Laws of Motion
Newton's Life and Times
Isaac Newton was born in 1643 on a farm. His father died before he was born.
His grandmother raised him after his mother got remarried.
As a teenager, his mother pulled him out of school to work on the family farm. Isaac wasn't good at farming and didn't like it.
He went back to school and started at Trinity College, Cambridge University, when he was 18.
In 1665, Newton got his college degree and won money to keep studying.
Soon after, the university closed for 2\,\mathrm{years} because of a bad sickness (the bubonic plague) spreading across England. Newton went home and spent this time studying math and science by himself.
Reading Check
When was Isaac Newton born?
Who raised Isaac Newton for much of his childhood?
Why did Newton leave school as a teenager?
What university did Newton attend?
Why did the university close, and what did Newton do during that time?
Newton's Studies and Influences
In college, Newton learned ideas that are not taught in schools today; some of these ideas were later found to be wrong.
One old idea was the geocentric view: people thought the Earth was the center of the solar system. They believed the Sun traveled around Earth because that's how it looks from Earth.
Copernicus (in 1543) suggested heliocentrism: that all planets move around the Sun.
In the early 1600s, Kepler shared three laws that explained the math of how planets move around the Sun.
Newton studied the ideas of Copernicus, Kepler, Galileo Galilei, and others on his own. The years he spent at home during the plague gave him lots of time to think about these new ideas and try to understand them.
Reading Check
What was the geocentric view of the solar system?
Who suggested that all planets move around the Sun?
What did Kepler's three laws describe?
Name two scientists whose work Newton studied.
How did the time during the plague help Newton with his studies?
Newton's Masterpiece: Principia Mathematica
In 1687, Newton wrote a very important book called Philosophiae Naturalis Principia Mathematica. This is Latin for "Mathematical Principles of Natural Philosophy," and people usually just call it Principia. Newton was 44 years old when it was published.
The Principia showed four scientific laws. A scientific law is a statement or a math description of something that happens in nature. It tells what happens but not why it happens.
Newton created a math model to describe how planets move. He combined the ideas and observations from scientists like Copernicus, Kepler, and Galileo.
This model was first made for planets, but it also explains how all objects move, not just those in space.
The four laws in Principia are the main parts of this model. One of them is the universal law of gravitation, which explains how gravity works and controls how planets move.
The other three laws explain the connection between pushes/pulls (forces) and how things move. These are now called Newton's laws of motion.
Newton's laws are all connected to each other and to the universal law of gravitation, but we often study them separately to make them easier to understand.
Reading Check
What was the name of Newton's famous book?
What does a scientific law describe?
Which scientists' ideas did Newton use to build his model of planetary motion?
What does Newton's universal law of gravitation explain?
How many scientific laws did Newton present in Principia Mathematica?
What is a Force? Scientific Meaning
In everyday talk, "force" has many meanings (like a strong storm or forceful speech).
In science, a force is a push or a pull on an object.
A force can make an object speed up, slow down, or change direction. This is called acceleration.
To "exert" a force means to apply it, like when you kick a ball, you exert a force on it.
Forces can also change an object's shape, like when you squeeze a sponge.
Forces always happen when two objects touch or affect each other.
Forces are described by how strong they are (their magnitude) and their direction. The strength is measured in units called newtons (N).
The unit "newton" is named after Isaac Newton.
For example, lifting a banana needs an upward force of about 2\,\mathrm{N}. Lifting a 27\,\mathrm{lb} child needs about 120\,\mathrm{N}$.
We can draw forces using arrows. The length of the arrow shows how strong the force is, and the direction of the arrow shows which way the force is going.
Reading Check
In science, what is a force?
What three things can a force cause an object to do?
When do forces always occur?
What two things describe a force?
What unit is used to measure the strength of a force?
Newton's Third Law of Motion (Equal and Opposite Forces)
When one object pushes or pulls on a second object, the second object pushes or pulls back on the first object with the same strength but in the opposite direction.
These two pushes or pulls are called a "force pair," and they always act on different objects.
Common examples:
A swimmer pushes against a pool wall with her feet. The wall pushes back on the swimmer with the same strength but in the opposite direction. This usually moves the swimmer away from the wall because the wall is much heavier and the swimmer is lighter.
A car crashes into a wall: the car pushes on the wall, and the wall pushes back on the car with the same strength. This damages both the car and the wall.
Newton used things like colliding pendulums (weights hanging on strings) of different sizes to measure these forces. His experiments showed that the force on one pendulum was always the same strength as the force on the other, proving the third law.
