Phys 1210 Conceptual Physics
Child's Question-of-the-Day Part 2
Weeks 6-9 Gravity, Projectile Motion, Properties of Matter: Density & Pressure, Temperature & Heat

"What would happen to the moon if the Earth suddenly disappear?"
If the Earth were to suddenly disappeared, there would be no gravitational force pulling the moon into the circular path that it had as it orbited the Earth. 
But don't forget that as the moon orbits the Earth, the Earth is orbiting the Sun! They are actually orbiting the Sun together.
The moon has the same average speed that the Earth has as it orbits the Sun. But on their journey together around the Sun, the moon's path wobbles a bit in order to orbit the Earth at the same time.  So if the Earth suddenly disappear - the moon would  no longer orbit the place where the Earth was, but it would continue to orbit the Sun

"Does gravity push or pull?"
The Earth's gravity pulls down on you. The Earth's mass is attracting your mass, and vice versa.

"Is gravity only 'on' when something is falling?"
Gravity is always pulling down on us and the things about us, even when we are sitting still and not falling.

"Do the people on the bottom of the world see the sky up above?"  
Yes, when the people on the bottom of the world look up they will see the sky. ("Looking up" on the bottom of the world means looking in the opposite direction away form the ground.) The sky goes all the way around the Earth. The sky is full of air, and gravity holds this air down to the Earth's surface, all the way around the Earth.

"What causes gravity?"

Gravity is our everyday word for what causes things to fall to the ground. But what causes gravity?
Does the rotation of the Earth cause gravity? NO!, if you are on rotating object (like a merry-go-round, or a turntable, or the Earth)  and you let go, you will 'fly off' in the direction that you were going when you let go (in a straight line, tangent to the curve of your path).
Does the  Earth's atmosphere cause gravity? NO!, The Earth's atmosphere does not cause Earth's gravity, the Earth's atmosphere does not even help it out, the atmosphere does not exert a net downward force on us. Earth's gravity does cause the Earth to have an atmosphere, it pulls down on the air molecules; Earth 'holds on to' its atmosphere using its gravity.
Does the magnetic core of the Earth cause gravity? NO! Do you need to wear steel boots in order to stay on the ground?!
What causes gravity?  Gravity is caused by mass attracting mass: The mass of the Earth and the mass of your body attract each other. The Earth's mass is so great that gravity is strong enough to keep you on the Earth, to keep you from 'flying off into space' in spite of the Earth's rotation, and to keep our atmosphere around it.

"If the Earth is round, why don't people on the other side fall off?"
Note: Children have a flat model of the Earth, and 'down' is from the top to the bottom. When they look at a globe model of the Earth, they might imagine gravity pulling from the top the globe to the bottom of the globe. When we say "gravity pulls down", we mean it  pulls down towards the surface of the Earth. The total mass of the Earth attracts the total mass of your body. This force of attraction pulls you towards the Earth; we call this 'down'). The people on the "bottom" of our round planet also experience gravity, they are also pulled toward the Earth (and they also call this 'down'!).
(Note: Gravity is not provided by the Center of the Earth. Gravity is provided by every little bit of the Earth: its dirt, its magna, its rocks, its water, etc. Every little bit has mass. The net force of all of these attractions is toward the center of the Earth.)

"If the mass attracts mass, then was doesn't the moon just fall into the Earth, or the Earth fall into the sun? Why does the moon keep going around in orbit?"
NOTE: The moon is not 'standing still'. If the moon was standing still, the Earth would pull the moon straight down.
The moon is moving, it is moving about 2300 mph as it orbits the Earth. The Earth's gravitational pull on the moon changes the direction of moon's motion as it orbits the Earth. This change is enough to redirect the moon at each point along its orbit about the Earth. Think of the moving moon like a baseball that you throw sideways. The baseball doesn't fall straight to the Earth - rather it falls in a curved path down to the Earth. Now imagine throwing a baseball so hard that as it falls its curved path matches the curve of the Earth. Now the ball won't fall all the way to the Earth, rather it will fall around the Earth. (You can't really do this with a baseball, because the ball will have to move through air and the air will bit by bit slow it down.) The moon is like a baseball thrown sideways while at the same time, Earth is pulling it down. The moon is falling, it is continually falling around and around the Earth.


