Newton's Three Laws of Motion for Middle School Science

How are Issac Newton, the three laws of motion, and a pandemic related? This pandemic didn't happen in 2020. It was all the way back in 1665. Between 1665 and 1666, the Great Plague of London killed around 100,000 people. The plague was spread by bacteria on the fleas that lived on the rats of London. There was no known cure, so the city shut down as much as possible to protect people.

Isaac Newton's college, Cambridge University, closed, so he went home to the country to wait for the pandemic to end. Many historians refer to this year as Newton's annus mirabilis (year of wonders) because of all of the amazing work he did. One of the theories he developed during this time were his three laws of motion.

You can learn more about Newton's three laws of motion by checking out the Google Slides below. If you would like a copy of the presentation, you can get it at Teachers Pay Teachers.

Newton's Three Laws of Motion

Sir Isaac Newton created three laws to explain all of the motion in the universe, from pebbles on Earth to massive supergiant stars. Interestingly, Newton did a lot of the work on his laws while stuck at home when his college was closed for two years because of the plague.

A common story tells of Newton discovering the law of gravity after an apple fell on his head. Falling apples may have contributed to Newton's interest in motion and gravity, but there is no evidence the story is true.

Newton's First Law of Motion

Newton’s first law of motion states that an object in motion will stay in motion, and an object at rest will stay at rest unless acted upon by an outside force. 

Newton’s first law of motion is also called the law of inertia because it describes inertia. Galileo originally wrote about inertia in the early 1600s. Realizing the importance of inertia, Newton made it his first law of motion.

Momentum is a measurement of how hard it is to change inertia. Larger and fast-moving objects have more momentum than small or slow-moving objects.

Friction

Newton’s first law of motion doesn’t appear to match what we observe on Earth. If we kick a soccer ball, the ball doesn’t keep rolling until another force stops it. The soccer ball slows down and eventually stops on its own.

Even though it doesn’t look like it, the soccer ball follows Newton’s first law of motion. It is being acted upon by an outside force. That force is friction.

Friction is the opposition to motion when an object moves across a surface. Friction always acts in the direction opposite motion, so it always slows down moving objects. Without friction, that soccer ball you kicked would keep rolling until it met another force.

The amount of friction affecting an object depends on the mass and surface area of the object and the roughness of the surface. Smooth ice has very little friction, while shag carpet has a lot of friction.

There are three types of friction, static, sliding, and rolling friction. Click on the links to read about each type of friction.

Intertia and Momentum

Inertia is the tendency of an object to stay in motion or stay at rest. An object’s inertia depends on its mass. Objects with more mass have more inertia, so it is harder to get them moving or stop them from moving.

Momentum is a measurement of how hard it is to change inertia. Larger and fast-moving objects have more momentum than small or slow-moving objects. For example, an elephant has a lot of momentum because it has so much mass. A bullet shot out of a gun also has a lot of momentum because it has so much velocity.

Momentum becomes important when two objects interact. The object with less momentum will have a greater change in velocity than the object with more momentum. For example, if a large truck runs into a small car, the small car will have a greater change in velocity than if it had been struck by another small car. 

A Newton’s cradle, like the one shown to the left, shows that the momentum within a system is conserved. If you drop a ball on one side of the cradle, the ball on the other side will rise to about the same height. On Earth, we lose some momentum to friction, but in a vacuum, a Newton’s cradle would rock back and forth forever unless acted upon by an outside force. 

Newton's Second Law of Motion

Newton’s second law of motion connects force, mass, and acceleration. The larger a mass, the more force it takes to speed it up or slow it down. Newton’s second law is usually known by the scientific formula force = mass x acceleration or F = m x a.

According to Newton’s second law of motion, if you apply the same force to a large object and a small object, the small object will accelerate faster than the large object.

This relationship is easy to see. We can easily toss pebbles across a river, but we can’t even budge a boulder.

Newton's Third Law of Motion

Newton's third law of motion states that for every action, there is an equal and opposite reaction. These are sometimes called action-reaction pairs or coupled forces.

When you push on a wall, the wall pushes back on you. If the wall didn't push back, your hand would go straight through it. 

The force's size in action-reaction pairs is the same, but the acceleration of each object might not be. Remember, small objects accelerate more than large objects under the same force. For example, when you jump, you are exerting force on the Earth, and the Earth is exerting force back on you. You move, and the Earth doesn't because the Earth is so much bigger than you. Similarly, you can knock over a stack of blocks because you are bigger than the blocks.

Space

The brilliance of Newton’s three laws of motion is that they apply everywhere, including in space. For example, the Earth revolves around the Sun because of Newton’s three laws of motion.

When the solar system formed over four billion years ago, the dust and debris of the cloud were spinning. Newton’s first law of motion says that objects in motion will stay in motion unless acted upon by an outside force. An outside force has not acted upon the planets formed from the spinning dust and debris of the early solar system, so the planets are still spinning.

In some ways, it is easier to observe Newton’s laws of motion in space because there is no friction in space. If we could throw a rock hard enough to escape Earth’s gravity, the rock would keep going in the same direction we threw it until it was acted upon by another force, perhaps the gravity from another planet.

Forces

Middle School Science