Science

How is force related to motion? What is the difference between balanced and unbalanced forces? These are questions that will be answered over the course of this unit. This unit will include concepts on forces, friction, Newton’s Laws of Motion, gravity, and weight and mass. You will also learn about each kind of force and its causes. Mathematical formulas and problems will be presented as a means of explaining the before-mentioned topics. We will discuss Newton’s Laws of Motion in detail and why they are important to the study of motion and forces..

GLOSSARY OF KEY TERMS
force	A force is a push or a pull on an object. A force happens when two objects interact—that is, when one object does something to the other object. When the interaction stops, the force stops, too.
contact forces	Contact forces happen when objects touch each other. For example, contact forces happen when a person kicks a ball or pulls a wagon. Other examples of contact forces are sandpaper rubbing on a piece of wood, wind blowing against a moving car, and a rubber band stretched around a newspaper.
Hooke’s Law	Law of Elasticity, which states that the stretching of a solid body (e.g., metal, wood) is proportional to the force applied to it. The law laid the basis for studies of stress and strain and for understanding of elastic materials.
newton	The newton is the Standard International (SI) unit of force. In physics and engineering documentation, the term newton(s) is usually abbreviated N. One newton is the force required to cause a mass of one kilogram to accelerate at a rate of one meter per second squared in the absence of other force-producing effects. In general, force (F) in newtons, mass (m) in kilograms, and acceleration (a) in meters per second squared are related by a formula well known in physics:
unbalanced force	the total force on an object is in one direction
balanced force	forces that act in opposite directions and are equal in size, thus canceling each other out
friction	Friction is a force that acts in a direction opposite to the moving object’s motion.
sliding friction	solid objects slide over each other: A sled sliding over the snow is an example of sliding friction.
rolling friction	Friction is reduced by placing objects on wheels or ball bearings. This is called rolling friction. Moving a file cabinet on wheels is an example of rolling friction.
fluid friction	force exerted by a fluid
First Law of Motion	an object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force.
inertia	property of matter that tends to resist any change in motion. Everything that has mass also has inertia.
Second Law of Motion	force, mass, and acceleration are related
Third Law of Motion	for every action, there is an equal and opposite reaction.
gravity	force of attraction between two objects: This force is responsible for accelerating an object towards the Earth.
Law of Universal Gravitation	all objects in the universe are attracted to each other through a force of gravity. The size of that force is dependent upon the mass of the objects and the distance between them.

In the previous unit, we discussed motion and its related topics. Motion and its change in position, characterized by speed and direction, affect what happens when force is applied. To discuss forces and gravity, it is necessary to have prior knowledge of the material presented in the previous unit.

A force is a push or pull. A force gives energy to objects, sometimes causing them to move, stop, or change direction. This definition shows how force is related to motion. Some examples of force might include:

This list could be endless. So we could say that a force makes things move faster or slower, stop, change direction, or even size or shape. What about forces causing an object to change size or shape? This is a new concept that we will discuss later in this unit.

A force can also affect objects without making them move. A force can act on a stationary object without making it move. An example: sitting on a cushion, which is a stationary object, and the sides bulge out. The force causes the object’s shape to change. Objects that return to their original shape when the force is removed are called elastic substances. An example of an elastic substance is a trampoline. When the forces that stretch the trampoline are removed, it returns to its original shape. This example relates to Hooke’s Law, which states that the stronger a force, the greater the stretch. However, if force causes an object to stretch beyond its elastic limit, Hooke’s Law no longer works. An example: stretching a rubber band beyond its elastic limit and the rubber band snaps. Some objects, such as clay, do not return to their original form after being stretched by a force. They change their shape permanently. This is called plastic behavior.

We know that forces can affect objects in various ways. We are all familiar with forces that are visible, such as kicking a football. There are four main types of invisible forces such as magnetism, electricity, gravity, and strong nuclear forces. We will not be discussing strong nuclear forces in this unit. These forces do not need objects to be touching. Magnetism, electricity, and gravity act at a distance. Magnetic forces are natural or invisible forces that attract two objects together, such as the north and south poles. Electrical forces are natural forces that are found all around us. This is the force between two electrically-charged particles. This helps bind the atoms together in matter. An example: static electricity that causes your clothes to stick together. When magnetic and electrical forces are combined together, we have what is called electromagnetic force. These forces work so closely together that they are usually studied as one force. They are closely related and are difficult to separate.

