
"
r2
r2 %
! M
$
2
g = # G Earth
& = 9.8 m / s
"
r2 %
What does g mean?
•
Weight is the downward gravitational force experienced by
an object
The weight force is related to the acceleration due to gravity - a
constant
5
See on-line version for print that is too small
Dropping keys (or
throwing them
upwards) on a
moving train
What does the
woman on the train
see?
What do we see
(on the platform)?
Why?
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EDUF 1017 Physical Phenomena L5
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EDUF 1017 Physical Phenomena - L5
Is there Gravity in space?
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If the initial
‘sideways’
sideways’ speed
is high enough,
an object will
orbit - Gravity
always pulls
towards the
centre of the
Earth, but that
direction keeps
changing.
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the space shuttle orbits and the people in it orbit independently
if they’
they’re travelling at the right speed
the shuttle doesn’
doesn’t exert any force on the people to keep them
moving - they have their initial speed and gravity provides the
force to change their direction.
•
If you were a school kid what would you ask about gravity?
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EDUF 1017 Physical Phenomena L5
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August 10, 2010
EDUF 1017 Physical Phenomena L5
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Phases of the Moon
Tides
8
Due to difference of the Moons’
Moons’
gravitational attraction on the
water and the Earth
Neap Tides
Spring/King Tides
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Summary
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Mass is the amount of matter in an object - doesn’
doesn’t change,
not matter where you are.
Weight is the downward gravitational force experienced by
an object - is different on different planets
There is gravity in space!
All objects accelerate because of gravity at the same rate
(if we ignore friction, air resistance)
NEXT: …..Energy
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EDUF 1017 Physical Phenomena L5
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See on-line version for print that is too small
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