Forces Around Us
All around us, changes in motion are constantly taking place. Cars move along the roadways,
people walk from place to place, leaves fall from trees, and engines turn. Any time you see
movement occurring, a force has provided the energy that causes the movement to start, stop, or
change.
Force is a push or a pull that causes a change in shape or motion. Sometimes an object will
change shape when a force is exerted on it. A tennis ball changes shape when we push down on it. A
rubber band changes shape when it is pulled. Sometimes exerting a force will cause a change in the
movement of an object. The object can experience a change in either speed or direction of
movement. The word scientists use for a change in speed or direction is acceleration. When force is
being exerted on an object, we say the object accelerates. In the case of a toy car, it speeds up when
you push on it. If you put your hand in front of a rolling tennis ball, it will change direction and slow
down when it bounces off your hand. In both cases the objects were accelerating.
Sometimes forces are exerted on an object and no change seems to be occurring. This
happens when two forces are working on the same object with the same amount of force, but in
opposite directions. We call these balanced forces. Let’s say that two teams of students are having a
tug-of-war. Both teams are pulling very hard, but the rope is not moving. Force is definitely being
exerted on the rope, but the forces are balanced. Both teams are pulling with equal force but in
opposite directions. Now suppose that one of the students gets a blister on his hand and has to drop
off of the rope. Suddenly the rope begins to move toward the team with more people. The forces
have become unbalanced and the rope begins to accelerate in one direction. Objects accelerate
when unbalanced forces act on them.
There are many different types of force. Often force is the result of direct contact, such as
picking up a book, pushing a shopping cart, or throwing a ball. Other types of force are more
invisible. Magnetic force is a force of attraction or repulsion that moves objects made from certain
types of metal. Static electricity is a type of electrical force. You have experienced this force if you
have ever received a shock by touching an object (such as a doorknob) after walking across carpet.
One type of invisible force that plays a huge role in our lives is gravity. Gravity is the force of
attraction between all objects in the universe. Usually this force is much too small to be noticed. For
example, your pencil and your hand have a gravitational force of attraction between them, but when
you let go of the pencil it does not stick to your hand. The force is too small to have a visible effect.
The amount of gravitational force between two objects depends on the mass of the objects.
Your hand and your pencil have a very small mass, so the force of attraction between them is small.
Objects that are massive in size exert a large gravitational force. In fact, the larger the object, the
more gravitational force it can exert. The Earth has a very large mass, so it attracts objects towards
its center with a great deal of force. This force is so strong it keeps us firmly in place on the surface of
the Earth. Without gravity, we would go flying off into space!
The amount of gravitational force also depends on how far apart the objects are from one
another. When you are standing on the surface of the Earth, gravity has a greater effect on you than
it would if you were in a shuttle orbiting the Earth. This is why the astronauts seem to be ‘floating’
when they are in space. Gravity is still working on the astronauts causing them to remain in orbit
around the Earth, but the effect of gravity is considerably less there than it is on Earth’s surface.
The amount of gravitational force exerted on the mass an object can be measured. This force
is called weight. When you weigh an object, you are measuring the amount of force that gravity is
exerting on the mass of that object. The more mass an object has, the more force gravity exerts on it.
For this reason, mass and weight are closely related. Sometimes they are even confused with one
another. Remember that mass is the amount of matter contained in an object and is measured in
grams. Weight is the amount of force that gravity exerts on the mass of an object and is measured
using a different metric unit: the newton. The newton is the basic unit of measurement of force in
the metric system and is named in honor of Sir Isaac Newton, who was the first scientist to describe
and study gravity. All forces are measured in newtons. On Earth it takes a force of 9.8 newtons to lift
a 1 kilogram mass. Said another way, the weight of a 1 kilogram mass is 9.8 newtons. As you lift
farther away from the Earth, the effect of gravity becomes less and so weight also decreases. The
mass, however, does not change. Mass remains the same no matter where you are.
We use an instrument called a scale to measure weight of an object. Scales use the principle
of balanced forces to measure how much something weighs. To get an idea of how a scale works,
different amounts of mass could be hung on a rubber band and the amount of stretch that occurred
can be measured. The length of the rubber band is a measurement of the amount of force placed on
it by hanging the different amounts of mass. The rubber band stretches until it is exerting an upward
force equal to the weight of the mass hung on it.
Scientists use a spring scale to measure all kinds of force including weight. A spring scale is an
instrument that contains a spring with a hook at that end of it. An object is hung or pulled on the
hook and the spring stretches. Numbers are marked on the scale and as the spring stretches, a
pointer indicates the amount of weight on the scale. The scale you see in the produce department at
the grocery store works this way. When you step on a bathroom scale, the force of your body
stretches a spring inside the scale. The stretched spring pulls on levers inside the scale until the
upward force of the scale equals the downward force exerted by your weight. The dial on the scale
moves as the spring stretches and stops moving when it reaches the number on the scale and
corresponds to your weight.
