Force and Motion


Force and Motion Facts

Force and Motion ['fȯrs] [\ən(d)] ['mō -shən]

Something that moves or stops a body, and the act of changing place or position.

Kid riding bicycle on city roads

Motion makes the world go 'round. Motion makes the moon go 'round too. In fact, motion makes lots of things go. When we think of motion we often think of cars, bicycles, kids running, basketballs bouncing, and airplanes flying. But motion is so much more. Motion is important to our lives and impacts so many things that we do. Motion is the changing of position or location. But motion requires a force to cause that change. Let's learn about force and motion and the effects of these physical laws in our world.

What is Force?

Force is just a fancy word for pushing or pulling. If I push on something or pull on it, then I am applying a force to it. Force makes things move or, more accurately, makes things change their motion. Two natural forces that we have experienced are the force of gravity and magnetic forces magnetic forces.

Peach dropped into water with bubbles

These two forces act at a distance and do not require direct contact between the objects to function. Gravity produces a force that pulls objects towards each other, like a person towards the ground. It is the force that keeps the Earth revolving around the sun and it's what pulls you toward the ground when you trip. See Science Trek's site on Gravity.

Magnetism produces a force that can either pull opposite ends of two magnets together or push the matching ends apart. A magnet also attracts objects made of metal.

Types Of Contact Forces

There are 6 kinds of forces which act on objects when they come into contact with one another. Remember, a force is either a push or pull. The 6 are:

  • normal force
  • applied force
  • frictional force
  • tension force
  • spring force
  • resisting force

Let's investigate how these forces can be seen in our lives.

Normal Force

paperback books on a table. Close up

A book resting on a table has the force of gravity pulling it toward the Earth. But the book is not moving or accelerating, so there must be opposing forces acting on the book. This force is caused by the table and is known as the normal force. You can “see” the normal force in some situations. If you place a thin piece of wood or plastic (a ruler works) so that it is supported by both ends (by books perhaps) and place a small heavy object in the center, the piece of wood will bend. Of course, it wants to straighten out so it exerts an upward force on the object. This upward force is the normal force. You can feel the force yourself if you push down in the center of the piece of wood. The harder you push, the more the wood bends and the harder it pushes back.

Applied Force

Kid pushing elevator button

Applied force refers to a force that is applied to an object such as when a person moves a piece of furniture across the room or pushes a button on the remote control. A force is applied.

Frictional Force

Runner feet running on road closeup on shoe

Frictional force is the force caused by two surfaces that come into contact with each other. Friction can be helpful as in the friction that allows a person to walk across the ground without sliding or it can be destructive such as the friction of moving parts in a motor that rub together over long periods of time.

Tension Force

Soft focus of a fencing wire tensioner at a grassy field

Tension force is the force applied to a cable or wire that is anchored on opposite ends to opposing walls or other objects. This causes a force that pulls equally in both directions.

Spring Force

Close-up Shock Absorber and Coil Spring of Car Suspension System

The spring force is the force created by a compressed or stretched spring. Depending upon how the spring is attached, it can pull or push in order to create a force.

Resisting Forces

Falling leaves

Resisting force, like air resistance or friction, changes motion. Whether the forces actually stop or slow something depends upon your point of view. Air friction makes a leaf travel along in the wind. When you pick up a pencil, it's friction with your fingers that gets the pencil in motion. In each case, the friction makes the two things (like the air and the leaf) move together.

What is Inertia?

Inertia is actually not a force at all, but rather a property that all things have due to the fact that they have mass. The more mass something has the more inertia it has. You can think of inertia as a property that makes it hard to push something around.

What is Friction?

Wooden toy train scene

Friction is a force that happens when objects rub against one another. Say you were pushing a toy train across the floor. It doesn't take much effort or force, because the toy is light. Now say you try to push a real train. You probably can't do it because the force of friction between the train and the ground is more intense. The heavier the object, the stronger the force of friction.

Velocity

Car wheel spinning wheel and smoking, Race car doing lot of smoke on race track, Car wheel drifting

Velocity is the speed of an object in one direction. If an object turns a corner, it changes its velocity because it is no longer moving in its original direction.


Newton's Laws of Motion

Closeup Of Newton Cradle With Swinging Metal Balls

Some consider Sir Isaac Newton to be the greatest English mathematician of his time and perhaps one of the greatest scientists the world has known. According to a story, Newton saw an apple fall to the ground and he figured out that the same force which caused the apple to fall also governed the motion of the Moon and the planets. In 1687 Newton published his three laws of motion in the “Principia Mathematica Philosophiae Naturalis.” His three laws explained how the concepts of force and motion work.

Newton's First Law

Bowling ball on alley

Newton's first law of motion states: A body in motion tends to remain in motion, a body at rest tends to remain at rest unless acted on by an outside force.

So, if an object is moving – its inertia (mass) will tend to keep it in motion, and if something is at rest, its inertia will tend to keep it at rest.

From the Glenn Research Center: learn more about Newton's First Law. Check out this PBS video on the subject.


Newton's Second Law

Baseball, sports and training where a bat hit a ball closeup for exercise, fitness or a workout

Newton's second law of motion states that a force, acting on an object, will change its velocity by changing either its speed or its direction or both.

If your basketball goes rolling into the street and is hit by a bike, either the ball will change direction or its speed, or both. It will also be true for the bike.

From the Glenn Research Center: learn more about Newton's Second Law.  Check out this PBS video on the subject.


Newton's Third Law

Civil War Cannon Firing

The third law is probably the best-known of Newton's laws. It states that for every force and action, there is an equal and opposite reaction.

