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VOCABULARY friction p. 85 fluid p. 88 air resistance p. 89
KEY CONCEPT
BEFORE, you learned
• Gravity is the attractive force masses exert on each other
• Gravity increases with greater mass and decreases with greater distance
• Gravity is the centripetal force keeping objects in orbit
NOW, you will learn
• How friction affects motion
• About factors that affect friction
• About air resistance
TH I NK ABOUT
As a person walks, she exerts a backward force on the ground.
A reaction force moves her forward.
But some surfaces are harder to walk on than others. Ice, for example, is harder to walk on than a dry surface because ice is slippery. How can different surfaces affect your ability to walk?
SUPPORTING MAIN IDEAS
Take notes about friction, including details and examples.
Have you ever pushed a heavy box across the floor? You probably noticed that it is easier to push the box over some surfaces than over others. You must apply a certain amount of force to the box to keep it moving. The force that acts against your pushing force is called friction.
Friction is a force that resists the motion between two surfaces in contact.
When you try to slide two surfaces across each other, the force of friction resists the sliding motion. If there were no friction, the box would move as soon as you applied any force to it. Although friction can make some tasks more difficult, most activities, including walking, would be impossible without it. Friction between your feet and the ground is what provides the action and reaction forces that enable you to walk.
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RESOURCE CENTER
CLASSZONE.COM
Learn more about friction, forces, and surfaces.
reminder
Remember that balanced forces on an object do not change the object’s motion.
If you look down from a great height, such as from the window of an airplane, a flat field appears to be smooth. If you were to walk in the field, however, you would see that the ground has many bumps and holes. In the same way, a flat surface such as a piece of plastic may look and feel smooth. However, if you look at the plastic through a strong microscope, you see that it has tiny bumps and ridges. Friction depends on how these bumps and ridges on one surface interact with and stick to the bumps and ridges on other surfaces. There are several factors that determine the friction between two surfaces.
Types of Surfaces Friction between two surfaces depends on the materials that make up the surfaces. Different combinations of surfaces produce different frictional forces. A rubber hockey puck sliding across ice has a smaller frictional force on it than the same puck sliding across a wooden floor. The friction between rubber and ice is less than the friction between rubber and wood.
Motion of the Surfaces You need a larger force to start something moving than you do to keep something moving. If you have ever tried to push a heavy chair, you may have noticed that you had to push harder and harder until the chair suddenly accelerated forward.
As you apply a force to push a chair or any other object that is not moving, the frictional force keeping it from sliding increases so the forces stay balanced. However, the frictional force has a limit to how
Before Object Moves While Object Moves acceleration applied force applied force friction friction
When an object is standing still, there is a maximum force needed to overcome friction and start it moving.
Any force less than this will be exactly balanced by the force of friction, and the object will not move.
Once the object is moving, the frictional force remains constant. This constant force is less than the maximum force needed to start the object moving.
86 Unit: Motion and Forces

Page 3 of 5 large it can be. When your force is greater than this limit, the forces on the chair are no longer balanced, and the chair moves. The frictional force remains at a new lower level once the chair is moving.
Force Pressing the Surfaces Together The harder two surfaces are pushed together, the more difficult it is for the surfaces to slide over each other. When an object is placed on a surface, the weight of the object presses on that surface. The surface exerts an equal and opposite reaction force on the object. This reaction force is one of the factors that determines how much friction there is.
If you push a chair across the floor, there will be a certain amount of friction between the chair and the floor. Increasing the weight of the chair increases the force pushing the surfaces together. The force of friction between the chair and the floor is greater when a person is sitting in it than when the chair was empty.
Friction depends on the total force pressing the surfaces together, not on how much area this force acts over. Consider a rectangular cardboard box. It can rest with its smaller or larger side on the floor.
The box will have the same force from friction regardless of which side sits on the floor. The larger side has more area in contact with the floor than the smaller side, but the weight of the box is more spread out on the larger side.
check your reading
What factors influence frictional force? Give two examples.
