Have you ever thought about what really happens when you throw a ball or fire a slingshot? What gets projectiles moving? And what brings them to a stop? You know that a baseball doesn’t just send itself flying over the outfield fence, a golf ball won’t move until it’s struck by a club, and a soda can won’t toss itself into a recycling container. For anything to move, a force needs to be applied. So let’s get moving!
A force can be something simple, such as pushing a skateboard or pulling a wagon, or it can be something more complicated, like the thrust from a jet engine. Forces are all around us, but they are invisible—you can only see their effect.
ESSENTIAL QUESTION
What are the different forces that control movement, and what would life be like without these forces?
When you apply a force to something, what happens? If the force is big enough, you can get things moving. If you push or pull something, does it keep going? Or does it eventually stop?
thrust: a force that pushes an object forward.
physicist: a scientist who studies physical forces, including matter, energy, and motion, and how these forces interact with each other.
mechanics: the working parts of something.
BCE: put after a date, BCE stands for Before Common Era and counts down to zero. CE stands for Common Era and counts up from zero. These non-religious terms correspond to BC and AD. This book was printed in 2018 CE.
natural state: according to Aristotle, the way an object behaves when nothing is acting on it.
astronomy: the study of the sun, moon, stars, planets, and space.
MECHANICS: STUDYING MOTION
Ancient scientists and philosophers from every culture were fascinated by how and why things move. These first physicists looked closely at the world around them and tried to explain what they saw. The study of motion and forces is called mechanics.
Aristotle (384–322 BCE) was a Greek philosopher and scientist who studied lots of things, including physics, math, and biology. By looking closely at how things moved in the world around him, he developed a theory of motion to describe what he saw.
Aristotle believed that everything had a “natural state” it wanted to be in. According to Aristotle, water flowed because its natural state was to move, and a stone was motionless because its natural state was to be at rest. Aristotle believed that to change something’s natural state, a force must be applied to it, and the force must continue to be applied to keep it moving. Without an applied force, the object would return to its natural state.
Feel the Force!
Although there’s no “Force” like the kind you see in science fiction, forces are everywhere. What forces are acting on you right now? How many can you think of?
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One of the first ways ancient peoples studied motion was by observing the sky. By carefully watching and recording the movements of the sun, moon, planets, and stars, many different cultures used astronomy to create calendars. These calendars helped the Maya of Central America determine when to plant crops and told the Egyptians when the Nile River was likely to flood. What did these early calendars look like? Are they still used today?
For example, a horse cart’s natural state is at rest. For it to move, a horse needs to continuously apply a force. If the horse stops, the cart will return to its natural state and stop moving. To throw a stone, you apply a force to the stone to get it moving. But, according to Aristotle, once you let go you’re no longer applying a force, and the stone will fall to the ground and return to its natural state.
Aristotle’s idea of a natural state makes a lot of sense, because it describes what we see around us.
A bust of Aristotle made from marble, a Roman copy of a Greek bronze original by Lysippos from 330 BCE
credit: Ludovisi Collection
Whether you roll a ball across a floor or push a book across a table, both objects will eventually come to a stop. And any projectile, whether it’s a home run ball or a bullet fired from a gun, eventually comes to rest.
calculus: a branch of mathematics involving calculations.
gravity: the force that pulls objects toward each other and holds you on Earth.
friction: a force that resists motion.
For almost 2,000 years Aristotle’s ideas were used to explain how and why things moved. It wasn’t until Isaac Newton (1643–1727) challenged these long-held beliefs about motion that the world discovered that Aristotle was only partly right.
Sir Isaac Newton was an English scientist who is famous for discovering many things, including where color comes from and an important kind of mathematics called calculus. But it’s his study of motion and gravity that might be the most important.
NEWTON’S FIRST LAW OF MOTION
Since Aristotle, people had believed that all objects have their own natural states. But Newton looked at motion differently. He believed that if an object was moving, it would keep moving until something happened to stop it. And if an object wasn’t moving, it wouldn’t move until something got it moving. This became his first law of motion.
A illustration from Newton’s A treatise of the system of the world, 1731
NEWTON’S FIRST LAW OF MOTION:
An object at rest will stay at rest, and an object in motion will stay in motion at the same speed and direction unless acted upon by another force.
Newton’s idea might seem strange at first. How could something in motion just keep moving in the same direction, forever? If you stop peddling, your bicycle eventually slows to a stop. If you shove a book across a table, it probably won’t even make it to the edge. And even the hardest hit home run ball comes down somewhere.
But Newton believed that without a force acting on them, bicycles, books, and baseballs would keep doing what they’re doing, forever. According to Newton, a force must be at work to slow an object down and eventually stop it from moving. For most things, the force that acts on objects to slow them down is friction.
