Unit 5 · Forces, Motion and Energy

Kick a soccer ball and it rolls away. Pull a wagon and it follows you. Both of these are examples of a force. A force is simply a push or a pull. You cannot see a force itself. You can only see what it does to an object. When you watch something start, stop, speed up, or turn, a force is at work.

Every force has two parts. The first is strength. A gentle tap on a ball is a weak force. A hard kick is a strong force. The second part is direction. A push to the left and a push to the right are different forces. Scientists draw a force as an arrow. A longer arrow means a stronger push. The arrow points the way the push goes.

Try this with a door. Push it near the handle and it swings easily. Push the same door right next to the hinges and it will barely budge. Same strength, different place, different result. Where and which way you push matters as much as how hard. When we describe a force, we always name both its strength and its direction.

Think back to the tug-of-war. Both teams pulled hard, but the flag stayed still. The two pulls were equal in strength and opposite in direction. We call forces like this balanced. Balanced forces cancel each other out. It is as if no force were acting at all. The flag did not move because there was no leftover pull in any direction.

Balanced forces are everywhere. A book sits on a table. Gravity pulls the book down. The table pushes the book up with exactly the same strength. The book stays put. You sit in a chair right now. The chair pushes up as hard as your weight pushes down. If it did not, you would sink through it.

Here is a surprise. Balanced forces do not only keep things still. They also keep moving things moving at a steady speed. A hockey puck sliding on very smooth ice keeps going in a straight line. No force is winning. The puck’s motion does not change. Balanced forces mean no change, whether the object is resting or moving.

When the girl in the tug-of-war slipped, the pulls stopped matching. The right team’s pull was now stronger. The forces were unbalanced. The rope moved toward the stronger pull right away. Any time forces do not cancel, motion changes. This is the rule that lets you predict what happens next.

An unbalanced force can change motion in three ways. It can make an object start or speed up. Think of a kick on a still ball. It can make an object slow down or stop. Think of a catcher’s mitt on a pitch. Or it can change direction, like a bat sending a pitch into the air. In each case, look for the force that is not being canceled.

The size of the change depends on two things. A stronger force makes a bigger change. A heavier object changes less for the same force. Push a shopping cart that is empty and it zips away. Push a full one with the same strength and it barely moves. Scientists call the amount of stuff in an object its mass. More mass means harder to change.

Drop a pencil and it falls to the floor. It never falls up or sideways. The force that pulls it is gravity. Gravity is a pull between objects that have mass. Earth has an enormous mass. So Earth pulls hard on everything near it. The pull always points toward Earth’s center. That is what we mean by "down."

You feel gravity as weight. Your weight is the strength of Earth’s pull on you. A scale measures that pull. Gravity acts even when you are not falling. Stand still, and gravity pulls you down while the floor pushes you up. Jump, and for a moment the floor stops pushing. Gravity wins, and you come back down.

Gravity does not need to touch you. It pulls across empty space. That is how Earth keeps the Moon circling around it. It is also why a ball thrown up always comes back. As the ball rises, gravity slows it. At the top it stops for an instant. Then gravity pulls it faster and faster toward the ground.

Slide a book across a table. It stops after a short distance. Slide it across ice and it goes much farther. The force that stopped the book is friction. Friction happens when two surfaces rub against each other. Even smooth-looking surfaces have tiny bumps. The bumps catch on each other and resist the motion.

Friction always pushes against the direction of motion. That makes it a great brake. Bike brakes squeeze rubber pads against the wheel. Car tires grip the road. Sandpaper grips wood. In Chicago winters, crews spread salt and sand on icy sidewalks. The grit adds friction so shoes and tires can hold on.

Sometimes we want less friction. Oil on a bike chain lets it spin freely. Wax on a sled lets it fly down the hill. A hockey rink is kept smooth so pucks glide. Rough surfaces make more friction. Smooth surfaces make less. Engineers add friction where things must grip and remove it where things must slide.

Tie a washer to a string and let it hang. Pull it to the side and let go. It swings back and forth, back and forth. This is a pendulum. Gravity pulls the washer down toward the middle. It overshoots, rises on the other side, and gravity pulls it back. The swings get smaller as air friction steals a bit of motion each time.

