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Friction: Introduction

Friction: Introduction

This CELL is designed to help students understand frictional force, Newton’s three laws of motion, and the process of scientific experimentation. Friction is the force that opposes motion between two surfaces. Frictional force is due to the mechanical and molecular interaction between the two surfaces. Friction occurs due to the movement of one surface over another (dynamic friction) or a force acts to move one of the surfaces but they remain stationary (static friction) (Figure 1). If you push hard on an object but it fails to move, it is because the static friction force is equal to (or greater) and opposite to the force you apply. The maximum static friction force is the maximum force that can be applied without the object moving. Once the maximum static frictional force is exceeded, the object will move and the friction force is dynamic and is less than the maximum static frictional force.

FRICT_Intro_Fig1

Figure 1: (a) dynamic friction, (b) static friction.

In the first activity in this CELL, you will push a penny with the aim of making the penny travel and stop on the end line. This task is an example of the role of friction and can be described using Newton’s three laws. With the penny at rest, you must apply an external force to the penny to accelerate it (speed it up in the forward direction). This is an example of Newton’s 1st Law which states that an object remains at rest or moves at a constant velocity (constant speed in a straight line) unless acted upon by an external force. According to Newton’s 2nd Law, the acceleration of the penny is directly proportional to the force applied and inversely proportional to the penny’s mass (see equations below).

f=ma

Once you let go of the penny, it experiences a negative acceleration (slows down in the forward direction) until it comes to a stop. This negative acceleration indicates that a force must be present in the opposite direction to the motion of the coin. Once the coin comes to a stop no frictional force is present.

We have mentioned Newton’s 1st and 2nd laws, but have failed to mention Newton’s 3rd Law, which states that for every action there is an equal and opposite reaction. How does Newton’s third law apply to the penny situation? When you push on the coin, the coin applies an equal and opposite force back on you. The reason this is not observed is because the large mass of the person pushing relative to the penny means the person only experiences a very small acceleration. The equal and opposite force to the friction force on the coin is the frictional force of the coin on the table. Again, the acceleration of the table is very small due to its mass and hence not observed.

In order to measure frictional force, you will pull on a spring scale attached to a wooden block. If someone pulls on the spring scale carefully increasing the force it is possible to observe the block remain stationary while the force increases up to the maximum static frictional force. Then the block accelerates (starts to move forwards). Now, if the block is pulled so that it moves at a constant velocity, the friction force equals the pulling force, and the spring scale indicates the dynamic friction force (when the forward force is equal and opposite to the backward force the block does not accelerate). In order to stop the block, the pull force is reduced below the friction force so that the friction force slows down the block’s motion. For Investigations 2, 3, and 4, you will measure the static friction force of the wooden block by pulling the block at a constant velocity between two points and recording the force on the spring scale.

As scientists, we will make hypotheses about what possible factors could influence frictional force and will test them in Investigations 2, 3 and 4. The four properties that we will consider are movement velocity (speed in particular direction), weight, contact surface area and surface type. The beliefs that either movement velocity or surface area influence friction are common misconceptions. We have included these two variables because it is equally important in the scientific process to find out what does not influence friction as what does. The students will examine all four properties by running experiments and analyzing the data. The results will reveal that friction force is the same for different surface areas, indicating surface area does not affect friction. Also, for different velocities the friction force is constant (some small noise in measurement are likely, but the values should be essentially the same). In contrast, increasing weight will lead to a linear increase in friction. Also, surface type will lead to different friction forces. These results suggest that friction is dependent on weight and surface type.

Students should understand Newton’s three laws as they apply to this situation. Students should also have specific knowledge of friction and variables that influence friction. In addition, the usefulness of equations in describing relationships between variables and the importance of what is not included in the equation should be stressed. Finally, students will have experienced the scientific process of examining variables to determine their influence.

In summary, through investigation and the use of mathematical formulas, students will begin to define the relationship between speed, velocity, and acceleration. Students will begin to understand that velocity is a measure of speed and direction, and students will find that an object with no change in velocity has no acceleration.

Students will perform investigations that involve pulling a wood block along a table at a constant velocity. Using the formula   ΣF = ma, students will discover that the sum of the forces on an object with constant velocity is zero. Through use of the formula ΣF = Ffriction + Fapplied, students will determine if the sum of the forces is zero, frictional force is equal and opposite to the applied force. Using this knowledge, students will determine the frictional force on a wood block and a wood box in a variety of scenarios.