Another type of invisible force occurs as the result of two objects coming into direct contact
with one another. This force is called friction. Friction is defined as a force that opposes the motion
of an object. When the surfaces of two objects are in contact, friction acts in a direction exactly
opposite of the moving object. The result is usually that the moving object slows down or comes to a
stop. For example, if you push a book across a table, it slides quickly at first, begins to slow down,
and eventually comes to a standstill. The cause of this change in motion is the friction that occurs
between the table and the book.
The amount of friction depends on the type of surface. Different types of material create
different amounts of friction. In general, the smoother the surface of an object is, the less friction it
will have. A book will travel much farther if you slide it across a glass table to than if you push it
across a concrete floor. The weight of the moving object is another factor that influences the effect
of friction. You can push an empty box across a room much more easily than you can push a box full
of books.
There are three types of friction. The first is sliding friction. When two solid objects slide over
each other, sliding friction acts between the surfaces. When wooden blocks slide down different
surfaces on a friction board covered with different surface types, the blocks slide down in different
amounts of time due to the amount of friction between the block and the surface type it is moving
across. You experience sliding friction every day when you walk, pick up a pencil, or butter a piece of
toast.
A second type of friction is rolling friction. Rolling friction occurs when a round object, such
as a ball, or a wheel turns on a surface instead of sliding. Rolling friction is usually much less than
sliding friction because only part of the wheel is in contact with the surface at any one time. For this
reason, wheels or ball bearings are often placed under objects to make them easier to move. Making
use of rolling friction allows us to use much less force to move objects from place to place. Imagine
how much force you would have to use to get your bicycle to move if you had to overcome sliding
friction instead of rolling friction.
The third type of friction is fluid friction. Water, oil, and air are examples of substances with
fluid movement. When an object moves through or across one of these substances, fluid friction is at
work, but it is usually much less than when two solid surfaces come in contact. The air resistance
parachutes experience through the air is a form fluid friction. Air resistance makes falling objects fall
more slowly. Substances which are ‘slippery’ such as grease and oil are often used to change sliding
friction to fluid friction. This causes a reduction in friction. A substance that reduces friction is called
a lubricant. Lubricants are used in engines to overcome the sliding friction between moving metal
parts. The wheels and axles of cars are lubricated so that they will turn more easily, reducing the
amount of force needed to make the car move. Skiers wax the bottom of their snow skis so that they
will slide more quickly and easily down the snow-covered mountain. Surfers wax the surface of their
surfboards to decrease friction and to allow them to move more easily across the surface of the
water.
Without friction, your life could be considerably different than it is now. It is friction that
allows the soles of our shoes to grip the ground so we can walk. Your shoelaces stay tied because of
friction. The ability to pick things up or hold on to objects comes directly from the effect of friction.
Friction allows us to control the movement of the things that we use in our everyday life.
Automobiles stop because of the friction created by brakes and friction allows you to control how you
move on your skateboard. Using and managing friction allows us to take advantage of the
environment in which we live.
There are many types of force that are present in the universe. It is easy to see that both
motion and force play a significant role in our everyday lives and are a part of everything we do.
Muscles in our body exert forces that allow us to move and function. The force of gravity holds us on
the ground. Friction is a force that makes it possible for us to get from one place to another.
Electrical force makes many modern conveniences possible. The world as we know it would not exist
without the forces that drive everything in the universe.
Name _______________________________________________ Date ________________ Hour _____ Table __
Questions: ‘Forces Around Us’
Answer each of the following questions in complete sentences.
1. What is the definition of force?
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2. What two features of movement can change as a result of the application of force?
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3. What is acceleration?
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4. In which of the following cases is the bicycle accelerating: (Explain your answer)
when the brake is being put on or
when the bicycle is turning a corner or
when the bicycle is being pedaled downhill?
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5. When two equal forces act in opposite directions, what do we call these forces AND what is their
result?
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6. An archer pulls back on her bow string and holds it in place. Use force to explain what is
happening.
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7. The archer lets go of the bow string and the arrow flies outward. Use force to explain what is
happening.
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8. Name two forces that do not involve one object coming in direct contact with another object.
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9. What is gravity?
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10. What two factors affect the amount of gravitational force between any two objects?
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11. Which has a stronger gravitational force, the Earth or the Sun? Explain your answer.
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12. What is weight?
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13. On the Moon your weight is one-sixth of what it is on Earth. Explain why this is true.
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14. What would happen to your mass if you were to go to the Moon?
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15. What is the metric unit used for measuring mass?
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16. What is the metric unit used to measure force?
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17. What is the weight of a 10 kg mass on Earth?
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18. What is the name of the instrument that scientists use to measure force or weight?
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19. What is friction?
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20. When does friction occur?
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21. What two factors affect the amount of friction?
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22. Use friction to explain why some people put rubber mats in their bathtubs.
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23. You have decided to move a bookcase. First you push it. Then you put it on a cart with wheels.
Finally, you add some grease to the bearings of the wheels to make them roll more smoothly. Explain
the type of friction associated with each of these actions.
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24. Why is rolling friction less than sliding friction?
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25. What type of friction occurs when a paper airplane is flown? Explain your answer.
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26. What is a lubricant and how does it work?
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27. Over the course of history, people have tried to build perpetual motion machines (a machine that
will run forever). Use what you know/have learned to explain why no one has ever succeeded in
doing this.
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