Newton’s third law of motion states that “for every action there is an equal but opposite reaction.” An example of action and reaction is two basket balls. If you roll one ball at the other ball, when it hits the other ball it will exert a force on the still one. In return, the still one exerted an equal, but opposite, force on the rolling ball. This reaction force caused the ball you rolled to slow down.

Before, your friend drove you off the ice by jamming the gas pedal. With the wheels spinning at full speed, you slowly got back on the pavement. Once the tires reached the pavement, there was enough friction for the wheels could grip the road, so you sped off at full speed. Your friend, still panicking over the ice, drove right into the back of his old truck. If you knew he was this bad with directions, you might not have let him drive. The force of the car hitting the truck was the action force. Which sure enough was met with a reaction force! This reaction force brought your car to an immediate stop.

I explained before that equal forces acting in opposite directions create a net force of zero. Action and reaction forces are equal forces acting in opposite directions. The reason they don’t cancel each other out is they are acting on different objects. When the ball you rolled hits the still one, the force of the moving ball acts on the still ball. The reaction force acts on the one you rolled. The two forces can’t be added together because they are acting on different ob

If you push something, the law of inertia says it should keep moving, but friction stops it. If you hold something in the air and let go, the law of inertia says it should stay there, but it falls. The reason it falls is gravity. Gravity is the force that pulls things toward Earth.

If something is in free fall, it has no forces acting on it except for gravity. Gravity is an unbalanced force. Because unbalanced forces can cause an object to accelerate, an object in free fall accelerates. Every second something is in free fall, it accelerates by 9.8 meters per second. So if an object free falls for two seconds, it would be traveling at 19.6 meters per second.

A demonstration of how air resistance can affect how fast a piece of paper and a ball fall to the ground

If a piece of paper and a marble are in free fall, they will fall at the same speed, so they should hit the ground at the same time. If you test this by just dropping a marble and a piece of paper you will find it is not true. This is because the objects are not in free fall. To be in free fall, gravity has to be the only force acting on the objects. When you just drop something, there is also air resistance. Air resistance is a type of fluid friction. Because friction acts in the opposite direction of the object’s motion, air resistance of an object falling downward is an upward force. This is because a falling object is coming down, so the opposite direction is up. If air resistance were equal for every object, objects would still fall at the same rate. Since we know they do not fall at the same rate, we know air resistance is different for different objects. The amount of air resistance acting on an object depends on the object’s surface area. If an object has a small surface area, it will have little air resistance. Because the piece of paper has a larger surface area than the marble, the marble will have less air resistance than the piece of paper. Just because an object has more air resistance than another object doesn’t mean it will fall slower. This is because of weight, the force of gravity on an object at the surface of the planet. Some people confuse weight and mass, they are different. Mass is how much matter is in an object, and weight is the force of gravity on an object. If the object with the high air resistance had a high weight, it might fall faster than the object that has little air resistance, like if you had a wrecking ball and a piece of paper. Even though the wrecking ball had more air resistance than the piece of paper, it would fall faster because the wrecking ball weighed more than the paper.

Terminal velocity

As the speed of a falling object increases, the air resistance does too. If an object falls long enough, the force of the air resistance will equal the force of gravity. Now the forces are balanced so the object will stop accelerating. When this happens it is called the terminal velocity. If a skydiver jumped out of a plane, he would start accelerating. After falling a while, the force of air resistance would equal the force of gravity. The forces are balanced, so the skydiver stops accelerating. This does not mean he will stop falling, he will just stop continuously gaining speed.

Universal Gravitation

Object	Weight in Pounds
Our Sun	26,933
Mercury	35
Venus	86
Earth	100
Mars	38
Jupiter	263
Saturn	111
Uranus	106
Neptune	141
Pluto	13
Our Moon	17

The force of gravity acts between all objects. This is the law of universal gravitation. This means you are attracted to everything else in the universe. You don’t notice it because the force is not very strong. The strength of the force depends on the masses of the objects. You are attracted to Earth because Earth has such a great mass. There are other things that have great masses, such as the moon and the sun. Why aren’t you attracted to these? The reason is the force of the gravity also depends on the distance between the objects. If you were on the moon, you would be close enough for it to attract you. Still the force of gravity on the moon is about 1/6 of what it is on Earth. On the moon you would weigh about 1/6 of what you do on Earth. Because the different planets have different masses, you would have a different weight on each.

