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Bite-Size Physics

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This lesson may give you a sinking sensation or it may rub you the wrong way but don’t worry about it. It’s only because we’re talking about gravity and friction. You can’t go anywhere without gravity and friction. Even though we deal with gravity on a constant basis, there are several misconceptions about it. Let’s get to an experiment right away and I’ll show you what I mean.

 

Experiment 1

Weight Doesn’t Matter

 

You Need:

Two objects of different weights. A marble and a golf ball, or a tennis ball and a penny for example.

A sharp eye

A partner

 

1. Take a careful look (observe) at both objects and make a hypothesis (you remember what that means right?). Which object will hit the ground first if they are dropped from the same height?

2. Test your hypothesis. Hold both objects at the same height.

3. Let them go as close to the same time as possible.

4. Watch carefully. Which hits the ground first, the heavier one or the lighter one? Try it a couple of times and watch carefully. It will be a little easier for the person who isn’t dropping them to see what happens.

 

What you should see is that both objects hit the ground at the same time! They both accelerate at the same rate of speed and as hit the ground at the same time. Any two objects will do this, a brick and a Buick, a flower and a fish, a cumquat and a cow! “But,” I hear you saying, “whoa Jim, if I drop a feather and a flounder, the flounder will hit first every time!” Ok, you got me there. There is one thing that will change the results and that is air resistance. The bigger, lighter and fluffier something is, the more air resistance can effect it and so it will fall more slowly. Air resistance is a type of friction which we will be talking about later. In fact, if you removed air resistance a feather and a flounder would hit the ground at the same time!!! Where can you remove air resistance? The moon!!! One of the Apollo missions actually did this (well, they didn’t use a flounder they used a brick). An astronaut dropped a feather and a brick at the same time and indeed, both fell at the same rate of speed and hit the surface of the moon at the same time.

Ask someone this question. Which will hit the ground first, if dropped from the same height, a bowling ball or a tennis ball? Most will say the bowling ball. In fact, if you asked yourself that question 5 minutes ago, would you have gotten it right? It’s “common” sense to think that the heavier object falls faster. Unfortunately, common sense isn’t always right. Gravity accelerates all things equally. In other words, gravity makes all things speed up or slow down at the same rate. We will be discussing acceleration more in a later lesson. If you would like more details on the math of this, it will be at the end of this lesson in the Deeper Lesson section.

Remember the pendulums? The weight of the bob made no difference on the rate of swing of the pendulum. Do you see why now? Gravity causes all things to fall at the same rate of speed. Since gravity was the only force causing the pendulum to fall, it didn’t care how heavy the bob was. Whether it was a coin or a Cadillac! All things drop at the same rate of speed.

This is a great example of why the scientific method is such a cool thing. Many, many years ago, there was a man of great knowledge and wisdom named Aristotle. Whatever he said, most people believed to be true. The trouble was he didn’t test everything that he said. One of these statements was that objects with greater weight fall faster than objects with less weight. Everyone believed that this was true. Hundreds of years later Galileo came along and said “Ya know...that doesn’t seem to work that way. I’m going to test it” The story goes that Galileo grabbed a melon and an orange and went to the top of the Leaning Tower of Pisa. He said, “Look out below!” and dropped them! By doing that, he showed that objects fall at the same rate of speed no matter what their size. It is true that it was Galileo who “proved” that gravity accelerates all thing equally no matter what their weight, but there is no real evidence that he actually used the Leaning Tower of Pisa to do it. Which is too bad, because it makes a great story and David Letterman would be proud!

 

Experiment 2

The Fling’s the Thing

 

You Need:

2 rulers or paint sticks. Anything wide and flat

2 coins or poker chips

A sharp eye and ear

A partner is good for this one too

 

1. Place one of the rulers flat so that it is diagonal across the edge of a table with half the ruler on the table and half sticking off.

2. Place one coin on the table, just in front of the ruler and just behind the edge of the table. Place the other coin on the ruler on the side where it’s off the table.

3. Put your finger right in the middle of the ruler on the table so that you are holding it in such a way that it can spin a bit under your finger. Now with the other ruler you are going to smack the end of the first ruler so that the first ruler pushes the coin off the desk and the coin that’s resting on the ruler falls to the ground.