An electromagnet is very powerful. One example would include an electromagnet powerful enough to pick up a car. Forces of gravity, or gravitational forces, can be large or small. Gravitational force is defined as a force of attraction between two objects which have mass. Since all objects have mass, gravity acts between all objects. The strength of the gravity between objects depends on the mass of the objects and the distance between them.

When force is measured, it is measured in newtons (N). A newton was named after the famed English scientist, Sir Isaac Newton (1642-1727). A newton is a metric measurement. One newton is the force needed to accelerate a mass of 1 kg by 1 m/s squared. Remember that when we talk about 1 meter per second squared, we are talking about 1 meter/second/second or 1 m/sec/sec. This is equal to 2.2 lb. or about the force needed to lift a glass of water. Newtons are commonly measured with a spring balance which gives the strength of the force being measured.

Forces have strength, or magnitude, and direction. In a previous unit, the term vector was introduced. In physics, objects or quantities with these quantities are said to have vector qualities. Some examples of vector quantities might include acceleration and velocity. The term scalar, was introduced in a previous unit as well. An example of a scalar quantity could include temperature and speed, which have magnitude but no direction.

When we talk about measurements involving motion, such as acceleration, velocity, and momentum, they also involve direction. Forces act in a particular direction as well. When two forces act in the same direction, they add together. The total force is the sum of the individual forces. If you and a friend pulled on an object in the same direction, you would be combining forces. When the total force on an object is in one direction, we call this an unbalanced force.

If your friend pulled in the opposite direction from you, the forces would combine in different ways. When two forces combine in the opposite direction, they are combined by subtraction. If you and your friend were pulling in opposite directions, and one force was greater than the other, the object would move in the direction of the greater force. That makes sense, but what would happen if your forces were equal? If you and your friend pulled with equal force in opposite directions, there would be no force acting on the object. When you subtract one force from another, you are left with zero, and the object will not move. Balanced forces are forces that act in opposite directions and are equal in size, thus canceling each other out.

Friction is a force that resists the motion of two surfaces that do not touch each other. Friction is a force that acts in a direction opposite to the moving object’s motion. It will cause moving objects to slow down and finally come to a stop. The amount of friction between the surfaces depends on how hard they are pressed together and the roughness of the surfaces. Friction can occur in liquids, solids, and gases (such as air).

When solid objects slide over each other, this type of friction is called sliding friction. A sled sliding over the snow is an example of sliding friction. However, keep in mind that sliding friction can oppose motion. Friction can also be reduced by placing objects on wheels or ball bearings. This is called rolling friction. Rolling friction tends to be less than sliding friction. Moving a file cabinet on wheels is an example of rolling friction. Friction can also exist in a fluid when an object moves across it or through it. This force exerted by a fluid is called fluid friction. All liquids and gases are considered to be fluids. Other examples would include water, oil, and air. Fluid friction is usually less than sliding friction. Slippery substances, or lubricants, such as grease, oil, or wax, change sliding friction into fluid friction.

Friction is often very helpful. Without friction between the tires and the surface of a highway, your car would be unable to stop. The grooves on a tire channel water and mud through them. Water and mud lessen friction because they act like lubricants. Remember that lubricants promote sliding. The sole on your snow boots helps to increase friction, so you don’t slide on the snow.

Push a book across a tabletop. The book encounters some friction, or resistance to motion. This is called sliding friction. Next, place some marbles or ball bearings under the book. Move the book. You should see that the book moves much easier and more smoothly. The marbles or ball bearings reduce the friction between the book and the tabletop. Can you think of some examples where ball bearings would help something move easier? Bearings are often found in machinery or axles of a car wheel. The bearings, or rolling friction, reduce the friction between surfaces. Besides creating a smoother surface, bearings also bear some of the weight of the moving parts as they rotate.

Sir Isaac Newton lived between 1641 and 1727. Although he is clearly the most influential scientist who ever lived, his humble nature is revealed in his own self description: “If I have been able to see further, it was only because I stood on the shoulders of giants.” (Taken from: www.lucidcafe.com/library/95dec/newton.html)

Most likely, Sir Newton felt his work achieved great heights because he respected the work of the “giants” who lived before him. In fact, he was born the year that another great scientist, Galileo, died. While he probably savored the work of others, he found science an assortment of isolated facts and laws, capable of describing some phenomena and predicting only a few. He left it with a unified system of laws that could be applied to an enormous range of physical phenomena and used to make exact predictions. Take, for example, his three Laws of Motion.