If you have ever walked on slick ice, you might know how easy it is to slip and fall. Ice is very slippery because it has little friction. The surfaces of things have little bumps and scrapes that can be so small you can’t see them. These bumps and scrapes are called irregularities. Friction is caused by these irregularities getting caught on each other as two surfaces rub together. Some things like glass and ice don’t have many irregularities to get caught on, so there is little friction. Without the friction, you slip. On concrete, there are many things for your shoes to get caught on so you don’t slip.

Friction is a force that always acts in the opposite direction of the object’s motion. For instance, if you were sledding down a hill friction would be opposing the sled.

The force of friction wouldn’t actually pull the sled up the hill. It would only bring it to a stop. Once the sled stopped there would be no movement between the irregularities, so there would be no friction.

Your friend wasn’t strong enough to move the truck, so he helped you get to the gas station. Now both of you are back on the road, driving through some snow. As you go farther south, the snow turns into freezing rain, and your tires start to slip. Your friend, who you let drive, panics and jams the gas. You start moving forward a little faster, but without the friction it wasn’t much. You slowly move far enough to get back on dry pavement, where your wheels stop slipping because there is friction.

A demonstration of how weight can affect the amount of friction acting on a chair.

Sometimes friction would stop the sled much faster. This would happen if the force of the friction would be stronger. Two things can cause friction to change its force. One is the type of surface. If instead of the sled having a hard, smooth, plastic bottom it had something like carpet for a bottom, there would be much more friction, so the sled would come to a stop faster. The other thing is how hard the surfaces are pressed together. If a 200 pound man was riding the sled there would be much more friction than if a 50 pound child was riding it. Sometimes in the winter we put snow tires on our cars. This is because the surfaces on snow tires cause more friction than regular tires do.

Friction is sometimes very useful, but sometimes it isn’t. Without friction it would be very hard to move around. Walking would be kind of like walking on ice, but on ice there is at least some friction. Without friction you wouldn’t be able to move. Sometimes friction is not useful. In a car engine, there are many moving parts. These parts rub together, and they produce friction. The friction produced when surfaces rub together is called sliding friction. The force needed to overcome sliding friction is more powerful than the force needed to overcome fluid friction. This is why we put oil in our engines. With the oil between the moving parts, the sliding friction becomes fluid friction. With the parts oiled, it is easier to overcome the friction. Without friction, we wouldn’t have to worry about putting oil in our engines.

Rolling friction is another type of friction. We use rolling friction to reduce the force needed to overcome sliding friction. If you had two boards with marbles between them it should be pretty easy to move the top board. Sometimes in machines there are ball bearings between moving parts to reduce friction. Ball bearings are like marbles made of metal.

Remember when you were pushing the car. When it was rolling down the road, it was an example of rolling friction. If the car was in park, and you tried to push it there would be sliding friction. Now, instead of the wheels rolling on the road, they slide across it. This makes it much harder to push.

A demonstration showing how different forces on the same mass affects acceleration.

Force, mass, and acceleration are all related. Newton’s second law of motion explains how. Acceleration = force divided by mass. If something is accelerating it is constantly gaining speed. If you were driving and your foot was on the gas pedal, and you were constantly gaining speed, you would be accelerating. Suppose your friend was driving the truck, and you were driving the car. You both ran out of gas. To get to the gas station you both use the same amount of force to push your vehicles. Which one will move faster? The car would, because it has less mass. The number ten represents your equal forces. If five represents the mass of the truck, and one represents the mass of the car, the car would have a higher acceleration. This is because ten (the force) divided by five (the truck’s mass) equals two (the truck’s acceleration). Ten (the force) divided by one (the car’s mass) equals 10 (the car’s acceleration). You can see that the car accelerates five times as fast as the truck because it has1/5 as much mass.

A demonstration showing how the same force acting on different masses affect the objects' acceleration.

You can also rewrite Newton’s second law so it will be force equals mass times acceleration. To find the amount of force acting on something we multiply the mass of what is producing the force by the rate at which it is accelerating. The product of the two is the amount of force. This amount is usually measured in newtons. A newton is equal to the force that is needed for a one kilogram object to accelerate at one meter per second per second. If you traveling at one meter per second per second, it would mean every second you were moving you would go one meter per second faster than the previous second.

Mass and acceleration change in opposite ways. If you want something to accelerate faster, you would need to decrease its mass. This also works the other way around, if you add to an object’s mass it will accelerate slower.

If you are playing hockey and the puck is sitting on the ice, it will stay there until you hit it or another force acts on it. Why? The answer is inertia. Inertia is the tendency of an object to resist change in its motion. This means the object does not want to move. Newton’s first law is based on inertia. This is why it is sometimes called the law of inertia. It states that an object at rest will remain at rest. Also, an object moving at constant speed will keep moving at constant speed, unless acted upon by an unbalanced force.