"How does an astronaut get prepared for the long period of antigravity that he is going to be put on? -- ASB, Chiapas, Mexico
When an astronaut is orbiting the earth, he isn't really weightless. The earth's gravity is still pulling him toward the center of the earth and his weight is almost as large as it would be on the earth's surface. What makes him feel weightless is the fact that he is in
free fall all the time! He is falling just as he would be if he had jumped off a diving board or a cliff. If it weren't for the astronaut's enormous sideways velocity, he would plunge toward the earth faster and faster and soon crash into the earth's surface. But
his sideways velocity carries him past the horizon so fast that he keeps missing the earth as he falls. Instead of crashing into the earth, he orbits it.
During his orbit, the astronaut feels weightless because all of his "pieces" are falling together. Those pieces don't need to push on one another to keep their relative positions as they fall, so he feels none of the internal forces that he interprets as weight
when he stands on the ground. A falling astronaut can't feel his weight. 
To prepare for this weightless feeling, the astronaut needs to fall. Jumping off a diving board or riding a roller coaster will help, but the classic training technique is a ride on the "Vomit Comet"--an airplane that follows a parabolic arc through the air that allows everything inside it to fall freely. The airplane's arc is just that of a freely falling object and everything inside it floats around in free fall, too--including the astronaut trainee.
The plane starts the arc heading upward. It slows its rise until it reaches a peak height and then continues arcing downward faster and faster. The whole trip lasts at most 20 seconds, during which everyone inside the plane feels weightless."

"How can a metal ship float on water?"
Metal can not float on water, but a metal ship can. The ship is designed with a lot of "empty space" within it (rooms with air!). The ship's total mass is not very much when you compare it to the volume of the ship. (The amount of "ship matter" is not very great compared to the amount of space the ship occupies.) The ship's overall density is less than the density of the water. What does overall density mean? It mean the average density of the metal of the ship + the air pockets within the ship + everything else inside of the ship.

"Why do the toy penguins float like that?" (Toy Question)
The toy penguins float on top of the blue liquid (water?) because they are less dense than the blue liquid. The toy penguins are at the "bottom" of the clear liquid (oil?), because they are more dense (overall) than the clear liquid. They float upright because the head of the toy penguin is less dense than its feet (and the head is less dense than the clear liquid!)
 If you wanted to design the toy so that the penguins floated upside down, you would have to "weight" the heads of the penguins instead of their feet. To weight something means to add a piece of dense material (such as lead) to one end of the object. That end will now have a greater density than the other side.

"Why doesn't it hurt when I sit on a seat of nails?"
It doesn't hurt because the pressure on your "seat" is not very large. It would hurt a lot if you sat on one nail. Instead of all of your weight on one nail, when you sit of a seat of nails your weight  is spread out over all of those nails. Your weight/ the force that gravity is pulling down on you with is spread out over all of those nails. The pressure on your "seat side" is your weight divided up over all of the nails' surface areas. (Pressure = force/ area)  

"Did the bread gain weight or mass as it was rising in the oven?"
(NOTE: The bread does lose a bit of moisture as it bakes. However, the child sees the bread growing and might assume from this that the bread is gaining mass. The bread is not gaining mass/ matter, it is gaining volume. The bread spreads out when part of its ingredients releases gases and these gas bubbles push the dough up and out of the pan.)
As the bread bakes it does not gain any more stuff, it is not gaining any mass. What stuff or matter the bread does have, this matter is spreading out and the bread is gaining volume (the bread is taking up, or that is occupying, more space. The bread's mass is staying the same, while its volume is increasing, and as a result the bread is becoming less dense. The bread's density is decreasing.