In the mid-1600’s, Sir Isaac Newton discovered how forces cause all states of matter. He developed three laws that describe rest, constant motion, and acceleration. Newton, called the father of modern science, was the first person to understand the concepts of force, motion, and gravity. His theory of universal gravitation, which will be discussed later in the unit, states that all objects are attracted to one another. His three laws of motion explain the relationship between mass, gravity, and the distance between objects and all objects in the universe.

Newton’s First Law of Motion states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force. This also means that an object in motion tends to remain in motion in a straight line at a steady speed. This first law is based upon a property of matter that was discovered by Galileo called inertia. Newton’s first law of motion is also called the law of inertia. Inertia is a property of matter that tends to resist any change in motion. Everything that has mass also has inertia. Remember that mass is roughly equal to weight. The more mass an object has, the more inertia it has, and the more it resists change. So we could say that the inertia of an object is related to its mass. The word inertia comes from the Latin word iners, which means “lazy” or “idle.”

Newton’s first law of motion tells us that acceleration and force are related. Acceleration is only present when forces are present. So if an object changes velocity or accelerates, a force must be acting on it. Remember that this law is based on the ideal situation with no air resistance or other kind of friction. Inertia is present in our everyday lives, all around us. If you are riding in a car that suddenly stops, you keep moving forward. The inertia could send you right through the windshield if you weren't wearing a safety belt. Again, if you are riding in a car, inertia affects you. A car travels along a road in a straight line due to inertia. What will happen if there is a curve in the road? The driver turns the wheel, and the car moves along the curve. But what happens to the passengers? They continue to move in a straight line. The result is that the passengers are pushed into the walls of the car. So the force exerted on the passengers by the walls of the car keeps the passengers in the curved path.

Place an index card on top of a plastic drinking glass. Place a quarter in the center of the index card. Use your finger and flick the card quickly across the glass. What happens? Your answer should be that the quarter has inertia. The quarter tends to stay at rest until acted on by a force. The quarter should have fallen into the glass. You can extend this inertia experiment by stacking five checkers in a pile. Place another checker about 3 inches from the pile. Flick the one checker quickly towards the stack. What happened? Remember the characteristics of inertia. Record your results of this experiment in question # 41. Remember the characteristics of inertia.

Newton’s Second Law of Motion states that force, mass, and acceleration are related. This can be represented by the equation:

We know from Newton’s first law that acceleration does not occur without a force. The second law shows that force and acceleration are related. In other words, the acceleration of an object depends upon its mass and the applied force. If you push a wagon along a sidewalk, it will begin to move. If you push harder, the faster the wagon will accelerate -the greater the force, the more acceleration. If you fill the wagon with heavy objects, you will need to push harder than you would if it was empty. This is because the wagon filled with heavy objects has more mass, or inertia. A greater force is needed to accelerate an object with greater inertia. So it is obvious that force and acceleration are related to an object’s mass. The equation above is applicable in that force equals mass times acceleration.

When mass is measured in kilograms (kg) and acceleration is measured in meters/second/squared (m/s^2), force is in newtons (N). One newton will equal the force required to accelerate one kilogram of mass at one meter/second/squared.

Normally the mass of an object is constant, and thus the force is proportional to the acceleration of the object. This sounds complicated but it’s really not.

ACCELERATION EXPERIMENT

Try this easy experiment representing Newton’s Second Law of Motion:

You will need a sheet of ruled notebook paper, a marble, one large and one small ball bearing, and a bar magnet. You also need a ruler with a groove in the center. This groove will serve as a ramp. If you do not have a ruler with a center groove, tape two rulers of the same size together and use the space between them as the ramp.

1. Using a tabletop, set up your experiment. Prop the ruler up at least one inch. Line up the notebook paper horizontally with the ruler.

2. Place a bar magnet vertically on the lined paper about one inch from the ramp’s exit. Roll the marble down the ramp past the magnet. Mark the spot where the marble left the notebook paper. What happened? Was the marble affected by magnetism?

3. Next, roll the small ball bearing down the ramp. Mark the spot where the ball bearing left the notebook paper. What happened to the ball bearing as it rolled past the magnetic force? You may need to reposition your magnet. The magnet needs to be close enough the affect the ball bearing.

4. Roll the larger ball bearing down the ramp past the magnet. You can adjust the magnet as needed. Mark the spot where the large ball bearing left the notebook paper. What happened?

