cahier physique - physics workbook

PART I: the weight, gravity

watch this movie about gravitation. go to gravity segment

1) Read: Weight is the force due to the attraction of gravity.
The force of gravity, called weight, is pulling you downward, toward the center of the Earth.
weight (N) = mass (kg) x g , g depends on the planet and it also
depends on the distance from the object to the center of the planet. For EArth,
at sea level, it is about 9.8 m/s². Sometimes, we round to 10m/s/s. The unit tells you what physical quantity is g
m/s² = m/s/s is the unit for ____________. (see first unit we studied) .
It means that an object falling on EArth, if we neglect air resistance, will speed up toward the ground at a rate of 9.8 m/s².
Every second, the object will go 9.8m/s faster. (or 32feet / second faster )

2) Complete the sentences
The force due to gravity on an object is the __________, unit is the _________ symbol is ___
The quantity of matter in an object is the _________ , unit is the _________symbol is ______
The amount of space an object occupies is the ____________, unit is ________ symbol is _____

3) Complete the table (take g=10 m/s/s (instead of 9.8)
1kg = 2.2 pounds

Object	Mass	Weight in newtons	weight in pounds
melon	1 kg	______ N	____________ lbs
apple	___________	1 N	_______ lbs
physics book	find out, use a balance	_________ N	__________lbs
uncle harry	60kg	__________ N	__________ lbs
automobile	1,150 kg	__________ N	__________ lbs

4) Find your weight on other planets. (I suppose your mass to be 60kg (about 120 pounds) )
Remember: weight = mass x (acceleration due to gravity) For example: On Earth, acceleration due to gravity g = 10m/s/s

Planets	m = Mass (Kg)	g_planet= acceleration due to gravity (m/s²) divide weight by mass	Weight (N)	ratio g_planet/ g_earth = 10/g_{planet =}
Earth	60kg	10 m/s/s	600 N	1
Moon	60kg	_________	99.6 N	_________
Mercury	_________	_________	226.8 N	_________
Venus	_________	_________	544.2 N	_________
Mars	_________	_________	226.2 N	_________
Jupiter	_________	_________	1416.0 N	_________
Saturn	_________	_________	549.6 N	_________
Uranus	_________	_________	533.4 N	_________
Neptune	_________	_________	672.0 N	_________
Pluto	_________	_________	35.4 N	_________

5) John's mass is 65kg on earth. What his metric (a) mass and (b) weight on Mars, where g= 3.72m/s/s.
m=____kg and W = _________N
On Mars, a falling object will speed up by ______ m/s every second. Acceleration is ______ @ ________

6) Find out your mass in Kg. (divide pounds by 2.2) _________kg.
What are your metric mass and weight on Jupiter where g = 25.9 m/s/s ?
m = _________kg and W = ________N.

7) Find the mass of a 11,500N automobile. m = _____kg.

8) A student use a balance scaled in newton to find the weight of 1kg and then 2 kg.

He finds that 1 kg = _______ N, 2kg = ________ N, 3 kg = __________ N ... 10kg = _________N
THe mass is proportional to the pull due to gravity (weight) and the constant of proportionality is_______

9) See the picture below.

Find the gravity on the moon: g = ______ . So if you travels to the Moon, you will lose weight.
your weight will be divided by ______ but your mass is the ______________.

10) source of picture : great website for cartoons/MAth
click on picture.
This picture shows that to escape the EArth attraction, you need the same energy
than to escape a well about 6km deep. At that scale, the other gravity wells of other planets
are shown. See below to see computations.

The The binding energy (potential energy) of a mass m on a planet M is given by : G mM / R G is a constant. (gravitational constant)
m is the mass of the prisonner and R is the radius of the planet.
To escape the planet you need to produce an equivalent energy. (G mM / R)
If you were on eArth, trapped in a well h m deep, you need an anergy m g h to escape.
G mM / R = mg h means h = G M / g R
So you can compute a well on EArth would be to match the escape energy of a planet/

PART II going further - introduction to Astrophysics :
general relativity / black hole/ dark energy/dark matter

0) watch this movie from discovery school: THe invisible force
answer these questions after you watched the movie.

1) watch this movie. introduction to astronomy/cosmology/universe

2) gravity or no gravity ? Enistein's theory of general relativity
what Einstein's theory of general relativity has to say? Maybe you don't need the force of gravity (+ principle of inertia) introduced by the Newtonian Mechanics to explain the motion of the EArth around the Sun.
Maybe the curvature of time and space can explain that motion. We now know that the curvature
of space-time is a fact. watch this movie about einstein's theory (segment about einstein).
The Sun bends the fabric of space. The EArth travels along a curvature of space-time.

watch that movie (from NOVA)

3) dark matter
Have you heard of dark matter? nothing to do with Star War. When studying, for example,
our own galaxy (the milky way) , (and other parts of the universe )the scientists found out something really strange.
We don't know what (about) 95% of the universe is made of. Scientists have computed the orbiting speeds of stars in our own galaxy,
around its center. These speeds depends on the total mass inside their orbits. The mass that accounts for the speeds scientists observe, is much larger
than all the mass they can " see " .
It is like they find that the force of gravity that holds all the star together is much stronger than predicted
by the mass we " can see ". There must be some mass " we can't see" that accounts for the observed binding.

This unknown mass is called the dark matter. Some say, it must be really small particles called wimps and some say it must be very large object
like neutron stars called machos. (like black holes). So far , scientists could not detect any of these wimps. (they are
supposed non reactive so hard to detect). and we can't find enough big objects (machos) to explain the strong gravity
that binds every thing together in the universe.
Check on the Internet what wimps and machos stand for.
5% of the universe we can explain . 1% of that we can see (visible matter) like stars. 4% of that we can't see like black holes and gas.
25% is dark matter
70% is dark energy

new findings about general relativity and dark matter. The theory might predict dark energy.