Friction happens when two things move against each other. There is friction between a bicycle tire and the road, and there is friction between a book and the tabletop it slides across.
inertia: the resistance of an object to a change in its motion.
mass: a measure of the amount of matter in an object.
accelerate: to increase the speed of an object’s movement.
velocity: a measure of an object’s speed and direction.
decelerate: to decrease the speed of an object’s movement.
Another way to describe Newton’s first law is to say that objects resist changes in motion.
Have you noticed that when a car comes to a sudden stop, you’re thrown forward against your seatbelt? And when the car lurches ahead, you’re thrown back into your seat? This resistance to a change in motion has a special term: inertia. Your body has inertia and wants to keep doing what it’s doing until another force is applied to change your motion.
The amount of inertia something has depends on its mass. Mass is the amount of stuff that makes up an object. Everything from ice cream to elephants has mass. The more mass an object has, the greater the amount of inertia.
Inertia is also the reason you wouldn’t want to kick a bowling ball like you would kick a soccer ball. A bowling ball has much more mass than a soccer ball and needs a much larger force to get it moving. And kicking it would be very painful! Ouch!
DID YOU KNOW?
Newton’s first law of motion is also called the law of inertia.
Sticking to the Ground
Have you ever been ice skating? The amount of friction between the skates and the ice is very low, letting you glide across the ice. Friction is also how your sneakers grip the pavement when you run. Without it, you wouldn’t get very far! If there was no friction, an object in motion would keep moving forever—unless something else came along to stop it.
Whenever something speeds up or slows down, it’s accelerating. Acceleration is a change in velocity. When you throw a ball, you’re accelerating it. When you catch a ball, you’re decelerating it. Acceleration can happen in any direction—forward or backward, up or down, even side to side. What determines how an object accelerates? Newton also tackled that problem, and figured it out with his second law of motion.
Velocity
Velocity is a measure of an object’s speed and direction. When a pitcher hurls a fastball, the ball’s speed is measured at 100 miles per hour. But the fastball’s velocity is 100 miles per hour toward home plate. Velocity is very important in ballistics—we want to know where projectiles are headed and how soon they’re going to get there!
What forces can you spot working in this photograph?
NEWTON’S SECOND LAW OF MOTION:
The acceleration of an object is proportional to and in the same direction as the force applied, and inversely proportional to the mass of the object.
That’s a mouthful. The second law of motion says that how something accelerates depends on just two things—the mass of the object and the force applied to it. How does the second law work, and what does it really mean?
The more force you apply, the more you can accelerate an object. But the more massive something is, the harder it is to accelerate and the greater force you’ll need to get it moving. The more things you put in the shopping cart, the more force you need to accelerate it. While it might not take much effort to push a skateboard, what would happen if you applied the same amount of force to a car? You wouldn’t be able to accelerate it like you can a skateboard!
WORDS TO KNOW
proportional: corresponding in size.
inversely: when something increases in relation to a decrease in another thing or vice versa.
DID YOU KNOW?
In math and physics, the word proportional means that when one thing increases, another thing increases, too. The term inversely proportional means that as one thing increases, another decreases. Can you think of something that is proportional? How about something that is inversely proportional?
When you throw or kick a ball, why does the ball, and not your arm or leg, go flying? Forces occur when things interact with each other. When you try to shoot a goal in soccer, your foot interacts with the ball. When an arrow hits a bulls-eye, the arrow interacts with the target. How do these pairs work together to get things moving or bring them to a halt? Newton solved the problem with his third law of motion.
Forces are an important part of archery competitions.
action force: the force created by one object that acts upon another.
reaction force: the force acting in the opposite direction to the action force.
recoil: to spring back suddenly as the result of an action force.
contact force: a force that occurs when two objects are touching each other.
NEWTON’S THIRD LAW OF MOTION
NEWTON’S THIRD LAW OF MOTION:
For every action (force), there is an equal and opposite reaction (force).
Forces always come in pairs. When your foot applies a force to a ball, the ball applies an equal force in the opposite direction to your foot! Your foot striking the ball is the action force and the ball pushing back against your foot is the reaction force.
If the forces are equal, how does one object move more, or accelerate more, than another? Why does a kicked ball sail into the sky instead of sending your foot backwards?
Remember, acceleration depends on force and mass. If the forces are equal, then the object with the least mass will accelerate the most. When you kick a ball, the forces between your foot and the ball are equal, but the masses are not the same.
Compared to you, the soccer ball has very little mass. When you apply the force, the ball accelerates quickly in the direction you kick. The reaction force of the ball accelerates you in the opposite direction, but it barely slows your foot down, thanks to your greater mass. But remember, if you tried to kick a bowling ball—ouch!