Through investigations, students will discover that frictional force and applied forces affect the motion of an object. Further investigating frictional force, students will find that it is dependent on the weight of the object being moved and on the smoothness of the surfaces in contact with one another. Students will also find that frictional force is independent of the surface area of the object being moved and the velocity at which the object moves. Through these Investigations, students will begin to draw conclusions about the way forces affect motion.

Newton's Three Laws of Motion

Sir Isaac Newton (1642-1727) was an English scientist and mathematician. Newton’s contributions to physics alone represent one of the most significant advancements in the history of science. Even today, it is impossible to discuss topics such as motion, light, gravity, or mechanics without referencing his work of applying his formulas and equations.

One of Newton’s most famous discoveries (after gravity!) is his Three Laws of Motion. Let’s look at each of these Laws of Motion below, with reference to friction.

Newton’s First Law of Motion:

An object remains at rest or moves at a constant velocity (constant speed in a straight line) unless acted upon by an external force.

On the one hand, this Law explains why a stationary soccer ball on the field remains stationary unless another force acts on it. This force may be a players foot kinking it, the referee picking it up, or perhaps a very strong gust of wind forces it into motion.

On the other hand, the First Law also explains that a soccer ball in motion will continue moving in a straight line forever unless other forces act on it. This second part of the Law may be a bit more difficult to imagine because it goes against what we typically observe. A flying soccer ball, no matter how hard it was kicked through the air does not travel in a straight line, it moves in a downward-curving arc. Alternatively, if you kick the ball hard along the ground, while it might move in a fairly straight line, it certainly doesn’t continue moving forever.

This is because in both cases, the ball moving through the air in an arc and the ball slowing and stopping after a kick along the ground, other forces act upon the moving ball. In the case of the arc-like motion, the force of gravity prevents the flying ball from traveling in a straight line. Gravity is a force that pulls on all objects. From the time the ball leaves the player’s foot, the force of gravity pulls it towards the Earth. If the ball was kicked perfectly up perpendicular to the ground, we would find that the ball does return to Earth in a straight line as the First Law demands. However, as the ball flys through the air in the direction of the opponent’s goal, two forces are at work on it –  the force from the kick and gravity pulling it down towards the Earth. The combination of the first, straight, motion, and the pull downwards by gravity results in a combination of forward and downward motion. This is what causes the ball to move in a downward arc.

But there is a second reason the ball slows and curves downward on the sailing ball – friction. As the ball moves through the air it must push trillions of air molecules out of the way. This causes friction, which is also a force acting on the ball. Friction also acts on the soccer ball rolling across the ground due to the surface of the ball and ground coming in contact with each other.

If you were in outer space, where no air molecules exist to cause friction and no gravity exists acts on it to bend its flight, it would continue moving in a straight line forever or until coming in contact with another large body like a star or planet.

Newton’s Second Law of Motion:

Acceleration is directly proportional to the force applied and inversely proportional to its mass (see equations below).

F = ma     or     a = F/m

F is force, m is mass, and a is acceleration

This simple Law explains why it is harder to pull a wagon with two friends sitting in it than with only one of them sitting in it. The mass is greater and the Second Law equation says that you will have to apply more force in order to pull it.

Of course, in this example, we also have friction acting on the moving wagon. Friction comes in the form of the wheel and sidewalk surface rubbing against each other as well as the air molecules that must be pushed aside to move. Once again, if you were in outer spaces, in the absence of gravity and friction, you could pull or push your friends and then let go and watch them continue moving in a straight line away from you forever!

The video above is a fun animation from NASA and PBS. The NASA Physical Science and Engineering Collection 2011 is a collaborative production of WNET, WGBH Educational Productions, the WGBH Media Library, and WGBH Interactive. Major funding for this project is provided by NASA.

Newton’s Third Law of Motion:

For every action, there is an equal and opposite reaction.

When you push on a brick wall, it pushes back at you with exactly the same amount of force but in the opposite direction. If you could exert more force than the wall, the wall would have to move. If, for example, you got in a powerful tractor and drove it into the brick wall, you might very well cause it to move.

If you sat cross-legged on a scooter with wheels and pushed against the wall, you would push yourself away from it and continue moving until friction (wheels on floor and molecules of air) slowed you down and you came to rest.

Newton’s Third Law comes into play whenever we design machines that can travel. For example, a rocket engine exerts a force against the ground so great that the equal and opposite action as to lift the metal rocket, that can be as long as a soccer or football field off the ground. After leaving the ground, the same thrust continues to push against the air (friction) and against the Earth’s force of gravity, and the rocket continues to climb toward outer space.

Automobiles move forward when their tires push against the ground and the ground pushes back. When you blow up a balloon and release it, the escaping air exerts a force on the air molecules around it which push back with the same forces causing the balloon to race wildly through the room.

