# Newton's Laws of Motion

## Newton's Laws of Motion

Sir Isaac Newton was an English physicist and mathematician who lived in the 1600s and 1700s. One of his most famous works is his three laws of motion. Newton's three laws were first introduced in his book *The Principia*, which was published in 1687. **Newton's three laws** explain how objects behave when still, when moving, and how forces relate to their movement. **Force** is a push or pull on an object. A person can see force when a ball is kicked; how that ball moves is explained by Newton's laws. It can also be seen when NASA launches a rocket or a child makes a toy rocket. The amount of force needed to move that rocket and how it moves is explained by Newton's laws.

## Newton's Laws of Motion

The study of kinematics (1D and 2D motion) was about describing how things move, from displacements to velocity to acceleration. However, when Newton began to do his work, he was concerned with explaining why things move the way they do. He achieved that by defining his three laws of motion. These use the concept of **force** (a push or pull) to explain the motion we observe in the world around us. Let's take a look at each of the laws of motion one by one.

## Newton's Three Laws

So, the question now is, what are Newton's three laws? Newton's three laws are this:

1. The **law of inertia** states that an object at rest (not moving) will stay at rest and an object in motion will stay in motion unless acted upon by an outside force. When moving, the object moves in a straight line and at a constant speed.

2. **Newton's second law** shows how force, the mass of an object, and acceleration are all related by giving a formula:

{eq}f = m \cdot a {/eq}, where *f* is the amount of force, *m* is the mass of the object, and *a* is the acceleration of the object. This law is also known as the **Law of Force and Acceleration**.

3. **Newton's third law** states that for every action there is an equal and opposite reaction.

These three laws affect every aspect of day-to-day life. They explain how force affects motion and objects. Remember, **force** is just a push or pull on an object. Newton's laws apply to everything from standing up to throwing a ball to driving a car. It is impossible to stand up from a chair without applying Newton's third law â€” a person's feet apply force to the ground, while the ground pushes back and allows the person to get up. When throwing a ball, it will continue to move in the same direction until another force is applied to it, following Newton's first law. The force needed to accelerate a car can be calculated based on Newton's second law, and his first and third laws factor into how that car is able to move.

### Newton's First Law

What is Newton's first law? As stated previously, the first law is the law of inertia. It states that an object at rest will stay at rest and an object in motion will stay in motion unless acted upon by an outside force. To put it in extremely simplified terms, **inertia** is objects being stubborn. If an object is moving, it wants to remain moving in a straight line and at the same speed. If an object is at rest (not moving), it wants to stay that way. Force has to be applied to get it to change.

Imagine being a passenger in a car and the driver suddenly slams on the brakes. What happens? The passenger's body keeps moving forward until the seatbelt stops the movement. This is because of inertia. The passenger's body was moving with the car. When the driver suddenly braked, a force was applied to the car but not the passenger. The passenger continued to move forward until the seat belt applied enough force to stop her. What about when the car starts moving again? The passenger goes back in their seat slightly, right? Again, it is because of inertia. The body was at rest and wanted to stay that way until enough force was applied to get it moving with the rest of the car.

If a block is slid across a table, it doesn't have a seatbelt to stop it, so why does it stop? That has to do with friction. The force of friction (the table rubbing against the block) acts on the block to slow it down and stop it. If there was no friction, the block would keep going and going.

### Newton's Second Law

What is **Newton's second law**? As stated above, his second law, or the **law of force and acceleration**, is a math equation.

{eq}f = m \cdot a {/eq}

This equation says that the force needed to move an object can be calculated by taking its mass and multiplying it by the acceleration. It also says the more mass an object has, the more force is needed to move it. Obvious, right? Acceleration is also factored in, though. Think of a soccer player. A soccer ball does not have a lot of mass, so not a lot of force is needed to get it to move. If a soccer player were to kick the ball gently, it would move but wouldn't accelerate hard or have a lot of speed. If the goalie kicked the ball across the field to a teammate, however, the extra force on that same amount of mass would cause much greater acceleration of the ball.

### Newton's Third Law

While Newton's first two laws are closely related to each other, Newton's third law tends to stand alone. His third law says that every action has an equal and opposite reaction. But what does that mean? These two forces, the action and reaction, are equal in strength but opposite in direction. When playing tennis, the racket applies a force to the ball, making it move across the net. But the ball also applied a force to the racket in the opposite direction. Since the ball has a smaller mass, the effect of that force is more clearly seen as it sails away from the racket.

How about bouncing a ball? When the ball hits the ground, it applies a force to the ground. The ground applies the same, but opposite, force to the ball, causing it to bounce up. The ground (part of the Earth!) is much bigger than the ball, so the effect of the force on the ground isn't even noticeable.

Remember the part in the beginning about standing up? To stand up, force must be applied to the ground by a person's feet, or to the chair by their hands and body. The chair and ground apply the same force back, allowing the person to stand up. Sometimes the chair moves, showing the effect of the force that was applied to it. Without these equal and opposite forces at work, it would be impossible to do almost anything, including standing up from a chair. Not sure about that? Try it! Use no force at all and see if standing up is possible.