3) here another evidence for dark matter: the Bullet Cluster

4) watch this movie about dark matter

5) dark matter can be mapped because of its effect it has on the matter around it.
Scientists used a method called gravitational lensing to map dark matter in 3D. These movies explain how.

without sound

6) Some scientists were able to map in 3D this dark matter.(using the method gravitational lensing)

8) What about dark energy ? THE UNIVERSE IS ACCELERATING ? doppler effect animation
Hubble showed that the universe is expanding. You can see this expansion by studying the redshifts versus distance
of far away objects. Like supernovae. It was thought that although the universe is expanding (the galaxies are moving
away from each other) it is expanding at a constant rate. And because the force of gravity was thought to be the only
force in town, the universe should even slows down. It was thought that the universe would be crunched together as
an ultimate fate. The force of gravity should pull every thing together, thus
slowing down the motion. Kind of like a friction force slows down an object in motion.
A team from from Berkeley College was trying to find this deceleration factor. (see Alex Fillipenko)
They used supernovae type Ia. What they found was really surprising. Not only the universe is not slowing down but
it is accelerating. the galaxies are moving from each other at an accelerated rate. So there must be a force
that counter balances the force of gravity. This force was introduced by Einstein in his theory of relativity then
rejected by him. (he called this the greater blunder of his life). Einstein was right after all.
He called this force the cosmological constant. We don't know what is this force. so we call it the dark energy.
(note, more advanced: before Hubble found that the universe was expanding, scientists thought the universe was static. that's why Einstein was trying to introduced a force to counter balance gravity. remember ? net force = 0 means no acceleration.
but then Einstein gave up his idea because Hubble said the universe is not static , it is expanding as an after math of the big bang.

Scientists use supernovae type Ia like standard candle. When some kind of white dwarfs explode they
become a supernova that produces a given brightness. By looking at the brightness they can find the distance of the supernova.
by looking at the redshift they know the speed of the supernova. they compare the relationship redshift vs distance of several supernovae and
they found that they move faster than predicted by Hubble law. It is a very tedious and long task. typeIA supernovae are rare.
They need to take 1000s of pictures of patches of the sky at 1 month interval. They compare the patches to find exploding stars.
this can only be done with sophisticated computer programs and telescopes.

For a white dwarf to explode, it needs a companion. The white dwarf suck the
material (gas) from its companion until the pressure is so high that a thermonuclear reaction is triggered and the star explodes.
scientists record the brightness of the explosion. They plot the brightness versus the wavelength and this curve is the same
for all typeIA. except that farther away the supernova is , more fainted it is. That's why they can find the distance.

A candle will appear fainted too as it moves away from you. You can estimate the distance of the candle just by looking at the brightness of it.
This is why it is called a standard candle )
watch that movie THE UNIVERSE IS ACCELERATING

9) black holes Watch the movie: black holes - the ultimate abyss from discovery school
A black hole is a singularity in the fabric of space-time. Some object are so dense that they curve the
space-time with an infinite slope. It is like a hole in the fabric. A black hole can form after
a massive star dies. the force of gravity tugs all the matter together and the
left over from the exploding star becomes incredibly dense. nothing can escape a black hole. not even light.
that's why we can't see them. To detect black holes, astromomers look for stars circling
massive compact object. The material (gas) from these spinning stars is sucked by (canabilzed) the black home.
The gas from these stars starts to spin very fast around the hole. forming a spinning disk of hot gas.
This gas gets so hot that it emits Xrays. (invisible light with high frequency, that is high energy)
It is by detecting these X-rays that we can tell where is the black hole. By computing the speed
of these spinning stars, scientists can find the mass of the black hole.

The place deep within a black hole, where everything is destroyed, is called a singularity.

If you get close to a black hole, you would be spagetized. The tug will be really strong on your head and less on your head.
See the cartoon the Simpsons 3D on you-tube.

Other weird things will happen if you were to travel in a black hole. Not only the space is stretched but also the time.
For a person on EArth looking at you, getting close to the black hole , your time gets dilated. Inside the black hole,
from the Earth's point of view, the time is frozen.

Some kinds of black holes (called supermassive black hole) emit jets of energetized particles that can also be detected by telescopes as radio waves.
The radio waves come from distant galaxies called radio galaxies.

Our SUn will not become a black hole. It is not massive enough. It will explode to a giant red
and will end up as a white dwarf.

If you want our sun to be a black hole you will need to squeeze it to the size of a ping pong.
However, the mass stays the same. Only the volume decreases and the density increases.
We would be ok, because the distance from the sun and the mass of the sun would stay the same.
The force of gravity depends only on masses tugging on each other and the distance between them.

The first " observed " black hole was Cygnus X-1 i in the constellation Cygnus. It was discovered in 1960s thanks to the detection of X-rays
it was emitting. When a black holes is sucking the gas from a companion star, the gas spins around and emit x-rays or gamma rays before
being lost in the black hole. There is a also a black hole at the center of our galaxy, a few million the mass of our Sun but 10% its size.
See Google Earth to find the galactic center. (called SAgittarius A)

take home quiz 4 part 2 from @discovery school, black holes, the ultimate abyss.

what happen in a black hole: watch this movie

EXCELLENT POWER POINT PRESENTATION ON LINE: different ways a black hole can kill you !

10) worm hole: time machine ?
gravity can slow down the time. One consequence of Einstein's general relativity is the
existence of worm holes that could allow us to move from one part of the universe to the other.
If we could connect black holes we could make these so called worm holes and we would be able
to travel bfater than the speed of light, from one place of the Universe to another.
learn more:

11) supernovae explosion

12) big bang