"Is a molecule an atom?"
No, a molecule is a group of atoms bonded together. Each atom is just one part of the group.
[Teacher: "A whole is the sum of it parts"; TEKS Ref: All grade levels (a) (4)]

"Do atoms shrink in cold temperatures?"
Atoms do not shrink in cold temperatures; the atoms in an object get closer together as the object gets colder (its temperature decreases). 
[Teacher Note: Be careful not to anthropomorphize the atoms! The atoms do not get closer together to keep warm!]
Any object, for example a penny, is made up of many atoms. A single atom doesn't have a temperature However, the atom is moving and it does have motion energy (kinetic energy). We can't see the atoms moving, but we can measure the temperature of the penny. The temperature of the penny is related to the average kinetic energy of the atoms and in the penny. If heat is removed from the penny (i.e., we cool it down), the atoms will on average move slower:
The penny's SIZE/ VOLUME will change: its atoms will be moving slower and when they "hit" each other, they will not hit as hard (hit less force), they will not spread out as far from each other.
The penny's TEMPERATURE will change: its atoms' will be moving slower and it average kinetic energies decrease; as a result there will be a decrease in the temperature of the whole penny.

"Why does a balloon shrink when I place it in my freezer?"
Some how the forces that were keeping the inflated balloon big, are now decreasing. You know about these forces, you experienced these forces when a balloon popped near you. Or you might have seen how an inflated balloon (that wasn't tied shut) flew through the air when you released it. The force inside the balloon is a result of the motion of the air molecules in the balloon. The faster these air molecules are moving, the greater the force they exert on each other and the inside walls of the balloon.
When you first put the balloon into the freezer, the temperature of the balloon (and everything in it) is greater than the freezer. As the balloon cools off more and more (i.e., its temperature decreases) the air molecules will move slower and slower. They hit the inside of the balloon walls with less and less force, and the balloon will shrink more and more.

"What causes the wind to blow?" [Trees sneezing?!]
Wind is a bunch of air that is moving altogether. We call any bunch of air an air mass. Something had to push this air mass to get it moving. What can push this air mass? Another air mass! Here's one way this could get started: As an air mass (or pocket of air) heats up, it starts to expand (Why? because the particles in that "pocket" start moving faster and faster, hitting each other more and more, and all the molecules spread out more and more) As this air mass expands, it becomes less dense. Because of gravity, the denser air around it pushes in on the less dense pocket of air causing the less dense air to be pushed up. Like a helium balloon, the less dense air will rise and the more dense air will move into its place. The air masses are moving and we have wind.

"Why does smoke always go up?"
Smoke is the warmed air and combustion particles from a burning material (for example, a burning candle). As the air in the flame is heated, it becomes less dense, the denser air around it pushes in and up on it causing this warmer air and combustion particles (smoke!) to rise.
[Teacher Note: By the way, as the new air moves in and replaces the warm, smoky air that rose up, the oxygen gas that used in the combustion of the candle wax and wick is replaced. If this didn't happen a burning candle would not be able to continue to burn!]

"How does the thermos know whether to keep my drink hot or cold?"
The thermos doesn't have a brain, so it doesn't know anything! Your drink stays hot because heat/ thermal energy can not leave the thermos. (Or on the other hand, your drink stays cold because heat/ thermal energy can not get into your thermos.) Your thermos is made to prevent any heat energy from moving in or out. How does it do this? First, it has double walls to isolate the drink from the surroundings. These walls separate the inside liquid from the outside air. If there is less contact between the these, less energy can be transferred. If the space between the double walls is a vacuum, then there will be even less energy transfer. One more way that energy can move form one place to another is by radiation (for example, light energy). Sometimes thermoses have a reflective, mirror like coating. This will reflect any heat radiation (infrared radiation) back into the hot drink, or vice versa.

”What are those gaps in the sidewalk for?”
Most objects expand when they are heated. Whenever the temperature outdoors increases, the sidewalk gains heat energy. As a sidewalk gains heat energy, the particles that make up the sidewalk (atoms and molecules) move faster. As they move faster, they hit each other harder and push apart further. The sidewalk as a whole will expand in size. The sidewalk gaps allow the sidewalk material room to "grow" as it expands in hotter weather.

