Unit 5 · Forces on Four Wheels

Stand on a skateboard with your front foot over the front bolts. Put your back foot on the ground. Press back against the ground. The board rolls forward. Your foot gave a push. A push presses something away from you. A push is one kind of force.

A pull is a force too. A pull brings something toward you. Hook your toe under the nose of a resting board and lift. The nose tips up toward you. Tug a board along by a rope, and you are pulling it. Every force is either a push or a pull.

Every force has a size and a direction. A gentle push starts the board slowly. A strong push starts it faster. Your back foot pushes backward on the ground. The ground pushes you forward in return. That is why a push off the ground moves you ahead.

A skateboard sits on a flat driveway. Gravity pulls it down. The ground pushes it up just as hard. The two forces are the same size. They point in opposite directions. They cancel each other out. These are balanced forces. The board stays still.

Now a skater steps on and pushes off. The push forward is bigger than anything pushing back. That is an unbalanced force. An unbalanced force changes motion. It can start a board, speed it up, slow it down or turn it.

Think of a tug-of-war. If both teams pull equally hard, the rope does not move. If one team pulls harder, the rope moves toward that team. A board works the same way. Only an unbalanced force makes its motion change.

A skateboard rests in the corner of a garage. It will sit there all week. It will not start rolling on its own. Something has to push or pull it first. Things that are still tend to stay still. That is one half of inertia.

Inertia has a second half. Things that are moving tend to keep moving. They keep the same speed and go in a straight line. Something has to push or pull to slow them, stop them or turn them. Inertia is not a force. It is how every object acts.

Heavier things have more inertia. An empty shopping cart is easy to start and easy to stop. A full cart is harder to start and harder to stop. A grown-up on a board is harder to stop than a small child at the same speed.

A rider rolls along a path. A front wheel hits a small rock. The rock pushes back hard on the wheel. The board stops almost at once. But nothing pushed back on the rider’s body. Inertia keeps the body moving forward. The rider steps or tumbles off the front.

This is why skaters scan the ground ahead. Look for pebbles, sticks and cracks before you reach them. A helmet that fits and is buckled protects your head. Wrist guards and knee pads protect the places that hit the ground most often.

If you do fall, fall small. First, bend your knees and get low. Next, tuck your chin. Then roll onto your side or slide on your knee pads. Do not catch yourself with straight, locked arms. Locked arms can hurt your wrists.

Remember the board in the gym. It slowed down and stopped with nobody touching it. A force did that. The force is friction. Friction happens when two surfaces rub or press together. It works against motion.

A rolling board has friction in more than one place. The wheels press and flex against the floor. The bearings inside the wheels rub a little as they spin. The air pushes on the board and rider. Each one takes away a bit of speed.

Smooth surfaces make less friction than rough ones. A board rolls far on a smooth gym floor. It rolls a shorter way on bumpy blacktop. It stops quickly in grass. Friction is why the board needs another push to keep going.

Look at the top of a deck. It is covered in grip tape. Grip tape is rough, like sandpaper. It makes lots of friction with the soles of your shoes. That friction keeps your feet from sliding off. Flat shoes with grippy soles help even more.

Friction also helps you stop. One way is the foot brake. First, keep your front foot over the front bolts, pointing forward. Next, bend your front knee. Then lower your back foot and let the sole touch the ground lightly. Press a little harder to slow more.

A common problem is stomping the back foot down too hard. The shoe grabs and you lurch forward. The fix is to brush the ground first, then press slowly. Practice the foot brake at walking speed on flat, smooth ground away from cars.

Drop a skateboard wheel and a pencil. Both fall to the floor. Something pulled them down. That pull is gravity. Gravity pulls everything toward the center of the Earth. It never turns off.

Gravity keeps a skater pressed onto the deck. When a skater hops, gravity pulls her back down. The strength of gravity’s pull on your body is your weight. A heavier rider is pulled down harder than a lighter one.

Gravity also pulls a board down a slope. On a hill, part of the pull points downhill. The board speeds up even without a push. That is why beginners stay on flat ground. Practice slowing down before you try even a gentle slope, with a grown-up nearby.

Isaac Newton was an English scientist. In 1687 he published a famous book called the Principia. In it, he wrote three laws of motion. The first one describes inertia. People call it Newton's first law.

Here it is in plain words. A still object stays still. A moving object keeps moving at the same speed in a straight line. Both stay that way until an unbalanced force acts on them. A force is needed to change motion, not to keep it going.

A skateboard shows the law every day. On a perfectly smooth floor with no air, one push would send a board on and on. On real ground, friction is always acting. Friction is the force that changes the board’s motion and stops it.

Early skateboards had clay or steel wheels. They were hard and slick. A small pebble could stop a steel wheel cold. Then the rider’s inertia carried him forward off the board. In the early 1970s, urethane wheels changed this. Urethane is a tough, rubbery plastic.

A soft urethane wheel squishes a little when it meets a bump. It wraps around a small pebble or the edge of a crack. The bump pushes back on the wheel more gently. The board slows less, and it keeps rolling. The ride also feels smoother and quieter.

Soft wheels also grip better. Soft urethane holds onto the ground better than hard clay or steel, so there is more friction sideways. That grip helps the board turn without sliding out. Riders on rough streets often pick softer wheels. Hard wheels feel faster on very smooth skatepark floors.

Turn over a skateboard wheel. Look on its side. You may see a number and a letter, such as 78A or 99A. That label tells how hard the wheel is. Hardness is measured with a tool called a durometer.

A durometer presses a small, blunt pin into the wheel. A soft wheel lets the pin sink in more. A hard wheel pushes back and lets the pin sink in less. The tool shows a number. Most skateboard wheels are measured on the A scale.

