Bite-Size Physics

Hi everybody. Well, now you asked for it! We’re beginning a block on energy. I’m not going to lie to you, this is tough. At least at first. Many of these concepts are quite abstract and it takes a while for them to sink in. The first several lessons will focus on some of the major definitions of energy so that you can get a feel for what energy is and what it does. Then we will begin to learn about some of the many forms of energy (sound, thermal, light, heat, electrical) and what they do. Are you ready? Let’s get going.

Everything in the entire universe can be categorized into two things, matter and energy. As a matter of fact, according to Einstein and some relatively new physics theories (string theory) even matter is actually energy incarnate. Energy can become matter and matter can become energy. Makes your head spin huh? So what is energy?

Energy has a number of different forms; kinetic, potential, thermal, chemical, electrical, electrochemical, electromagnetic, sound and nuclear. All of which measure the ability of an object or system to do work on another object or system. Energy is defined in the physics books as the ability to do work. Uh huh, great...so what’s work?

Work is the exertion of force over a distance. In our mechanics block we defined force as a push or a pull. So, work is when something gets pushed or pulled over a distance against a force. Mathematically it’s Work = Force x Distance or Work = fd. If I was to lift an apple up a flight of stairs, I would be doing work. I would be moving the apple against the force of gravity over a distance. However, if I were to push against a wall with all my might, and if the wall never moved, I would be doing no work because the wall never moved. There was a force, but no distance. Another way to look at this, is to say that work is done if energy is changed. By pushing on the non-moving wall, no energy is changed in the wall. If I lift the apple up a flight of stairs however, the apple now has more potential energy then it had when it started. The apple’s energy has changed, so work has been done. (We’ll define potential energy more next lesson.)

Here's a movie showing work (fast connection, slow connection). Here's a movie showing no work (fast connection, slow connection.)

If we wish to talk about energy further, we need to have a unit of measurement. For energy, a couple of units are the Joule and the Calorie. A Joule is the energy needed to lift one Newton one meter. “Arrghh, Jim, there’s another new word!!! What in the world is a Newton?!” Sorry, you’re right. OK, a Newton is a unit of force. One Newton is about the amount of force it takes to lift 100 grams or 4 ounces or an apple. It takes about 66 Newtons to lift a 15 pound bowling ball and it would take a 250 pound linebacker about 1000 Newtons to lift himself up the stairs! So, if you lifted an apple one meter (about 3 feet) into the air you would have exerted one Joule of energy to do it. The calorie is generally used to talk about heat energy and you may be a bit more familiar with it due to food and exercise. A calorie is the amount of energy it takes to heat one gram of water one degree Celsius. Four Joules are about one calorie. Try this experiment and see if you can feel the burn!

Experiment 1

Do your exercises!

You Need:

Something that weighs about 100 grams or 4 ounces, or just grab an apple.

A meter or yard stick

This might seem sort of silly but it’s a good way to get the feeling for what a Joule is and what work is.

1. Grab your 100 gram object, put it on a table.

2. Now lift it off the table straight up until you lift it one meter (one yard).

3. Lift it up and down 20 times.

A 100 gram object takes about one Newton of force to lift. Since it took one Newton of force to lift that object, how much work did we do? Remember work = force x distance so in this case work = 1 Newton x 20 meters or work = 20 Joules. You may ask “but didn’t we move it 40 meters, 20 meters up and 20 down?” That’s true, but work is moving something against a force. When you moved the object down you were moving the object with a force, the force of gravity. Only in lifting it up, are you actually moving it against a force and doing work. Four Joules are about 1 calories so we did 5 calories of work.

“Wow, I can lift an apple 20 times and burn 5 calories! Helloooo weight loss!” Well...not so fast there Richard Simmons. When we talk about calories in nutrition we are really talking about kilo calories. In other words, every calorie in that potato chip is really 1000 calories in physics. So as far as diet and exercise goes, lifting that apple actually only burned .005 calories of energy,...rats. It is interesting to think of calories as the unit of energy for humans or as the fuel we use. The average human uses about 2000 calories (food calories that is, 2,000,000 actual calories) a day of energy. Running, jumping, sleeping, eating all uses calories/energy. Running 15 minutes uses 225 calories. Playing soccer for 15 minutes uses 140 calories. (Remember those are food calories, multiply by 1000 to get physics calories). This web site has a nice chart for more information Calories used in exercise.

Everything we eat refuels that energy tank. All food has calories in it and our body takes those calories and converts them to calories/energy for us to use. How did the food get the energy in it? From the sun! The sun’s energy gives energy to the plants and when the animals eat the plants they get the energy from the sun as well. So, if you eat a carrot or a burger you are getting energy from the sun! Eating broccoli gives you about 50 calories. Eating a hamburger gives you about 450 calories! More information can be found here if you’d like Calories in food. (By the way, the Bite-Size Physics has nothing to do with those web sites. They are just a couple of sites that I found that give a bit of information. The Bite-Size Physics is not responsible for their content.) We use energy to do things and we get energy from food. The problem comes when we eat more energy than we can use. When we do that, our body converts the energy to fat, our body's reserve fuel tank. If you use more energy then you’ve taken in, then your body converts fat to energy. That’s why exercise and diet can help reduce your weight.