Time the swings and you will find a pattern. Each full swing takes almost the same time as the one before. A short string swings quickly. A long string swings slowly. Strangely, a heavy washer and a light washer on the same string keep the same time. The length is what matters, not the weight.

Once you know the pattern, you can predict. If ten swings took 20 seconds, the next ten will take about 20 seconds too. Long ago, people used this pattern to build clocks. A swinging pendulum ticked off the seconds. Any time motion repeats, watch for the pattern. The pattern is what makes the future easy to guess.

Roll a ball down a ramp. It starts slow and gets faster and faster. Gravity pulls it down the slope the whole way. The pull is steady, so the speed keeps growing. About 400 years ago, an Italian scientist named Galileo studied this. He wanted to know exactly how falling objects speed up. Free fall was too fast to time. So he slowed it down with a ramp.

Galileo rolled bronze balls down a long, smooth groove. He timed each roll. He found a clear pattern. In the second stretch of time, the ball went three times as far as in the first. In the third, five times as far. The distance grew by odd numbers: 1, 3, 5, 7. This pattern held no matter how steep the ramp was.

You can test this yourself. Mark a ramp every 10 centimeters. Let the ball go and count out loud at a steady beat. Notice where the ball is on each count. Then make the ramp steeper and try again. A steeper ramp means a bigger pull down the slope. The ball speeds up faster. But the odd-number pattern shows up again. Patterns like this let scientists predict motion before it happens.

Hold a magnet near a paper clip. The clip jumps across the gap and sticks. Nothing touched the clip until it moved. The pull came from the magnet itself. Magnets attract some metals, mostly iron and steel. They ignore copper, aluminum, wood, and plastic. Test this around you. A steel refrigerator door will hold a magnet, but an aluminum soda can will not.

Every magnet has two ends called poles, north and south. Bring two north poles together and they push apart. You can feel the push in your fingers. Turn one magnet around and the north and south poles snap together. Opposite poles attract. Like poles repel. The push and pull get stronger as the magnets get closer.

Earth itself acts like a giant magnet. That is why a compass needle turns to point north. Sailors used compasses for hundreds of years before anyone knew why they worked. In the year 1600, an English doctor named William Gilbert tested lodestones, which are natural magnets. He argued that the whole Earth was one great magnet. His evidence was the compass.

Rub a balloon on your hair. Hold it near a wall and it sticks. Hold it near tiny bits of paper and they leap up. Your hair may even stand on end. This is static electricity. Rubbing moves tiny bits of electric charge from one object to the other. The balloon ends up with extra charge. The extra charge pulls on the paper from a distance.

There are two kinds of charge, called positive and negative. Like charges push apart. Opposite charges pull together. That sounds a lot like magnets. But charge and magnetism are not the same thing. A charged balloon does not pick up a paper clip. A magnet does not attract paper bits. Each one has its own kind of force.

In a dry Chicago winter, static shows up everywhere. Pull off a wool hat and your hair crackles. Touch a doorknob and you feel a tiny zap. The zap is a little spark. Charge that built up on you jumps to the metal. Lightning is the same thing on a giant scale. Charge builds in a storm cloud and then jumps in one huge spark.

A cabinet door in a classroom keeps swinging open. Books fall out. The class decides to fix it. They need something that holds the door shut but lets it open with a small pull. A magnet is perfect. A small magnet on the frame and a steel plate on the door make a latch. The magnet’s pull holds the door. A tug stronger than that pull opens it.

This is engineering: using science to solve a problem. First the class defines the problem clearly. The door must stay shut on its own but open easily. Then they test ideas. One magnet is too weak; the door drifts open. Three magnets are too strong; small hands cannot open it. Two magnets are just right. Testing tells them which design works.

Magnets solve many problems this way. Refrigerator doors seal with a magnetic strip. Some phone chargers snap on with magnets. Cranes in scrap yards lift whole cars with huge electromagnets. Some trains float above their tracks on magnetic pushes. Every one of these designs started with a problem and a test.