## Newton's Laws of Motion Examples

Now let's look at some of Newton's laws of motion examples.

### Example of Newton's First Law

When ice skating, rollerblading, skateboarding, or anything similar, it requires force to get a person moving. Once moving, the person goes in a straight line unless another force is applied to make the skates or board turn. This is the law of inertia at work. Eventually, the friction between the wheels (or blade) and whatever surface they are on applies enough force to slow down or stop the object, so another push with the feet (force being applied) has to happen to keep moving. Again, this is Newton's first law. Forces are getting the skater moving and friction (another force) is stopping the movement.

### Example of Newton's Second Law

What about this scenario? An intense game of basketball is going on in the park. A player leaps into the air and shoots a basket to the cheers of his teammates. *If he applied 9 Newtons (a measurement of force) to the ball that weighs 0.6 kg, how much acceleration was the ball experiencing?*

Using the formula for Newton's second law, it is {eq}9 = 0.6 \cdot a {/eq} Solving for *a*, 9 divided by 0.6 is 15. The ball was experiencing an acceleration of 15 meters per second squared.

What if the ball accelerated at only 12 meters per second squared? How much force did the basketball player apply?

{eq}f = 0.6 \times 12 = 7.2 {/eq} Newtons of force

### Example of Newton's Third Law

At the other end of the park, a kid is pushing a friend on the swings. As the child pushes, the swing flies higher into the air, but the child has to take a step back. What happened? She applied force to her friend's swing, which made it move forward into the air. The swing also applied force to the child, though, enough to make her take a step back to keep her balance.

## Lesson Summary

Sir Isaac Newton was a physicist and mathematician who came up with laws describing the relationship between **force** (a push or pull on an object), objects, and how they move. **Newton's three laws of motion** are:

**The law of inertia**states an object in motion will stay in motion (in a straight line and constant speed) and an object at rest will stay at rest unless acted upon by an outside force.**Newton's second law**, also known as the**law of force and acceleration**, shows the relationship between force, the mass of an object, and its acceleration. {eq}f = ma {/eq}

**Newton's third law**states that every action has an equal and opposite reaction.

## Newton's First Law

**Newton's first law** says that an object in motion will remain in motion, at a constant velocity and in a straight line, unless acted upon by an unbalanced (or net) force.

What does this mean? Well, we tend to think of forces as being things that keep objects moving. To keep a shopping cart moving, it seems like you have to keep pushing it and applying a force. However, according to Newton, that's not true. When you let go of a shopping cart, there is still a force acting on it: the force of friction. If you push a shopping cart in space and then let go, it will keep going in a straight line, at a constant velocity, forever. It's the unbalanced force of friction that makes it stop when you push it along the ground on Earth.

Thanks to Newton's first law, we know that if we compare the forces acting on an object in each direction, we can figure out whether the motion will stay the same or change. If the forces are balanced, a stationary object will stay stationary, and a moving object will keep moving at the same velocity. If the forces are unbalanced, the object will accelerate, which means it will either slow down or speed up.

For example, if there is more force to the left than there is to the right, the object will have an acceleration pointing to the left.

## Newton's Second Law

**Newton's second law** is related to the first law, but tells us exactly how much acceleration we get because of those forces being unbalanced. It says that the acceleration of an object is proportional to the net force on that object and inversely proportional to the mass of the object. This can be written as an equation which looks like this:

*F = ma*

Here, *F* is the net force on the object measured in newtons (N), *m* is the mass of the object measured in kilograms (kg), and *a* is the acceleration of that object measured in meters per second squared (m/s.

This means that an object with a bigger mass (heavier) takes a greater force to accelerate it. It also means the larger the unbalanced (or net) force, the more acceleration will occur. It's an equation we can use to solve a lot of problems.

## Newton's Third Law

Finally, **Newton's third law** is a little different to the others and stands on its own. Newton's third law can be summarized by the famous phrase, 'Every action has an equal but opposite reaction.' But that sounds kind of vague - we can get a little more specific than that. A better way of saying it is as follows:

If body A applies a force to body B, body B applies an equal but opposite force back on body A.

This means that if you push on a table with a force of 3 N, the table pushes back on you with the force of 3 N. This might seem strange, because people wonder how anything can move anywhere with such a law. However, you and the table are quite different to each other. You have different masses, and you have different amounts of grip on the ground. You're also capable of moving because you can push the earth in one direction slightly as you move the other way. This push doesn't affect the rotation of the earth in a noticeable way, but it's enough to allow you to move.

Think about it this way: if you're driving down the road and a fly gets smashed on your windshield, both you and the fly feel a force given by the equation *F = ma*. The car has a large mass and undergoes a small acceleration. The fly has a small mass and undergoes a large acceleration. However, the force you feel is exactly the same. Just because the force is the same, doesn't mean the two objects feel the same acceleration.