4. Now, before you smack the ruler, make a hypothesis. Will the coin that falls straight down or the coin that is flying forward hit the ground first?

5. Try it. Do the test and look and listen carefully to what happens. It’s almost better to use your ears here than your eyes. Do it a couple of times.

 

 

Are you surprised by what you see, and/or hear? Most people are. It’s not what you would expect. The coins hit the ground at the SAME time. Is that odd or what? Gravity doesn’t care if something is moving or not. Everything falls at the same rate of speed. A bullet fired parallel to the ground from a gun and a bullet dropped from the same height at the same time will both hit the ground at the same time! Even though one may be a mile away! Seems incredible but it’s true. Gravity doesn’t care what size something is or whether or not it is moving, it treats all things equally and accelerates them downward with equal rates. Notice that I don’t say pulls on all things equally, because that’s not true. Gravity does pull on things differently. That’s why you weigh more then a chihuahua. Gravity is the attraction between two bodies, and all bodies have gravity between them. You and your chair have a gravitational pull between you. The coins and the ruler have a gravitational pull between them. Gravity, however, is a very weak force and one of the bodies has to be very, very, very, very big before a force is created. The Earth is very, very, very, very big and so things are attracted to it. The larger something is, the greater gravitational pull that something has. By the way, even though we have known about gravity for many years, scientists still have no idea what it really is. No one really knows why one thing is attracted to another and what pulls one thing towards another thing. Pretty strange huh? Maybe it will be you who figures this out!

 

Friction

Now let’s talk about the other ever present force on this Earth, and that’s friction. Friction is the force between one object rubbing against another object. Friction is what makes things slow down. Without friction things would not stop unless they hit something else. Without friction, you would not be able to walk. Your feet would just slide backward all the time like you’re doing the moon walk. Friction is a very complicated interaction between pressure and the type of materials that are touching one another. Let’s do a couple of experiments to get the hang of what friction is.

 

Experiment 3

A Soleful Experiment

 

You Need:

 

About 5 different shoes (they do not need to be stinky)

A board, or a tray, or a large book at least 15 inches long and no more then 2 feet long.

A ruler

Paper

Pencil

A partner

 

1. Put the board (or whatever you’re using) on the table.

2. Put the shoe on the board with the back of the shoe touching the back of the board.

3. Have a partner hold the ruler upright (so that the12 inches end is up and the 1 inch end is on the table) at the back of the board.

4. Slowly lift the back of the board leaving the front of the board on the table. (You’re making a ramp with the board). Eventually the shoe will begin to slide.

5. Stop moving the board when the shoe slides and measure the height that the back of the board was lifted to.

6. Look at the 5 shoes you chose and test them. Before you do, make a hypothesis for which shoe will have the most friction. Make a hypothesis. On a scale from 1 to 5 (or however many shoes you’re using) rate the shoes you picked. 1 is low friction and 5 would be high friction. Write the hypothesis next to a description of the shoes on a piece of paper. The greater the friction the higher the ramp has to be lifted. Test all of the shoes.

7. Analyze the shoes. Do the shoes with the most friction show any similarities? Are the bottoms made out of the same type of material? What about the shoes with very little friction?

 

Experiment 4

What a Drag

 

You need:

A 6 inch long piece of 2 x 4 wood, or a heavy book

A string

A spring scale or a rubber band and a ruler.

Paper

Pen

5 or so different surfaces, table tops, carpet, chairs, etc.

 

1. Write the different surfaces that you chose on a piece of paper.

2. Make a hypothesis. On a scale from 1 to 5 (or however many surfaces you chose) rate the surfaces you picked. 1 is low friction and 5 would be high friction. Write your ranking next to the surfaces on the paper.

3. Take your block or your book and attach a string to it.

4. Place your block on the surface to be tested.

5. If you have a spring scale, attach it to the string and carefully pull on your block until it just moves. What you will probably see, is that you will keep pulling and pulling until suddenly your block moves. Try to record the number that the scale said just before the block moved. It takes a little bit of practice to read that number so keep trying.