A slow marble nudges a domino. A fast marble knocks it flying. The fast marble carries more energy. Energy is what lets an object make something happen. It can move a thing, warm a thing, light a thing, or break a thing. You cannot see energy itself, but you can see its effects. The bigger the effect, the more energy was used.

The energy of a moving object is called motion energy. It depends on two things. One is how fast the object moves. The other is how much mass it has. Double the speed and the energy grows a lot, more than double. A bike at 20 miles per hour has four times the motion energy it had at 10. That is why a fall at high speed hurts so much more.

Mass matters too. A bowling ball rolling slowly can have more energy than a marble rolling fast. Bowling pins fly because the ball is both heavy and quick. When you look at a moving object, ask two questions. How fast is it going? How much mass does it have? Together they tell you how much energy it carries.

The marble hits the first domino. The marble slows. The domino starts to fall. Energy has moved from the marble to the domino. This is a collision, when two objects hit each other. In every collision, some energy passes from one object to another. The one that was moving loses some. The one that was hit gains some.

Watch a game of pool. The white ball rolls and smacks a striped ball. The white ball stops or slows. The striped ball zips off. Nearly all the motion energy jumped from one ball to the other. The energy did not vanish. It just changed owners. Scientists say energy is transferred, which means moved from one thing to another.

Not all the energy goes into motion. Listen to the click when the balls hit. That sound is energy too. Touch the balls after a long game and they feel slightly warm. Some energy became heat. When you add up the motion, the sound, and the heat, the total stays the same. Energy is never created or destroyed. It only changes form or moves along.

The marble at the top of the tower is still, yet it can make the bell ring. It holds stored energy because it is high up. The higher it starts, the more it stores. Let it go and gravity turns the stored energy into motion. A stretched rubber band stores energy the same way. Let it go and the energy snaps out.

Food stores energy too. Your body takes energy from bread, rice, and beans. That energy moves your muscles and keeps you warm. A car’s engine takes energy from fuel like gasoline. Gasoline burns, and the energy pushes the car. The energy in food and fuel is locked inside the tiny bits they are made of. Burning or digesting sets it loose.

A battery stores energy in chemicals sealed inside a case. Connect it to a wire and the chemicals slowly change, pushing electricity out. When the chemicals are used up, the battery is dead. Stored energy is useful because you decide when to use it. A marble on a shelf, a sandwich, a tank of gas, and a battery are all waiting for you.

The bell in the marble run rings. Across the room, you hear it. Energy traveled from the bell to your ear with nothing carrying it that you could see. The bell starts to vibrate, shaking back and forth very fast. The shaking bumps the air next to it. That air bumps the next bit of air. The bumps spread out like ripples on a pond. This traveling shake is sound.

Sound needs something to travel through. Air works. Water works even better. Solids work best of all. Put your ear on a table and tap the far end. The tap sounds loud and sharp. Sound carries real energy. A loud speaker can make a glass of water ripple. A big drum thumps in your chest. Very loud sounds can even shatter a glass.

Sound is fast, but not instant. Watch a batter hit a ball from far away. You see the swing, then hear the crack a moment later. In air, sound travels about 343 meters each second. That is about the length of three football fields. Light gets to your eye almost at once. So the sight arrives first and the sound trails behind.

Stand in a sunny window. Your skin feels warm. That warmth came from the Sun, about 150 million kilometers away. It rode here on light. Light is energy that travels from a source, such as the Sun, a flame, or a bulb. Unlike sound, light does not need air. It crosses empty space with ease. That is how sunlight reaches Earth.

When light lands on something, it gives up energy. A dark car seat gets hot in the sun. Plants catch light and use its energy to make food. A solar panel turns light into electricity. Some light bounces off instead. That bouncing light is what lets you see. Your eye catches light that came from a source and bounced off the world.

Light is the fastest thing we know. It travels about 300,000 kilometers every second. Sunlight takes a bit over 8 minutes to reach Earth. When you look at the Sun’s light, you are seeing energy that left the Sun 8 minutes ago. Light from distant stars left them long before you were born. The site’s telescope collects that light and lets you see it.