When you hit the puck it moves across the ice. Inertia would cause it to keep moving at the same speed until it hit the boards, but the friction between the rubber puck and the uneven ice causes it to slow down. Friction is the unbalanced force that stops the puck from continuing at the same speed.

You now have your friend pushing the car with you, not against you, and you’re a quarter of the way there. After you got the car moving, you noticed it was easier to keep it moving than it was to originally start it moving. This is because it has inertia. When it wasn’t moving, its inertia made the car resist rolling. Once it was moving, the inertia made it resist slowing down.

Mass

Have you ever noticed how it is harder to move something heavy versus something light? Like if you have a balloon filled with water, and a balloon filled with air, the one that is filled with water is harder to move. This is because it has more mass. Mass is the amount of matter in an object. Mass makes it harder to move the water balloon because the more mass an object has the more inertia it has.

Now that you know this, you are happy you weren’t driving a semi truck in the car running out of gas example. You remember how the car’s inertia made it hard to push it. If the amount of inertia an object has depends on the mass of the object, the truck would have a lot of inertia because of its great mass. It would be much harder to get the semi-truck rolling than it was to get the car rolling.

What is a Force?

The scientific definition for force is simply a push or a pull. For example, when you do homework you exert a force on your pen or pencil because you push and pull it across the paper.

Sometimes two forces act on something. Like if two people were pushing a shopping cart. If they were pushing in the same direction, you would add the two forces together. The sum of the forces is called the net force. In this case the net force is an unbalanced force. An unbalanced force is a force that changes an object’s motion or causes it to accelerate. The arrows show different forces and their direction, the wider the arrow the stronger the force.

Another way to look at this is you and your friend are riding in a car. It runs out of gas one mile from a gas station. You need to push the car to the gas station. If your friend is smart, he will push in the same direction you push, which is toward the gas station. If he does, your forces are added together. With this larger force, the car will move faster.

You can also have two forces acting in different directions. When the forces are equal and acting in different directions they balance each other out. When this happens, there is no net force because it is a balanced force. If one person was at the front of the shopping cart pushing backwards, and one person was pushing with equal force in the opposite direction, the forces would be balanced, and the shopping cart would not move. Back to the car running out of gas example, your friend might push the car in the opposite direction you are. If he does, your two equal forces in opposite directions balance out to zero net force. This means the car will not move.

Something else can happen when forces push or pull in opposite directions. If one force is more powerful than the other, they will not balance out to zero net force. This is because one force is stronger than the other, so the weak force is not strong enough to completely balance out the stronger force. Suppose there is a person in the front of a shopping cart and one on the other end. They are pushing in opposite directions, but one of them is pushing with a much greater force. Because the forces are not balanced, the shopping cart is moving in the direction that the person with the stronger force is pushing. Back to the car running out of gas example again, it would be like if your friend was pushing in the opposite direction you were, but this time you weren’t pushing with equal forces. If your friend was pushing harder than you, the forces would not balance each other. The car would roll away from the gas station.

Force:

Every time you move something, use your pencil, or walk, you are using force. These forces are examples of everyday life because that is what a force is, everyday life. Every day you use force to make your life easier. Sometimes it is hard to understand how forces help you.

Newton’s First Law of Motion:

Newton’s first law of motion explains inertia, another force. Inertia helps explain why things stay where they are. Think about how many times you would lose your papers if inertia didn’t make it stay where you put it.

Newton’s Second Law of Motion:

Sir Isaac Newton thought that you could find the acceleration of an object, so he wrote this law to explain how. It states that acceleration equals force divided by mass.

Friction:

Friction is a force that slows things down. This force can be both useful and harmful. Friction has two affects on a capsule reentering Earth’s atmosphere. The friction between the capsule and the atmosphere cause extreme heat. This is the harmful part. After the capsule comes through the atmosphere it uses parachutes to slow it down. The parachutes use air resistance, a type of friction, to slow it down. Without fiction the capsule would not slow down.

Gravity:

You may want to become an astronaut so you could float around, but do you know why you would float? The reason is there is no gravity in space. Gravity is the force that pulls you towards Earth.

Newton’s Third Law of Motion:

Newton’s third law says that for every action there is an equal and opposite reaction. Rockets are propelled upward because of a reaction force. The exhaust gases cause a reaction force that pushes the rocket into the air.

Momentum:

Momentum is a force that makes it hard to stop an object. If you were running, you would have momentum. Because of this, if you tried to stop really fast, you might fall because the momentum wants you to keep going forward.