Less Weight More Weight applied force weight applied force friction friction
The force of friction depends on the total force pushing the surfaces together. Here the weight of the chair is the force pressing the surfaces together.
The weight of the chair increases when someone sits in it. The force of friction is now greater than when the chair was empty.
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Friction produces sparks between a match head and a rough surface. The heat from friction eventually lights the match.
Friction between surfaces produces heat. You feel heat produced by friction when you rub your hands together. As you rub, friction causes the individual molecules on the surface of your hands to move faster.
As the individual molecules in an object move faster, the temperature of the object increases. The increased speed of the molecules on the surface of your hands produces the warmth that you feel.
The heat produced by friction can be intense. The friction that results from striking a match against a rough surface produces enough heat to ignite the flammable substance on the head of the match. In some machines, such as a car engine, too much heat from friction can cause serious damage. Substances such as oil are often used to reduce friction between moving parts in machines. Without motor oil, a car’s engine parts would overheat and stop working.
As you have seen, two objects falling in a vacuum fall with the same acceleration. Objects falling through air, however, have different accelerations. This difference occurs because air is a fluid. A fluid is a substance that can flow easily. Gases and liquids are fluids.
Write a hypothesis that explains how shape affects the speed of falling objects. Design an experiment that tests your hypothesis.
PROCEDURE
1 Figure out how you can use the three sheets of paper to test your hypothesis.
Remember to control all other variables, including the mass of the paper.
2 Write up your procedure.
3 Conduct your experiment.
WHAT DO YOU THINK?
• What were the results of your experiment?
• Did the results support your hypothesis? Explain your answer.
• Write a statement that summarizes your findings.
CHALLENGE What other variable might affect falling time? How could you test it?
88 Unit: Motion and Forces
SKILL FOCUS
Designing experiments
MATERIALS
3 identical sheets of paper
TIME
30 minutes

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When an object moves through a fluid, it pushes the molecules of the fluid out of the way. At the same time, the molecules of the fluid exert an equal and opposite force on the object that slows it down. This force resisting motion through a fluid is a type of friction that is often called drag. Friction in fluids depends on the shape of the moving object. Objects can be designed either to increase or reduce the friction caused by a fluid. Airplane designs, for example, improve as engineers find ways to reduce drag.
The friction due to air is often called air resistance.
Air resistance differs from the friction between solid surfaces.
Air resistance depends on surface area and the speed of an object in the following ways: air resistance
• An object with a larger surface area comes into contact with more molecules as it moves than an object with a smaller surface area. This increases the air resistance.
• The faster an object moves through air, the more molecules it comes into contact with in a given amount of time. As the speed of the object increases, air resistance increases.
When the force of air resistance equals the force from gravity, a skydiver falls at a constant speed.
When a skydiver jumps out of a plane, gravity causes the skydiver to accelerate toward the ground. As the skydiver falls, his body pushes against the air. The air pushes back—with the force of air resistance.
As the skydiver’s speed increases, his air resistance increases. Eventually, air resistance balances gravity, and the skydiver reaches terminal velocity, which is the final, maximum velocity of a falling object. When the skydiver opens his parachute, air resistance increases still further, and he reaches a new, slower terminal velocity that enables him to land safely.
gravity check your reading
How do speed and surface area affect air resistance?
KEY CONCEPTS
1.
How does friction affect forward motion? Give an example.
2.
Describe two ways to change the frictional force between two solid surfaces.
3.
How does air resistance affect the velocity of a falling object?
CRITICAL THINKING
4. Infer What two sources of friction do you have to overcome when you are walking?
5. Synthesize If you push a chair across the floor at a constant velocity, how does the force of friction compare with the force you exert?
Explain.
CHALLENGE
6. Synthesize If you push a book against a wall hard enough, it will not slide down even though gravity is pulling it. Use what you know about friction and Newton’s laws of motion to explain why the book does not fall.
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