This is what causes a cannon to recoil when it fires. The 'kick' from the firing of the ammunition is what makes the cannon jump backwards.

From the Glenn Research Center: learn more about Newton's Third Law.  Check out this PBS video on the subject.

Top 10 Questions

March 2016

Thanks to Thanks to Dr. Kathryn Devine, assistant professor of physics, College of Idaho; and John Gardner, professor of mechanical engineering, Boise State University for their answers. for the answers.

  1. What is force, and how is it made?

    A force can be thought of as a push or a pull that makes something change its speed or direction. So, if you want to speed something up, you have to push or pull it to make it speed up or change direction. If you drop something, gravity acts as a force and makes it speed up toward the ground. Static electricity is another force that can suck your hair toward a balloon. There are many types of forces, but they all act as a push or pull that can make something speed up or down, or make it change its direction. (From Blake at Kamiah Elementary School in Kamiah)

  2. Why are Newton's three laws so important to science?

    One of the reasons why Newton's laws are so important is because Newton observed a lot of things around him. All of the scientists and naturalists of the age were trying to figure out how things move and why. He was able to look at all of these observations and fold them together into a set of rules that fit with one another and fit the observations. It's a great example of the scientific theory and the scientific method. These laws are very important in helping us understand planets, gravity, and just how things move. Newton laid the foundation for all of this. (From Emily at Owyhee Elementary School in Boise)

  3. What makes a ball bounce?

    When you throw a ball to the ground, you give it energy. That energy is reflected in its velocity. Think of it as speed. When the ball hits the ground, it stops all of a sudden. That energy doesn't go away but gets absorbed into the ball. The ball squishes up a little bit, but it wants to un-squish. It expands and bounces back up. It won't come back in exactly the same way, because some of that energy gets lost in the material of the ball. (From Creighton at Owyhee Elementary School in Boise)

  4. How big is the gravitational force and why?

    Technically speaking, a gravitational field would extend away from a mass forever. So, Earth's gravitational field would extend infinitely out. Physicists tend to think of it as how far does it extend until it matters less or some other force matters more. Once you get far enough away from Earth, gravitational forces from other objects, planets or stars start to take over and be more important. So around Earth, our gravitational field matters a lot. Once you start moving out to the solar system, the gravitational field from Earth still exists, but other things are going to play a larger role and their gravitational fields matter more. (From Alex at Sagle Elementary School in Sagle)

  5. Why does something drop when I throw it up?

    When you throw something, like a ball, up in the air, it has mass. The earth under your feet has mass, and gravity acts between the two masses. So, as you throw the ball in the air, gravity is always acting on the ball and changing the speed of it as it moves up. Gravity is always trying to pull it back to the center of Earth. As the ball goes up, it slows down as the force of gravity slows it down. It will eventually reach its highest point and stop altogether and then come back down toward you. It doesn't get to the center of Earth because the surface, or the ground, stops it. When it sits there, gravity is still acting on it, but the surface of Earth is pushing back on the ball with the same amount of force, so it doesn't move anymore. (From Austin at West Park Elementary School in Moscow)

  6. I was wondering why a slinky follows itself down the stairs?

    A slinky, like everything else, has mass and is affected by Earth's gravity. So, when you push a slinky off the stairs, Earth's gravity is going to pull the top end of the slinky over and create the force that pushes the front end down. Because the slinky is connected, that connection force causes the rest of the slinky to go with it. If it's a little bit off center, the force pulls the slinky with it, causing it to continue down the stairway and you start the whole process over again. Gravity is the key force working here. (From Ayden at Willow Creek Elementary School in Meridian)

  7. How did Earth's gravity form, and where did it ever change?

    Gravity is directly related to mass. Earth has gravity because it is made up of matter and has mass. Planets, including Earth, formed because there were a couple of smaller pieces of mass that were attracting each other. They came together and they stuck together because that's what gravity does. Gravity pulls objects together. So two objects together have more mass than when they are apart. Those two objects together then create a greater gravitational force pulling more objects toward them. Once there is enough mass, other types of molecules are pulled upon that make up gas. This makes up the atmosphere. We need gravity to keep an atmosphere on a planet as well. (From Jacob at Basin Elementary School in Idaho City)

  8. How does the moon circle around Earth?

    The moon is being pulled toward Earth at all times. So, why doesn't the moon just crash into Earth? Well, the moon is being pulled toward Earth, but it also has speed around Earth, or velocity. If Earth would vanish, the moon would start traveling in a straight line, because things travel in a straight line unless acted on by another force. Earth exerts a gravitational force on the moon, pulling it a little bit, causing it to change direction and rotate around Earth. (From Jaden at Owyhee Elementary School in Boise)

  9. How does a magnet work?

    The force at work is called a magnetic force. It is related to the electrostatic force. If you were to look at the tiny pieces that make a magnet, like a bar of iron that has been magnetized, you would see individual atoms that make up the iron in the magnet. All of the electrons in those atoms are lined up so their orbits are oriented in the same direction. That's the key to how a magnet works. All of the electric fields of the electrons are in the same direction, and all of those electrons are moving. This generates a magnetic field. So, because all of the electrons are aligned, a magnetic field is generated. If they are not aligned, there is not a magnetic field. (From William at Owyhee Elementary School in Boise)

  10. Are all metals affected by magnets?

    No, not all metals are affected by magnets. It has to do with how the atoms line up in an individual grain of metal. We classify metals as ferrous metals, which is related to iron containing metals, and nonferrous. So, it's mostly metals that contain iron that are attracted by magnets. (From Jenson at Owyhee Elementary School in Boise)