5. Mentally, make a comparison about what happened to the small and large ball bearings as they rolled past the bar magnet’s magnetic force.

You should have made these assumptions during this experiment. The magnet force of the magnet will affect its position. You probably already know that a marble is not affected by magnetism, so the direction and acceleration were not affected. As the small ball bearing exited the ramp, the magnetic force should have affected the direction and acceleration. If it did not, you might need to redo this part of the experiment and move the bar magnet accordingly. When the large ball bearing exited the ramp, its acceleration may or may not have been affected. Even though the ball bearing was of a different size, or mass, the magnetic force remained the same. Did the larger ball bearing have the same results as the smaller one? Remember that mass makes a difference.

You read earlier in this unit that ball bearings help to reduce friction, especially with machine parts. When bearings are manufactured, they are made under controlled conditions and are shaped to within 0.00003 millimeters (1/1,000,000 of an inch) of the needed size. They are sometimes demagnetized so that they will not attract dust that could cause friction.

Newton’s Second Law of Motion explains why a heavy object and a lighter object dropped from the same height will hit the ground at the same time. Remember that they are both accelerating at the same rate. Because the lighter object has less mass, the gravitational force on it is much smaller. However, the larger object has more mass, so it has a greater gravitational force acting on it. Remember that falling objects accelerate at a rate of 9.8 m/sec/squared.

Newton’s Third Law of Motion states that for every action, there is an equal and opposite reaction. So, in other words, every force has to have an equal and opposite force. Remember that forces come in pairs and are always mutual. Newton’s third law is the easiest to understand. An example of this law would be a baseball bat hitting a baseball. The forces do not cancel each other out. The ball bat is exerting a force on the ball and accelerates it in an opposite direction. The ball is exerting an equal and opposite force on the bat. This is felt as the sudden slowing down of the bat. Another example might include turning on your garden hose with the water spurting out at high speed. If you do not hold the hose, it moves in an opposite direction and wiggles like a snake. You need to hold the hose firmly to get the desired results. This same concept is used with firefighters during a fire. There is so much water pressure coming from the water hose that it wants to move uncontrollably in the opposite direction. It usually requires two firefighters to hold the hose. In this example, the force of the water pump pushing the water out of the hose produces the action force. The reaction force is the expelled water pushing back against the hose. Both forces are equal in strength. As long as the firefighters hold the hose and push it with all their strength, theyuse their muscles to produce a force that balances the reaction. As long as the forces are balanced, the net force on the hose is zero; consequently, the hose doesn’t move. If you were out in a boat on a lake you could demonstrate the same principle. What would happen if you were standing on the edge of the boat and jumped off? Of course, the equal and opposite reaction would take place. As you jumped one direction, the boat would move in the opposite direction. Each of these examples demonstrates an equal but opposite direction.

BALLOON ROCKET LAUNCH EXPERIMENT

Try this simple experiment that demonstrates Newton’s Third Law of Motion. Remember when you are doing the experiment that real rockets and jet engines work following this same concept. Gas flows out of the rear end of the rocket, and it moves forward in the opposite direction.

1. Blow up a balloon to its maximum size. Tape a plastic drinking straw (about 2inches) inside the neck of the balloon to act as a rocket nozzle. If you want to make a smaller nozzle, you can pinch or flatten the edge of the straw.

2. Hold the balloon over your head and let it go. What happened? Did you observe anequal and opposite reaction? Yes, you should have.

You can extend this experiment by completing the following steps:

You will need a balloon, tape, two chairs, and a piece of string.

1. Tie a string between the backs of two chairs, at least 20 feet apart. Place a strawover one end before it is secured. Blow up a balloon and secure it with a paper clipso that the air cannot escape.

2. Tape the inflated balloon to the straw.

3. Make sure the string between the two chairs is tight. Release the paper clip andwatch the rocket take off. What happened? Did you observe an equal and oppositereaction? As the gas (air) escaped, the balloon rocket moved in the opposite direction.

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Do you think you can change this experiment?

Follow the steps as listed above except slope your string upwards. Is there as much thrust? Does the slope of the string make a difference? Does the amount of air in the balloon make a difference in the results?

A balloon-powered rocket and rockets traveling in outer space are good examples of Newton’s third law in action. The air is forced out of the open end when the balloon is stretched. This is the action force. An equal and opposite reaction occurs inside the balloon as the air pushes back on the balloon. This reaction force pushes the straw forward along the string.

