15-May-15 Physics 1 (Garcia) SJSU Chapter 9 Gravity.
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Transcript of 15-May-15 Physics 1 (Garcia) SJSU Chapter 9 Gravity.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Newton and the Moon
Newton realized that Earth’s gravity was the centripetal force that kept the moon in orbit.
Also discovered that gravity was weaker at that great distance.
Gravityforce
Apr 18, 2023 Physics 1 (Garcia) SJSU
Gravity & Distance
We don’t notice that gravity gets weaker as we move away from Earth because we rarely go very far.
Moon is 30 Earth
diameters away
Apr 18, 2023 Physics 1 (Garcia) SJSU
Universal Law of Gravity
Force of gravity has magnitude given by
(Gravity Force) = (G) x
ObjectA
ObjectB
( Mass of Object A ) x ( Mass of Object B)
( Distance ) x ( Distance )
DISTANCE
Force Force
Equal and opposite forces(Newton’s Third law)
Apr 18, 2023 Physics 1 (Garcia) SJSU
Universal Gravity Constant, G
In the formula for gravity force, we have
G = 0.0000000000667 N m2 / kg2
= 6.67 x 10–11 N m2 / kg2
The formula and the constant are called “universal” because, up to now, this theory predicts gravity anywhere in the universe.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Cavendish Experiment
For non-astronomical objects gravity force is very small.
Need sensitive balance to detect.
First measured by Cavendish in his home laboratory in 1797 at age 67.
Apr 18, 2023 Physics 1 (Garcia) SJSU
“Intelligent Falling”"Things fall not
because they are acted upon by
some gravitational force, but because
a higher intelligence, 'God' if you will, is pushing
them down,"
The Onion, August 17, 2005
Universal Law of Gravity
Apr 18, 2023 Physics 1 (Garcia) SJSU
Sample Problem
Here is an example of using the formula
(Gravity Force) = (G) x( Mass of Object A ) x ( Mass of Object B)
( Distance ) x ( Distance )
DISTANCE = Earth’s Radius
ForceObject B (Earth)
Object A (1 kg mass)
Apr 18, 2023 Physics 1 (Garcia) SJSU
Sample Problem
Find gravity force for a 1 kg mass on surface of Earth.
(Force) = (6.67 x 10–11) x
Value comes out to 9.8 Newtons (check this with your calculator; it’s good practice).
( 1 ) x ( 6 x 1024 )
( 6.38 x 106 )2
Universal Gravity Constant, GEarth’s Radius
Earth’s Mass
Apr 18, 2023 Physics 1 (Garcia) SJSU
Sample Problem (cont.)
Find gravity acceleration on a 1 kg mass.
Using Newton’s Second Law,
(Acceleration) = =
Answer is 9.8 m/s2, which we’ve been rounding off as 10 m/s2.
(Force)(Mass)
( 9.8 N )(1 kg )
Apr 18, 2023 Physics 1 (Garcia) SJSU
Inverse Square Law
Gravity force weakens with distance as the inverse of the square of the distance.
Geometric property of area and distance. Outer circle is twice Earth’s radius
so it has 4 times the area
Earth Gravity
1/4 Earth Gravity
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Perspective
These four figures are equally spaced indistance and, in perspective,you are standing that distance from the first.
At twice the distance,the face is ¼ the size.
At four times the distance face is 16th the size.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Perspective Cards
Hold large card at arm’s length.
Close one eye then hold small card at a distance such that it is same size as large card.
That distance will be half way between your eye and large card.
1
1 2
3 4
Arm’s length
Half
Apr 18, 2023 Physics 1 (Garcia) SJSU
WeightlessnessIn deep space, far away
from all stars, planets, etc. there is almost no gravity force.
In orbit near Earth, gravity is still strong (only 10% less than on surface).
Why are Shuttle and Space Station astronauts “weightless”?
Earth is nearby
Apr 18, 2023 Physics 1 (Garcia) SJSU
Freefall & WeightlessnessFreefall is a state of weightlessness, even though gravity is present.
Apr 18, 2023 Physics 1 (Garcia) SJSU
Check YourselfSit in a seat on Ferris
wheel.
Where do you feel heaviest?
Where do you feel lightest?
E
A
B
C
D
Velocity
CentripetalForce
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Ferris Wheel in Hand
Place a heavy object in your hand, hold your arm out and rotate your arm, palm upward, in a small circle.
Object will feel lightest at the top of the circle and heaviest at bottom.
SupportForce
SupportForce
Apr 18, 2023 Physics 1 (Garcia) SJSU
Demo: Falling Slinky
Hold a Slinky and its weight stretches it out.
Now drop it.
In freefall it’s in a state of weightlessness so Slinky immediately contracts.
Apr 18, 2023 Physics 1 (Garcia) SJSU
NASA’s “Vomit Comet”
NASA has a special airplane for training astronauts in free-fall weightless conditions.
The “Vomit Comet” nickname tells you it’s quite a wild roller-coaster ride. The plane flies between 20,000 and
30,000 feet, same as commercial flights.