The term “force pair” means the two opposite, equal forces that happen when two things interact.
Another example: on an airboat, the fan pushes air backward. In return, the air pushes the fan (and the boat) forward. Because the fan is attached to the boat, this push from the air helps the boat move forward.
Reading Check
What does Newton's third law of motion state?
On what do the two forces in a force pair act?
Describe the force pair example of a swimmer pushing off a pool wall.
How did Newton use pendulums to test his third law?
In the airboat example, what pushes the fan and boat forward?
Equal and Opposite Forces: Key Takeaways
Newton's third law shows that forces always happen in pairs when objects interact.
The results of these two forces are seen on different objects.
In many daily situations, it's easier to notice one force (the push or pull you make) than the opposite reaction force. The reaction force might be harder to see unless the objects are similar in weight or you are watching closely (like in pendulum experiments or car crashes).
Reading Check
What does Newton's third law highlight about forces?
Do the forces in a pair act on the same object or different objects?
Why is it sometimes harder to observe the reaction force?
What kind of objects make the reaction force easier to observe?
What concept is emphasized by Newton's third law?
Connections and Implications
The four laws (the three laws of motion and the universal law of gravitation) create a complete way to explain things happening both on Earth and in space.
Newton's laws show how measurements, math, and what we see in the physical world all come together to explain how objects move and interact.
The change from believing the Earth was the center of the solar system (geocentric) to understanding that planets orbit the Sun (heliocentric) shows how science gets better by testing ideas, using evidence, and improving what we know.
The ideas of force, acceleration (speeding up/slowing down), and action-reaction pairs explain many real-world things, from everyday movements to how planets orbit.
This work shows that science grows by getting rid of old ideas when new evidence appears. The Principia was a major turning point where using math became super important for describing nature.
Reading Check
What two types of phenomena do Newton's four laws explain?
What three things come together to describe how objects move and interact, according to Newton's laws?
What scientific shift is mentioned as an example of scientific progress?
What everyday occurrences are explained by concepts like force and acceleration?
Why was Principia considered a turning point in science?
Notable Examples and Figures Mentioned
Figure 2: A drawing showing a foot pushing a ball with a force of 50\,\mathrm{N}. The ball speeds up in the direction of the push. The force only exists while the foot is touching the ball.
Figure 3A: A swimmer pushes on a pool wall; the wall pushes back on the swimmer with the same strength but in the opposite direction.
Figure 3B: Pendulums crashing into each other; they swap equal and opposite forces; the pendulums swing away with the same speed.
Airboat example: The fan pushes air backward, while the air pushes the fan (and boat) forward. This shows a force pair in a real-world situation.
Reading Check
In Figure 2, what causes the ball to accelerate?
What happens to the force in Figure 2 when the foot stops touching the ball?
What is the action-reaction pair described in Figure 3A (swimmer and wall)?
What did the pendulum illustration in Figure 3B demonstrate?
How does the airboat example show a force pair?
Key Dates and Figures to Remember
Isaac Newton lived from 1643 to 1727.
He got his degree from Trinity College, Cambridge, in 1665. The college closed for 2\,\mathrm{years}.
Principia Mathematica was published in 1687 when Newton was 44 years old.
The four laws: three laws of motion + universal law of gravitation. The term “force” and the unit “newton (N)” are named after Newton.
Reading Check
What are the birth and death years of Isaac Newton?
In what year did Newton receive his college degree?
How old was Newton when Principia Mathematica was published?
What are the two main types of laws presented in Principia?
What scientific unit is named after Isaac Newton?
Quick Recap: Core Concepts in One Place
A force is a push or pull that can cause an object to accelerate (speed up, slow down, or change direction). It's measured in newtons (\mathrm{N}) and shown with arrows that have length (strength) and direction.
The Second Law of Motion (F = ma$$) explains how a force changes the motion of an object given its mass. The Third Law of Motion (action-reaction) says that forces always come in equal and opposite pairs that act on different objects.
The universal law of gravitation describes the force of gravity that controls how planets move and how things fall on Earth.
Newton's work connected observations of how planets move with everyday physics, creating a universal way to understand how things move and interact.
Reading Check
What are three ways a force can cause an object to accelerate?
What unit is used to measure force?
What is unique about the forces described in Newton's Third Law?
What does the universal law of gravitation explain?
What did Newton's work combine to create a universal framework for dynamics?