The space shuttle has rocket boosters attached to it that push it into orbit in much the same way. When the rocket boosters are ignited, the fuel burns and produce vast amounts of hot gases that rush out of the back of the booster, this is the action force. The reaction force of the hot gases pushing back on the booster is equal in strength, but in opposite directions. The reaction force sends the shuttle and rocket boosters upward in direction.

Keep in mind that weight and mass are different. Mass is the amount of matter in any given object. The mass will always stay the same unless it is physically changed. Weight is a measurement of the force placed on an object by gravity. Since weight is a force, its unit of measurement is the newton (N). If you were on Earth, the moon, or another planet, your mass would remain constant; however, your weight would change. We already know that weight and mass are related. We already know that more massive objects weigh more than less massive objects. So when we talk about weight, Newton’s second law can be rewritten to show the relationship between the two:

Keep in mind that the unit of weight is the Newton and the unit of mass is the kilogram.

In the previous unit, we discussed Galileo and his experiment of dropping two cannonballs at precisely the same time from the top of the Leaning Tower of Pisa. At this time in history, most people believed that the heavier cannonball would hit the Earth first. Galileo’s experiment proved that the basic laws of nature govern motion. Isaac Newton evaluated Galileo’s experiment and determined that both cannonballs speeded up at the same rate, regardless of their mass. Today we know that all falling objects accelerate at the same rate. Newton tells us that a force must be present if objects accelerate. We call this force gravity. Gravity is a force of attraction between two objects. This force is responsible for accelerating an object towards the Earth..

Gravity pulls everything down. If you drop a ball, it returns to Earth. A falling leaf floats down to Earth. And even a speeding bullet will obey the laws of gravity and eventually fall to Earth. Gravity is the force of attraction between two objects that have mass. And since all objects have mass, gravity will act between them. Gravity’s strength depends on two things: the mass of the objects and the distance between them. The strength of gravity is affected by the distance between them. If the objects are closer, the gravitational force will be stronger. If the objects are farther apart, the force will be weaker. Air resistance can affect the rate at which an object falls to the Earth. The shape of an object causes air resistance.

Newton began to wonder about other objects and force and gravity. He had questions about the forces that kept the moon in orbit around the Earth. He knew that the moon was accelerating even though its direction was constantly changing in its path. So he knew that a force of some sort had to be involved. Newton calculated the moon's acceleration using laws already developed by Johannes Kepler, an astronomer. Many people of Newton’s day believed that different forces affected places other than Earth. Newton discovered that they were indeed the same forces on Earth or in outer space. This particular discovery represented the first universal law of forces. When we talk about a universal law, we are talking about a law that applies to all objects in the universe. Newton developed the Law of Universal Gravitation. This law states that all objects in the universe are attracted to each other through a force of gravity. The force's size depends on the mass of the objects and the distance between them.

Gravity is a weak force, even though it may not seem that way. It is not even strong enough to pull together two objects placed next to each other. Yet gravitational forces can be felt over great distances when we talk about space physics. Sometimes phenomenal things can occur such as with a black hole. As a star shrinks in size, its fuels die out, its core becomes very dense, and the gravitational pull is strong enough to suck light into it. This is what we call a black hole. In our solar system, the strong gravitational force of our Sun, holds the nine planets together. The planets hurdle through space at speeds that just balance the Sun’s gravitational pull. This locks them into a perpetual circle around the Sun. Since the Sun weighs about 4.4 million billion billion pounds, its gravitational pull is enough to hold all the planets in orbit, even Pluto, which is 3.7 billion miles away. A satellite orbits the Earth in much the same way. It orbits Earth very fast, so gravity never brings it any closer. The force of gravity plays an essential role in the evolution of stars and the behavior of galaxies. So in a sense, it is gravity that binds the universe together.

In this unit, forces friction, and Newton’s Three Laws of Motion were introduced and discussed. Each of Newton’s three laws were discussed in detail and related to real-life examples. Friction was introduced as being helpful in our everyday lives, as well as problems caused by friction. Gravity, mass, and weight were related to forces and how they affect those forces. Mathematical formulas were used to determine measurements with force.

Forces that need two or more objects to be touching each other are called contact forces. Some examples of contact forces might include:

kicking the soccer ball (makes the ball begin to move)	catching a soccer ball ( catching the ball with your hands causes the ball to slow down and stop)	stepping on a soccer ball (causes equal forces pushing up and pushing down)