Pictured here is an animated GIF of a device that you may have seen before. It is called a Newton’s Cradle. It is a fun demonstration where one moving ball transfers its energy to the next ball and so on down the line. It nicely demonstrates the conservation of energy and momentum.

Friction and the Three Laws of Motion

From the above discussion, it should be clear that any moving object in our atmosphere (as opposed to outer space) must content with frictional forces. Thus, when engineers wish to design machines that will move us from one place to the other, they must take friction into account.

CONTENT

  • Fun Facts
  • Learn the Lingo
  • Get Focused

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FUN FACTS

See For Yourself

As you will learn in this CELL, friction occurs anytime two surfaces move when in contact with each other. You will also learn that one of the factors that influence friction is the nature of the two surfaces that are moving in contact with each other. A simple demonstration of this can be performed by running on a gymnasium or other smooth floor, perhaps a titled hallway, and then attempting to slide as far as possible.

If this simple experiment is attempted first with stocking feet and then wearing rubber sole sneakers, you will immediately experience the major impact of surface type (cloth versus rubber in this example) on frictional force!

You may extend this experiment by attempting the stocking feet slide, not on a smooth surface but on a concrete surface. Compare how far you are able to slide in stocking feet on the concrete surface versus the smooth gymnasium floor. Once again, you will immediately experience the drastic difference that the properties of surfaces have on friction.

Why Automobiles Need Oil

We fill our cars with gasoline or diesel fuel. It is obvious that, just like food for our own bodies, automobile engines use the petroleum fuel to burn and release energy. But have you ever considered why we need to add oil to automobile engines as well? First, let’s look at the most important part of an internal combustion engine, the piston and cylinder.

The Piston is the round steel part that moves up and down in the round cylinder. It compresses the gas vapors in the cylinder for the spark plug to ignite it. Notice that both the piston and cylinder are in constant contact with each other and made of metal. Therefore, two metal surfaces are rubbing against each other. Now, consider that this up and down movement occurs over 1,000 times per minute! All this friction would quickly destroy the engine. In fact, without oil, most automobile engines would fail well within 30 minutes. The metal surfaces would actually get so hot from the friction that they would begin to fuse with each other and lock the engine permanently. The oil that is added to an automobile lubricates the metal surfaces that come in contact with each other and drastically reduces friction and subsequent engine damage.

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Just for the fun of it, take a look at the paper below from the journal Proceedings of the World Congress on Engineering. Hopefully, you will notice two things. First, friction and motor oil are extremely important issues for automotive engineers. Second, you will immediately notice the importance and level of mathematics used in engineering science. This is an outstanding illustration of the importance of math in science and engineering.

LEARN THE LABLEARNER LINGO

The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.

 

Investigation 1:
  • Force: a push or pull on an object
  • Acceleration: the rate an object changes velocity
  • Velocity: the rate an object changes its direction
  • Applied force: force that is applied to the object from an outside force
Investigation 2:

Note: No new terms are introduced in Investigaton 2.

Investigation 3:
  • Frictional force: the force that prevent two objects from easily moving against one another
Investigation 4:
  • Coefficient of friction: a number that describes the degree of mechanical and molecular interaction between two surfaces
  • Normal reaction force: the force of gravity on that object when pulling an object across the table
  • ΣF = ma: the sum of forces are equal to mass multiplied by acceleration

GET FOCUSED

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

 

Investigation 1:
  • What is the relationship between speed, velocity and acceleration? A change in velocity occurs when an object in motion either changes speed or changes direction. Acceleration denotes a change in velocity in a period of time.
  • How does frictional force affect motion? Frictional force decreases the velocity of an object. That is, it decreases the speed of an object.
Investigation 2:
  • How does velocity of an object affect the frictional force between it and the surface with which is comes in contact? Frictional force is independent of velocity.
Investigation 3:
  • What is the relationship between weight and frictional force? As the weight of a load increased, the frictional force between the load and the surface it moves on increased.
  • What is the relationship between the surface area of an object in contact with another surface and the frictional force between two surfaces? As the surface area of a load increased, the frictional force between the load and the surface remained constant.
Investigation 4:
  • How does the smoothness of the two surfaces in contact affect the frictional force between the two surfaces? The smoother the two surfaces, the less frictional force. The less smooth the two surfaces, the more frictional force.

Note: These are succinct responses to the Focus Questions and are placed here for easy reference. Fully developed responses to the Focus Questions can be found on each PostLab page.

Note: Some questions may be revisited as the CELL progresses. As students acquire additional knowledge, their responses should reflect this.