Equal and Opposite
Equal and opposite force pairs are everywhere. When a bat hits a ball, the bat exerts an action force on the ball and the ball exerts a reaction force on the bat. But because the ball has less mass than the bat, the ball accelerates the most in the direction of the swing! The acceleration of the bat in the opposite direction is called recoil.
So far, you’ve learned that forces are needed to get things moving, and to bring them to a stop. But what’s going on when things aren’t moving? Are there still forces acting on a ball before it’s struck or on a car parked on the street? You might already know the answer—gravity!
DID YOU KNOW?
Have you ever high-fived someone and felt it sting your hand? Did they feel it too? That’s because you’re both exerting forces on each other!
GREAT GRAVITY!
Gravity is everywhere. It keeps planets circling their stars and makes black holes black. Here on Earth, gravity is the reason you fall when you trip. It’s also why we say, “What goes up, must come down.” Everything that has mass, from the smallest atom to the farthest galaxy, feels the force of gravity.
Most forces we see in action are forces that push or pull things, such as the forces you use when steering a shopping cart through the grocery store or playing a game of tug-of-war. These are contact forces. Contact forces happen when things contact, or touch, each other. But gravity is different.
Gravity works at a distance, without objects having to touch each other. It pulls everything together, even the farthest galaxies. Isaac Newton discovered that gravity is universal, which means that almost everything in the universe exerts a force on everything else. And just as with other forces, we can’t see gravity, but we can see it bring a baseball into a fielder’s glove or feel it when we leap to take a jump shot.
The amount of gravity an object has depends on its mass. Big things, such as planets and stars, have a lot of gravity, while small things, such as people and basketballs, have much less. Because Earth is so massive, you feel its pull much more strongly than the planet feels your pull on it.
Mass and Weight
Sometimes, people confuse mass and weight. While mass is a measure of how much matter makes up an object, weight is a measure of how strongly gravity pulls on a mass. On the moon, your weight would be just one-sixth of your weight on Earth, but your mass would still be the same!
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Gravity also depends on the distance between things. The closer things are, the stronger the force of gravity is between them. But if they move apart, the pull between them weakens.
Both the planet and the astronaut are exerting a gravitational force on each other. Which force is stronger? Why?
For centuries, people have been studying gravity. In the sixteenth century, Galileo Galilei (1564–1642), an Italian scientist and philosopher, discovered something very important about gravity and how it affects falling objects. In Galileo’s time, people believed that heavy objects fell faster than lighter ones. Galileo wanted to test this, so he came up with an experiment.
Legend has it that Galileo climbed to the top of the Leaning Tower of Pisa in Italy and dropped two spheres of different masses. He timed how long each took to reach the ground—the two spheres hit the earth at the same time! This means that a falling object’s acceleration doesn’t depend on its mass. The earth’s gravity accelerates everything at the same rate, whether it’s a pebble or a person.
ESSENTIAL QUESTION
What are the different forces that control movement, and what would life be like without these forces?
Gravity is always there, pulling everything back toward the earth. So how do all these laws and forces affect the movement and paths of projectiles? The different forces that control motion, including gravity, all affect the way projectiles move through the air. In the next chapter, we’ll look at some projectiles and how they behave!
As the World Turns
Why don’t we feel the moon’s gravity? Even though the moon is 238,900 miles away, the moon’s gravity pulls on Earth and everything on the planet. Because we’re so far from the moon and our mass is so small, we don’t notice the moon’s pull. But big things on Earth—including oceans—do! In fact, ocean tides come from the sun and moon tugging on the oceans as the Earth rotates.
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OBSERVING FORCES OF MOTION
When something moves, a lot is going on that we don’t see. But we can make deductions based on our observations of different movements! Try moving different objects on different surfaces and see what you learn.
›Find a large, flat surface, such as a floor or tabletop. Make sure the surface can’t be damaged by water or by rolling or sliding objects! Try rolling a round object, such as a ball, and watch its motion carefully. How far does it go? Does something stop it, or does it stop on its own?
›Next, try sliding a book or other flat object. How does its motion compare to a rolled object? Does something stop it, or does it seem to stop on its own?
›Now, try sliding or rolling a plastic container. First, get it moving while it’s empty. How does its motion compare to the first two objects? Now, put water into the container (be sure to put the cap on or seal it) and get it moving. How does the full container’s motion compare to the empty container?
Questions to think about
What stops the object’s motion? Is it the same for each item?
Which object stops the quickest? Why?
Which object goes the farthest and why?
Why do you think the motions of the objects are different?
Does the type of motion (rolling or sliding) make a difference?
Does the weight of an object affect its motion?
Which object takes the most force to move? Which takes the least? Why?
Try This!
Try exploring the movements of other objects. What happens when you throw a ball? What gets it moving and what stops it? Do bikes or skateboards roll forever?
WORDS TO KNOW
deduction: a conclusion reached by reasoning or evidence.