6. If you don’t have a spring scale, tie a rubber band to the string. Now put a ruler with the first inch at the end of the rubber band farthest from the block. Now pull on the rubber band holding it next to the ruler. When the block moves, record the number on the ruler where the end of the rubber band was. In other words, you are measuring how far the rubber band stretches before the board moves.

7. Remember, with the scale or the rubber band, this takes some getting used to so try not to get frustrated.

8. Write down your results next to your hypothesis.

9. The higher the number, the more friction there is between your board and the surface the board is on. In other words, the harder you had to pull to get the board moving, the more friction there is between the board and the surface.

10. Now analyze your data and see how the data matches your hypothesis. Which surface really had the most friction and which had the least. Write numbers 1 to 5 (or however many surfaces you chose) next to the results.

11. How did the data correlate with your hypothesis? Any surprises?

Friction and gravity. You just spent an hour with two topics that some folks spend their entire lives exploring. These are two of the most important and least understood forces in the world. When you ride your skateboard, it’s gravity that wants to pull you to the side walk and friction that makes you scrape your knees when you get there! There are many discoveries to be made in these two areas of physics. Go out and make some!

 

 

Gravity is an attraction between two pieces of matter. The heavier the matter the stronger the force of gravity.

Gravity accelerates all things at the same rate (32ft/s2 or 9.8 m/s2). The weight of the item doesn’t matter.

Gravity doesn’t care if something is moving or not. All things are accelerated downward at the same rate.

Friction is the force between two objects in contact with one another.

 

 

 

Alert! Alert! Serious math ahead!! If you are interested in going deeper with the concept of acceleration due to gravity, please enjoy the following section. However, feel free to skip this section and go straight to the “Did You Get It?” section at the end of this chapter, if math is not your cup of tea.

Okay, let’s see where we can go here. Gravity accelerates all things equally...what does that mean! All things accelerate at 32 feet per second squared due to gravity. In metric, it accelerates 9.8 meters per second squared. What that means is, every second something falls, its speed increases by 32 feet/second or 9.8 meters/second. Believe it or not, that’s about 22 miles per hour!! Gravity will accelerate something from 0 to 60 mph in about 3 seconds. Faster then all but the fastest sports cars!

Can you see where the squared comes from in ft/s2? Speed is the amount of distance something travels in a certain amount of time. Miles per hour or feet per second for example. Acceleration is how much the speed changes over time. So acceleration would be miles per hour per hour or feet per second per second. Acceleration is a rate of change of speed or, in other words, how fast is the speed changing. Feet per second per second is the same as ft/s/s which is the same as ft/s2. (I told you we were going deeper!)

 

If we want to find out how fast something is going after it has been dropped, we use the formula v=gt. The letter “v” stands for velocity (which basically means speed. We’ll talk about it more later.) “g” stands for the gravitational constant and “t” stands for time. If we want to find out how fast a golf ball is dropping after it falls for 3 seconds we multiply 3 seconds by 32 feet/second squared and that equals 96 feet/second. So, if I dropped a golf ball off a building, it would be going 96 feet per second after 3 seconds of dropping. The formula looks like this when we fill in the numbers:

v=3s x 32 ft/s2

If we do more math, we’ll see that after one second something will be going 32 ft/s, after 2 seconds it will be going 64 ft/s, after 3 seconds 96 ft/s after 4 seconds 128 ft/s. Get it? Anything dropped will be going that speed after that many seconds because gravity accelerates all things equally (air resistance will effect these numbers so you won’t get exactly the numbers in practice that you will mathematically).

All right, lets go even deeper. We now know how to calculate how fast something will be going if it is dropped, but what happens if we throw it up? Well, which way does gravity go? Down right? Gravity accelerates all things equally so, gravity will slow things down as they travel up by 32 ft/s2. If a ball is thrown up at 64 ft/s how long will it travel upwards? Well, since it is negatively accelerating (in physics there’s no such thing as deceleration) after the first second the ball will be traveling at 32 ft/s and after 2 seconds the ball will come to a stop, turn around in midair, and begin to accelerate downwards at 32 ft/s2. Using this, you can tell how fast you can throw by using nothing more then a timer. Get a timer and a friend. Count to three, throw the ball and start the timer at the same time. Stop the timer when the ball hits the ground. Let’s say your throw took 3 seconds to hit the ground. The first thing you have to do is divide 3 in half. Why? Because your ball traveled 1.5 seconds up and 1.5 seconds down! (By the way, this isn’t completely accurate because of two things. One, air resistance and two, the ball falls a little father then it rises because of the height of the thrower.) Now, take your formula and figure out the speed of the throw. v=gt,

so v=32 ft/s2 x 1.5 sec or

v = 48 ft/s.