Let’s take the concept of work a little bit farther. If Bruno carries a 15 pound bowling ball up a 2 meter (6 foot) flight of stairs, how much work does he do on the bowling ball? It takes 66 Newtons of force to lift a 15 pound bowling ball 1 meter. Remember work = force x distance. So, work = 66 Newtons x 2 meters. In this case, Bruno does 132 Joules of work on that bowling ball. That’s interesting, but what if we wanted to know how hard poor Bruno works? If he took a half hour to go up those stairs he didn’t work very hard, but if he did it in 1 second, well then Bruno’s sweating! That’s the concept of power. Power is to energy like miles per hour is to driving. It is a measure of how much energy is used in a given span of time. Mathematically it’s Power = work/time. Power is commonly measured in Watts or Horsepower. Let’s do a little math and see how hard Bruno works. In both cases mentioned above Bruno, does 132 Joules of work, but in the first case he does the work in 30 minutes (1800 seconds) and in the last case he does it in 1 second. Let’s first figure out Bruno’s power in Watts. A Watt is 1 Joule/second so:

For the half hour Bruno’s Power = 132 Joules/1800 seconds = .07 Watts

For the second Bruno’s Power = 132 joules/1 second = 132 Watts

You can see that the faster you exert energy the more power you use. Another term for power is horsepower. You may have heard the term horsepower in car ads. The more powerful car can exert more energy faster, getting the car moving faster. A Dodge Viper has 450 horsepower which can accelerate a 3,300 pound car from 0 to 60 mph in 4.1 seconds...WOW!

One horsepower is 745 Watts or one Watt is .001 horsepower. So converting Watts to horsepower poor Bruno exerts:

.07 x .001 = .00007 horsepower over the half hour

132 x .001 = .132 horsepower over the second (not exactly a Dodge Viper!)

Let’s find out your power.

Experiment 2

Do you have the power?

You need:

Same object from experiment 1

Meter or yard stick

A stopwatch or timer

1. Grab your 100 gram object, put it on a table.

2. Now lift it off the table straight up until you lift it one meter (one yard).

3. Start the timer and at the same time start lifting the object up and down 20 times.

4. Stop the timer when you’re done with the 20 lifts.

So, do you have the power of the Dodge Viper? Hmmm, probably not but let’s take a look.

First of all figure out how much work you did. Work = force x distance so take the force you used and multiply that by the distance you moved it. In this case, you can multiply 1 Newton x 20 meters and get 20 Joules of work.

Now figure out how much power you used. Power is work divided by time so take your work (20 Joules) and divide it by how much time it took you to do that work.

For example, if you lifted the block 20 times (doing 20 Joules of work) in 5 seconds, you did 20 Joules/5 seconds = 4 Watts of power. To convert Watts to horsepower we multiply by .001 so in this example, you did 4 x .001 = .004 horsepower. Not exactly vroom vroom!

Energy is the ability to do work.

Work is moving something against a force over a distance.

Mathematically, work = force x distance.

Work can be measured in Joules or calories.

Power measures how quickly work can be done.

Mathematically, power is work divided by time.

Power can be measured in horsepower or Watts.

Next lesson we will deepen our study of energy by investigating the two main categories of energy, potential and kinetic.

1. Everything in the universe can be categorized as what two things?

2. What is energy?

3. What is work?

4. If someone carries a lawn chair to their roof to watch the meteor showers, is work done on the chair?

5. What if the chair falls off the roof? Is work done on the chair then?

6. If someone pushes a train with all their might, but the train doesn’t move, is work done?

7. What are two units used to measure work?

8. What is power?

9. What are two units to measure power?

10. Where does all the energy you get from food originate from?

Answers to Did You Get It

1. Matter and energy.

2. The ability of an object or system to do work on another object or system. Energy is defined in the physics books as the ability to do work.

3. Work is moving an object against a force over a distance. Work = force x distance

4. Yes. The chair has been moved a distance, against the force of gravity.

5. Nope, the chair moves a distance, but it moves with the force of gravity. Work is moving something a distance against a force. In this case, the chair does not move against a force. No work is done.

6. Nope again! There’s no distance moved so...no work done.

7. Joules and calories.

8. The amount of work done in a given amount of time. Power = work divided by time.

9. Watts and horsepower.

10. The sun. You are powered by the sun!

Remember work=force x distance and power = work/time

1. A mouse that weighs 4 ounces, jumps, step by step, up a 2 meter tall flight of stairs. What kind of work did that little guy do? (1 newton is 4 ounces)

2. If it took him 3 minutes (180 seconds) to do it, what power did he exert?

3. Bob’s car breaks down. He needs to push on the car with a force of 1000 Newtons to get the car to go 30 meters (about 100 feet). How much work does he do?

4. If Bob takes 5 seconds to do it, how much power does he use?

5. Just for fun, let’s convert that to horsepower. 1 Watt = .001 horsepower

Answers

1. work = force x distance so

work = 1 newton x 2 meters

work = 2 Joules

2. Power = work/time

power = 2/180

power = .01 Watts

3. work = force x distance

work = 1000 x 30

work = 30,000 Joules (go Bob!)

4. power = work x time

power = 30,000/5

power = 6000 Watts (Wow! Big Bob!)

5. 6000 Watts x .001 = 6 horsepower (No Viper, but pretty impressive!)