Newton’s first law and the idea of inertia can help explain how and why things move, and why some things are harder than others to get going—and to stop. Can Isaac Newton help you get a coin into a cup without touching the coin?
›Place a playing card or an index card on top of a glass or clear plastic cup. Place a coin on top of the card.
›Can you think of a way to get the coin into the cup without touching the cup? First, try pulling the card slowly. What happens to the coin? What else can you try?
›Try flicking or hitting the index card. What happens to the coin?
Magic!
A classic magician’s trick is to pull a tablecloth out from under the dishes on a table, leaving them on the table! Try this at home with plastic or paper plates and cups. How is this like the coin and cup? What law of motion does a magician use in this trick?
Questions to think about
Can you flick the card fast enough so the coin simply drops into the cup?
Can you describe what’s happening using Newton’s first law and inertia?
What happens when you move the card more slowly? Why does the speed of the card matter?
What force makes the coin fall into the cup?
Try This!
When the card is removed quickly, the coin resists moving with the card because its inertia is greater than the force applied by the card. With the card out of the way, the coin falls into the cup. Challenge someone to see if they can figure out how to get the coin in the cup. Resist giving them hints!
Newton’s second law says that the acceleration of an object depends on the object’s mass and the force applied to it. Can we see the second law in action?
›Find a flat, open area to move a wagon or other small cart on wheels. Push or pull the empty cart several times in different directions. Get a feel for how the cart moves when it is empty.
›Place an object or two in the cart. Try moving the cart again. Is there a difference?
›Keep loading the cart up with more objects! How does this change the movement?
Questions to think about
When do you apply the most force and the least force?
Is it easier to accelerate the cart when it is empty or full?
When you push the cart, how does it move?
Is the cart harder to stop when it’s full or when it’s empty?
Can you use Newton’s laws to describe the difference?
Try This!
If your cart didn’t have wheels, would it be easier or harder to push? Find a container without wheels and repeat your experiment. What are your observations?
Newton’s third law says that for every action, there is an equal and opposite reaction. Test this yourself!
›Inflate a balloon. Once you’ve blown up the balloon, pinch the open end closed with a binder clip or clothespin. Don’t tie the balloon closed!
›Attach your balloon to a small toy car. Make sure the opening of the balloon is pointing in the opposite direction you want your car to move. Use tape to hold the balloon firmly in place on the car.
›Aim the car in the direction you want it to travel. Make sure its path is clear! You don’t want anything to interfere with the car’s motion.
›Remove the clip from the balloon and watch the car go!
Questions to think about
What’s happening when you remove the clip?
What direction does the air travel compared to the motion of the car?
What direction does the car move compared to the escaping air?
How does this demonstrate Newton’s third law?
Try This!
How does changing the amount of air in the balloon affect the car’s acceleration? Try using balloons of different sizes and shapes. How do they affect the motion of the car? How does the surface the car is on affect its speed and distance moved?
In the late sixteenth century, Galileo Galilei climbed to the top of the Leaning Tower of Pisa to conduct an experiment to find out if heavy objects fall faster than lighter ones. What do you think? Will a heavier object fall faster than a lighter one? Choose two balls of the same size but different masses to experiment with.
›Find a safe height for your ball drop. Make sure the objects you’re dropping can’t damage what they land on, or anything else!
›If you use a video camera to record the experiment, be sure it’s set up far enough away that it can’t be damaged.
›In your engineering notebook, number each test so you can record what happens. What do you think will happen when you drop the two balls at the same time? What is your hypothesis?
›From a safe height, carefully drop the two balls at the same time.
›Repeat your experiment several times! It’s important that you repeat your experiment so your results are consistent.
DID YOU KNOW?
Although nobody is really sure if Galileo performed his experiment from the Leaning Tower of Pisa, the tower really does lean!
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›Carefully observe how the balls fall. If you have a video camera that records in slow motion, use it!
›Record the results of each test in your engineering notebook. Does one hit before the other, or do they land at the same time?
Watch Out Below!
The air applies friction to falling objects, which can affect the outcome of this famous experiment. What happens when you take the air away? While we can’t all travel to the moon to drop bowling balls and feathers, some people do have access to an airless chamber!
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Questions to think about
How did the balls drop?
Did one hit before the other?
Did they hit at the same time?
Try This!
Repeat your experiment and use balls or objects of the same mass, but different size. Do the results change? Why or why not? What about objects of the same mass, but different shapes?
Galileo’s experiment showed that the force of gravity accelerates everything downward at the same rate. It doesn’t matter how much mass an object has. A bowling ball and a basketball dropped at the same time from the same height will hit the ground together. This means that whether you kick a ball, fire an arrow, or launch a rocket, gravity treats them all the same way.
Veritasium forces acting