On the A scale, a lower number means a softer wheel. Soft wheels are about 78A. They roll smoothly over cracks and grip well. Hard wheels are about 99A to 101A. They slide more easily, which some skaters want for slides on smooth concrete.

A skater carries her board up the steps to the deck of a small ramp. The deck is the flat top at the edge. Up there, she is not moving. But she has something she did not have at the bottom. She has stored energy from being high up.

Energy is the ability to make things move or change. Energy stored in something lifted up is called potential energy. Potential means ready to happen. Gravity is ready to pull the skater down. When it does, the stored energy will turn into speed.

The higher something is, the more energy it stores. A board on the deck of a tall ramp has more stored energy than one on a short ramp. A heavier rider at the same height also stores more. Height and weight both add stored energy.

Climbing to the top of a ramp takes effort. Your legs work hard. You may breathe faster. Your muscles are using energy from the food you ate. Some of that energy gets stored in your body as height.

Energy does not appear out of nothing. It also does not vanish. It changes from one form to another. Food energy becomes muscle energy. Muscle energy lifts you and becomes stored energy. Scientists call this idea conservation of energy.

This is why the top of the ramp is a kind of energy bank. You put energy in by climbing up or by pushing hard to roll up. Gravity lets you take it back out as speed. You can only take out what you put in.

A rolling board can knock over a cone. A still board cannot. The rolling board has energy because it is moving. Energy of motion is called kinetic energy. The word kinetic comes from a Greek word about moving.

Speed matters a lot for kinetic energy. A board rolling fast has much more motion energy than the same board rolling slowly. That is why a fast fall hurts more than a slow one. More energy has to go somewhere when you stop.

Weight matters too. A grown-up rolling at walking speed has more motion energy than a small child at the same speed. Speed is how far something goes in a certain time. Faster and heavier both mean more kinetic energy.

A skater stands on the deck of a small ramp. She has lots of stored energy and no motion energy. She rolls down the curved wall. As she gets lower, she goes faster. Her stored energy is turning into motion energy.

At the bottom of the ramp, she is at her lowest point. She is also at her fastest. Almost all of her stored energy has become kinetic energy. The flat part at the bottom is called the flat bottom.

Now she rolls up the other wall. She slows down as she climbs. Her motion energy turns back into stored energy. At the top she stops for a moment. This back-and-forth change is an energy trade. Gravity does the trading.

Think about a playground slide. You climb the ladder. You sit at the top. Then gravity pulls you down the slope, faster and faster. A skate ramp works the same way. The board is like you, and the curved wall is like the slide.

A ramp’s curved wall is called a transition. It goes from steep near the top to flat at the bottom. The curve guides the board. It turns the drop into forward rolling. Without the curve, the board would hit the ground hard.

A slide has a ladder. A ramp may have steps up to its deck. Either way, you use your muscles to get to the top. The ride down gives you back that energy as speed. Ride only ramps sized for beginners, with pads and a buckled helmet.

Two friends try the same idea at a skatepark. One starts at the top of a small bank. The other starts at the top of a taller bank. They roll down without pushing. The rider from the taller bank is going faster at the bottom.

A higher start stores more energy. More stored energy turns into more motion energy at the bottom. More motion energy means more speed. So height buys speed. That is why big ramps are for skaters with lots of practice.

The speed does not grow as fast as the height, though. A ramp twice as tall does not make you twice as fast. It still makes you faster every time. Scientists can figure out the speed from the height. The chart shows some worked example numbers.

A low bank is a good first ramp. Wear a buckled helmet, wrist guards and pads, and have a grown-up nearby. First, push slowly toward the bank on flat ground. Keep your front foot over the front bolts and your back foot over the back bolts. Bend your knees.

Next, roll a short way up the bank. Feel the board slow down. Gravity is trading your motion energy for stored energy. Keep your weight centered between your feet. As you stop, turn your head to look back down the bank.

Then let the board roll backward down the bank. Riding backward with your feet in your normal stance is called fakie. Stay low and centered. A common problem is leaning back toward the bank, so the board slips out. The fix is to keep your weight over the board, not behind it.

Rub your hands together fast. They get warm. The rubbing is friction. Friction changed the motion of your hands into heat. A rolling skateboard does the same thing, only more slowly.

Each time a wheel squishes against the ground, a little energy turns to heat. The bearings warm up a tiny bit as they spin. The wheels make a rolling sound. Sound energy is energy too. The air pushes against the rider and takes some energy away.

None of this energy is destroyed. It just spreads out as heat and sound. The heat and sound cannot easily turn back into speed. So the board has less motion energy than before. Every ride slowly hands energy over to the world around it.

Think back to the marble in the bowl. It started at the rim. It never rolled higher than the rim. Each trip ended a little lower. That pattern tells us energy was leaving the marble every trip.

Each time the marble rolled, friction and air took a small share of its energy. That share turned into heat and sound. Less energy was left for climbing. So the next trip was lower. After many trips, almost no motion energy was left, and the marble stopped.

A skater on a ramp works the same way. If she rolls without adding energy, each wall she reaches is a little lower. She can never roll higher than where she began. To keep going, she has to add energy with her own legs.

A halfpipe is a U-shaped ramp. It has two curved walls facing each other and a flat bottom between them. A skater rolls up one wall, comes back down, crosses the bottom and rolls up the other wall. Energy trades from height to speed and back again.

Like the marble, a skater loses a little energy each trip. So how do skaters keep going? They pump. Pumping means bending low on the way into the curve, then standing up tall as the curve lifts you. The legs push, and that push adds energy.

Good pumping can add more energy than friction takes. Then each trip goes a little higher. That answers Sofia’s question: the skater does not stop because her legs keep adding energy. Learners practice pumping on a small ramp first, low and slow.