Hold a mug of hot cocoa. Your hands warm up. The cocoa cools down. Energy moved from the hot drink to your cooler hands. This moving energy is heat. Heat always travels from warmer to cooler, never the other way on its own. A cold spoon in hot soup warms up. A hot spoon in cold water cools down. The energy flows until both are the same temperature.

Heat moves in a few ways. It passes through solids, which is why a metal spoon in soup gets hot at the handle. It rides on moving air and water, which is how a radiator warms a whole room. And it travels as light, which is how a campfire warms your face from a distance. Metals pass heat quickly. Wood, plastic, and air pass it slowly.

A material that slows heat down is called an insulator. A winter coat traps air to keep your body’s heat in. A foam cup keeps cocoa hot. An ice chest keeps the cold in and the summer heat out. In Chicago, homes stuff insulation in walls and attics. Less heat leaks out in January, so the furnace burns less fuel.

Flip a switch and a lamp across the room glows. The energy for that light did not start in the lamp. It came through a wire. Electricity is energy moving through wires as a flow of tiny charges. It is a fast and handy way to move energy from place to place. Power plants far outside Chicago send energy along wires to homes across the city.

Electricity needs a complete path, called a circuit. Think of it as a loop. Energy leaves a battery, travels along a wire, passes through a bulb, and returns along another wire. If the loop has a gap, the flow stops and the bulb goes dark. A switch is just a gap you can open and close. Open the gap, off. Close it, on.

Try it with a battery, two wires, and a small bulb. Touch one wire to each end of the battery and the other ends to the bulb. The bulb lights. Pull one wire off and it goes dark. Now put a paper clip in the loop; it still lights. Put a plastic straw in the loop, and nothing. Metals let electricity through. Plastic and rubber block it.

A flashlight is a tiny energy machine. Chemicals in the battery hold stored energy. Close the switch and the circuit carries that energy to the bulb. The bulb turns it into light and a little heat. Touch a flashlight bulb that has been on a while. It is warm. The energy took three forms: stored, then electric, then light and heat.

Swap the bulb for a small motor and the same energy becomes motion. A motor uses electricity and magnets to spin a shaft. Add a fan blade and you have a breeze. Add wheels and you have a toy car. Swap in a buzzer and the energy becomes sound. Every device in the circuit changes electric energy into a form you want.

Think about your day. An alarm clock changes electricity into sound. A toaster changes it into heat. A phone screen changes it into light. An electric train on Chicago’s rails changes it into motion. Each device is a converter. Follow any device back and you find a battery or a plug. Follow the plug back and you reach a power plant.

Here is a challenge. Build a device that uses a marble’s stored energy to ring a bell. You have tubes, tape, dominoes, a bell, and a marble. That is exactly what the cousins built. It is also what engineers do all day. They design devices that change energy from one form into a form people need. A wind turbine changes moving air into electricity. A solar oven changes light into heat.

Good design starts with a clear goal. What must the device do? Then comes a prototype, a first rough version. The cousins’ first prototype flung the marble off the table. That failure gave them information. They added a curved tube. Test, look at what happened, change one thing, test again. Each round makes the design better.

Try one yourself. Line a shoebox with foil. Paint the bottom black or cover it with black paper. Cover the top with clear plastic wrap. Set it in the sun with a chocolate chip inside. This is a solar oven. It changes light into heat. Does the chip melt? How long does it take? What one change would make it hotter? That question is where engineering begins.

You built a solar oven and the chip melted in 20 minutes. Now you want it faster. You could change the foil, the box size, the color, and the plastic all at once. But if it works better, which change did it? You would not know. A fair test changes only one thing at a time. Keep everything else the same. Then the result has one clear cause.

Make a data table. Write down what you changed and what you measured. Trial 1: white paper inside, 20 minutes. Trial 2: black paper inside, 12 minutes. Trial 3: black paper and a second layer of plastic, 9 minutes. Now the numbers tell a story. Black paper helped. The second layer helped more. Evidence, not guessing, guides the next step.

Engineers also weigh trade-offs. A bigger oven catches more light but is harder to carry. A thicker box holds heat but costs more. There is rarely one perfect design. There is the design that best fits the goal. Share your data with another team. Compare results. Science and engineering are team sports. The best ideas come from many tests and many minds.