15-May-15 Physics 1 (Garcia) SJSU Chapter 9 Gravity.

Apr 18, 2023 Physics 1 (Garcia) SJSU

Chapter 9 Gravity


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


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


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)


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.


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.


“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


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)


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


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 )


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


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.


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


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


Freefall & WeightlessnessFreefall is a state of weightlessness, even though gravity is present.


Movie: Roller Coaster


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


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


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.


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.


Pow

erC

limb

WeightlessFreefall

Pull out

of Dive

Flight of the “Vomit Comet”

At the top of the arc, the plane’s trajectory is projectile motion.

Boeing 707 (modified)

15-May-15 Physics 1 (Garcia) SJSU Chapter 9 Gravity.

Documents

Transcript of 15-May-15 Physics 1 (Garcia) SJSU Chapter 9 Gravity.