So if that’s how fast it left your hand...how fast was it going when it hit the ground? Yup, 48 ft/s. It has to be going the same speed because it had just as much time to speed up as it had to slow down, 1.5 seconds.

Ok, hold your breath, just a little deeper now. Let’s talk about distance. If something starts from rest you can tell how far it drops by how long it has dropped. This formula is d=1/2gt2 or distance equals one half the gravitational constant multiplied by time squared. Let’s try it. If I drop a ball and it drops 3 seconds how far has it dropped?

d=1/2 32ft/s2 x 32 or

d = 16 ft/s x 9s2 or

d=144 ft So it has dropped 144 ft

 

Phew, now take a deep breath. We’re done. Would you like to try out your new formulas? All right, give these problems a try. Don’t worry about air resistance for these. See the back of the chapter for answers.

Since you are finding velocity use this formula for these problems, v=gt. v is velocity, g is the gravitational constant (32 ft/sec2), t is time.

1. You dropped a ball off a building 3 seconds ago. How fast is it going now?

2. 6 seconds have passed since your meat ball rolled off the roof. How fast is it going?

3. If you shoot a model rocket into the air and it takes 8 seconds before it hits the ground how fast was it going when it left the launch pad?

Now for these you’re looking for distance, so use the formula d=1/2gt2. d is distance, g is the gravitational constant, and t is time.

4. If you dropped a ball off the edge of the roof of your house to your buddy on the ground and it took 5 seconds to get to your friend, how tall is your house?

5. If you’re in the outfield and a fly ball takes 3 seconds to go from the highest point of the hit to your mitt, how high was the ball hit?

 

 

 

Try these questions and see if this info has sunk in. See the last pages for the answers. If you missed any, just reread the lesson. You’ll get it!

1. Of the following objects, which ones are attracted to one another by gravity?

A. Apple and Banana

B. Beagle and Chihuahua

C. Earth and You

D. All of the above

 

2. Gravity accelerates all things differently...True or False??

3. Gravity pulls on all things differently...True or False??

4. If I drop a golf ball and a golf cart at the same time from the same height, which hits the ground first?

5. If one squirrel falls out of a tree at the same time as one jumps straight out from the tree, which one hits the ground first?

6. Why don’t a feather and a brick hit the ground at the same time?

7. What is friction?

8. Walking would be easier without friction....True or False

 

Answers for Did You Get It:

 

1. D. All bodies are attracted to other bodies by gravity. But a body has to be really stinkin’ big before it’s noticeable.

2. FALSE!!! Gravity accelerates all things at the same rate. All things fall at the same rate of speed no matter what (ignoring air resistance, that is).

3. True. That’s why some things weigh more then other things. Gravity pulls more on the big stinky guy sitting next to me on the bus, then it does on me.

4. They hit the ground at the same time. Gravity accelerates all things equally.

5. They both hit the ground at the same time. Gravity doesn’t care if something is moving or not. Their falling speed is the same. (Don’t worry, no squirrels were harmed in the making of this question.)

6. They do...if you’re on the moon! On Earth, the friction between the air and the feather causes the feather to slow down and the brick to win the race.

7. Friction is the force between one object rubbing against another object. Air resistance, by the way, is the friction of one object rubbing against air molecules.

8. FALSE!!! Walking would be impossible without friction. Your feet couldn’t push back against the floor to move you forward.

 

 

Answers for Problems:

 

I’ve converted feet/second to miles/hour for you so that you can get more of a feel for the speed.

1. 96 ft/s which is 64 mph

2. 192 ft/s or 131 mph (thatsa fasta meata balla!)

3. 128 ft/s or 87 mph (remember that you have to half the time. It took 4 seconds to go up and 4 seconds to fall down.

 

4. 400 ft. Ok, so, you have a big house!

 

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