## Lesson Summary

In this lesson, we summarized Newton's famous laws of motion, which explain how motion is caused by forces - pushes or pulls. **Newton's first law** tells us that an object will remain moving at a constant velocity and in a straight line unless the forces on it are unbalanced. This means that, contrary to popular belief, you don't need to apply force to keep something moving unless you are overcoming friction (for example when pushing a shopping cart).

**Newton's second law** tells us how much acceleration is produced when forces are unbalanced. It tells us that the net (or unbalanced) force is equal to the mass of the object times its acceleration. Finally, **Newton's third law** tells us that if you apply force to an object (and action), it applies an equal but opposite force back onto you (a reaction).

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## Newton's Laws of Motion

The study of kinematics (1D and 2D motion) was about describing how things move, from displacements to velocity to acceleration. However, when Newton began to do his work, he was concerned with explaining why things move the way they do. He achieved that by defining his three laws of motion. These use the concept of **force** (a push or pull) to explain the motion we observe in the world around us. Let's take a look at each of the laws of motion one by one.

## Newton's First Law

**Newton's first law** says that an object in motion will remain in motion, at a constant velocity and in a straight line, unless acted upon by an unbalanced (or net) force.

What does this mean? Well, we tend to think of forces as being things that keep objects moving. To keep a shopping cart moving, it seems like you have to keep pushing it and applying a force. However, according to Newton, that's not true. When you let go of a shopping cart, there is still a force acting on it: the force of friction. If you push a shopping cart in space and then let go, it will keep going in a straight line, at a constant velocity, forever. It's the unbalanced force of friction that makes it stop when you push it along the ground on Earth.

Thanks to Newton's first law, we know that if we compare the forces acting on an object in each direction, we can figure out whether the motion will stay the same or change. If the forces are balanced, a stationary object will stay stationary, and a moving object will keep moving at the same velocity. If the forces are unbalanced, the object will accelerate, which means it will either slow down or speed up.

For example, if there is more force to the left than there is to the right, the object will have an acceleration pointing to the left.

## Newton's Second Law

**Newton's second law** is related to the first law, but tells us exactly how much acceleration we get because of those forces being unbalanced. It says that the acceleration of an object is proportional to the net force on that object and inversely proportional to the mass of the object. This can be written as an equation which looks like this:

*F = ma*

Here, *F* is the net force on the object measured in newtons (N), *m* is the mass of the object measured in kilograms (kg), and *a* is the acceleration of that object measured in meters per second squared (m/s.

This means that an object with a bigger mass (heavier) takes a greater force to accelerate it. It also means the larger the unbalanced (or net) force, the more acceleration will occur. It's an equation we can use to solve a lot of problems.

## Newton's Third Law

Finally, **Newton's third law** is a little different to the others and stands on its own. Newton's third law can be summarized by the famous phrase, 'Every action has an equal but opposite reaction.' But that sounds kind of vague - we can get a little more specific than that. A better way of saying it is as follows:

If body A applies a force to body B, body B applies an equal but opposite force back on body A.

This means that if you push on a table with a force of 3 N, the table pushes back on you with the force of 3 N. This might seem strange, because people wonder how anything can move anywhere with such a law. However, you and the table are quite different to each other. You have different masses, and you have different amounts of grip on the ground. You're also capable of moving because you can push the earth in one direction slightly as you move the other way. This push doesn't affect the rotation of the earth in a noticeable way, but it's enough to allow you to move.

Think about it this way: if you're driving down the road and a fly gets smashed on your windshield, both you and the fly feel a force given by the equation *F = ma*. The car has a large mass and undergoes a small acceleration. The fly has a small mass and undergoes a large acceleration. However, the force you feel is exactly the same. Just because the force is the same, doesn't mean the two objects feel the same acceleration.

## Lesson Summary

In this lesson, we summarized Newton's famous laws of motion, which explain how motion is caused by forces - pushes or pulls. **Newton's first law** tells us that an object will remain moving at a constant velocity and in a straight line unless the forces on it are unbalanced. This means that, contrary to popular belief, you don't need to apply force to keep something moving unless you are overcoming friction (for example when pushing a shopping cart).

**Newton's second law** tells us how much acceleration is produced when forces are unbalanced. It tells us that the net (or unbalanced) force is equal to the mass of the object times its acceleration. Finally, **Newton's third law** tells us that if you apply force to an object (and action), it applies an equal but opposite force back onto you (a reaction).

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#### What are Newton's three laws of motion?

The first law states an object at rest will stay at rest and an object in motion will stay in motion unless acted upon by an outside force. His second law is an equation, f = m x a, where f is force, m is mass, and a is acceleration. His third law states that every action has an equal and opposite reaction.

#### What is Newton's 2nd law called?

Newton's second law is as known as the law of force and acceleration. It is a math equation stating: f = m x a, where *f* is force, *m* is mass, and *a* is acceleration.

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