"Why did my tongue freeze to the ice cube that I just took out of the freezer?"
That ice cube was very cold! A freezer's temperature is ~ -24°C and the ice cube will also be ~ -24°C. When you licked the ice cube, heat energy from your tongue moved into the ice cube. Your saliva lost enough heat energy to decrease its temperature to 0°C and then to change your saliva from a liquid into a solid (your saliva froze!). (At the same time, the ice cube gained heat energy its temperature changed from -24°C  to 0°C . At this point it is still ice and will need to gain even more heat energy before it can melt.)

"Where does dew come from?"
[Note to Teacher: Water is a very "polar molecule", i.e. it is a molecule that has  an uneven distribution of its electric charges. (Its net charge is still zero!) One side will be more negatively charged and the other more positively charged. This is due to the arrangement of the oxygen and hydrogen atoms. Since unlike charges attract, the negative side of one water molecule can attract the positive side of another water molecule. If the molecules are moving slow enough, they will not have enough kinetic energy to escape this electrical force of attraction. This is why water molecules cling or stick together in a water drop. The molecules in room temperature water are not moving fast enough on average to escape this force of attraction, thus water is a liquid at room temperature. A few of the molecules might have enough kinetic energy to escape the liquid, hence there will be some evaporation.  (The special arrangement of the atoms within the molecule also gives rise to the beautiful shapes of snowflakes.)]
As warm, moist air cools off during the night, the water vapor/ water molecules in the air, on average, move slower and slower. Their motion, and thus motion/ kinetic energy decreases. When these slow moving molecules hit each other,  or a blade of grass, or a leaf on a tree, they will stick together, not having enough energy to escape the forces of attraction (electrostatic forces) between opposite charges. If enough molecules gather in this place, you will be able to see the dew drops.

"How far away is the sky?"
[Note to Teacher: Is student thinking of the sky where birds fly, or where planes fly, or where clouds form, or where the moon is, or where the Sun is, or where the stars twinkle at night? or just the "blue part"?]
The Earth is surrounded by an atmosphere of air. In this air, birds fly, airplanes fly, the blue part of the white sunlight is scattered about, giving the sky its blue color. Birds need the air to flap and push their wings through. Airplanes need the air to turn their propellers through and be pushed forward. The denser air below the clouds pushes the clouds up, while gravity pulls them down. The Earth's gravity holds this atmosphere of air around Earth. But the atmosphere doesn't gone on forever. The higher you go, the less air there will be, until you get up to about 100 miles. Up at that height, there will be essentially no air molecules at all. Most of the air in the Earth's atmosphere is below 15 miles. Birds fly anywhere from sea level up to ~ 4 miles. Propeller airplanes can fly as high as ~ 4 miles, commercial jets as high as ~ 7 miles. Clouds form any where from less than a mile up to about 10 miles high.  The moon is  240,000 miles away and the Sun is 93,000,000 miles away. The nearest star is 4.2 light-years away from us. (It takes light from that star 4.2 years to reach us. This star is 23,800,000,000,000 miles away!)

"Why is the air cooler near the clouds even though the clouds are closer to the sun?"
The air up around cloud level does not absorb as much of the Sun's radiation, as the Earth does down below. In turn, the air closer to this warmed Earth will be warmed by the Earth. The portion of the air that is closer to the Earth traps most of this heat. The higher you go in the sky, the further you are away from this warmth.

"When I am in the mountains its colder, but I sunburn more easily, Why?"
It is the Sun's ultraviolet rays that cause sunburns. As the Sun's radiation passes into and down through the atmosphere, more and more of its rays are absorbed by the particles (air molecules) in the atmosphere. The further up you are in the atmosphere, the less absorption has taken place. The greater the intensity of these rays, the greater the exposure of your skin to these rays, the greater the likelihood of sun burning.