TEKS Lesson 7.7A: Work and Force
When Is Work Done?
If you pull your books out of your backpack, you are doing work on your books. If you push a child on a swing,
you are doing work on the child. In scientific terms, you do work anytime you exert a force on an object that
causes the object to move. Work is done on an object when the object moves in the same direction in which
the force is exerted.
No Work Without Motion It’s not difficult to recognize that if you stand still next to your backpack, you are
doing no work. In the same way, if you stand still next to the child on the swing, you are doing no work.
However, suppose you push on a car that is stuck in mud. You certainly exert a force on the car, so it might
seem as if you are doing work. But if the force you exert does not make the car move, you are not doing any
work on it. Remember, to do work on an object, the object must move some distance as a result of your force.
If the object does not move, no work is done, no matter how much force is exerted.
Force and Motion in the Same Direction Now think about carrying your backpack to school. You know that
you exert a force on your backpack when you carry it, but are you doing any work? To do work on an object,
the force you exert must be in the same direction as the object’s motion. When you carry an object, you exert
an upward force on the object. However, the motion of the object is in the horizontal direction. Because the
force is vertical and the motion is horizontal, no work is done.
1.Compare Study the pictures. Explain why lifting a bin full of newspapers
is work, but carrying the bin is not.
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How much work do you do when you pull a suitcase? When you pull a
suitcase, you pull on the handle at an angle to the ground. Therefore your
force has both a horizontal part and a vertical part. When you pull this
way, only part of your force does work—the part in the same direction as
the motion of the suitcase. The rest of your force does not help pull the
suitcase forward.
2.Identify Examine the pictures below. In which situation is NO work being done?
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Calculating Work Which do you
think involves more work: lifting a 40newton tuba up three steps (about
0.5 meters) or lifting a 5-newton
trumpet up the same three steps?
Your common sense may suggest that
lifting a heavier object requires more
work than lifting a lighter object. This
is true. But is it more work to lift an
instrument up three steps or up to
the top story of a building? As you
might guess, moving an object a greater distance requires more work than moving the same object a shorter
distance.
The amount of work you do depends on both the amount of force you exert and the distance the object
moves. The amount of work done on an object can be determined by multiplying force times distance.
Work = Force × Distance
When you lift an object, the upward force you exert must be at least equal to the object’s weight. So, to lift
the trumpet, you would have to exert a force of 5 newtons. The distance you lift the trumpet is 0.5 meters.
The amount of work you do on the trumpet can be calculated using the work formula.
Work = Force × Distance
Work = 5 N × 0.5 m
Work = 2.5 N•m
To lift the tuba, you would have to exert a force of 40 newtons. So the amount of work you do would be 40
newtons × 0.5 meters, or 20 N•m. You do more work to lift the heavier object. When force is measured in
newtons and distance in meters, the SI unit of work is the newton-meter (N•m).
3.Solve To help rearrange the furniture in your classroom, you exert a force of 20 N to push a desk 10 m. How
much work do you do? Show your calculations below. ______________________________________________
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How do machines affect work?
What do you picture when you hear the word machine? You may think of machines as complex gadgets with
motors, but a machine can be as simple as a ramp. Machines are devices that allow you to do work in an
easier way.
Machines do not reduce the amount of work you do. Instead, they just change the way you do work. For
example, you would do the same amount of work moving a box onto a stage whether or not you use a ramp.
However, the ramp would make your work easier. A machine makes work easier by changing at least one of
three factors:
• the amount of force you exert,
• the distance over which you exert your force, or
• the direction in which you exert your force.
When you do work, the force you exert is called the input force. You exert your input force over the input
distance. In the illustration below on the right, the girl’s input force is the force she uses to pull the box up the
ramp. The input distance is the length of the ramp. The machine exerts the output force over the output
distance. The weight of the box is the output force. The height of the ramp is the output distance. Input force
times input distance equals input work. Output force times output distance equals output work. Because
machines do not reduce the amount of work, output work is never greater than input work.
In the illustration on the left, the girl is not using a machine. Without a machine, her input force is the same as
her output force. Her input distance, or how far she moves the box, is the same as her output distance.
Lesson Check
1. Define Define work in your own words.
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2. Contrast Contrast situations in which work
is done with different amounts of force to
situations in which no work is done.
a.Where is work being done when you ride a
bicycle?
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b.What could you do to apply more force when
riding a bicycle? What would happen as a result?
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c. Describe the work being done when two people on bicycles coast down a hill while holding their feet
still on the pedals.
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3. Identify Which statement best explains the work done when a child presses a finger hard against a solid
brick wall?
A Work is done on the wall.
B No work is done on the wall.
C Some work is done on the child’s finger.
D Force is exerted on the child’s finger by the wall.
4. Identify Jeff and Matt are both pushing boxes across the floor. Jeff is pushing a small, light box. Matt is
pushing a large, heavy box. Which term is used to identify the force each person is exerting?
A input force
B distance force
C output force
D machine force
5. Compare Why does a ramp make the work of moving a piano from a sidewalk to a truck easier?
A It lessens the amount of work being done.
B It reduces the amount of force you must exert.
C It reverses the direction in which you exert your force.
D It decreases the distance over which you must exert your force.
6. Compare Suppose you are moving a rolling suitcase. Describe three ways of moving it that require
different amounts of force, and thus different amounts of work.
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7. Contrast Contrast the work you do in picking up a 5-kg bag of ice with the work you do in carrying the
bag to your family’s car.
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8. Define In which situation is work being done?
A memorizing a scientific law
B lifting a paper clip from a desk
C reading a passage in your science book
D standing and delivering a speech to your class
9. Identify List the three factors that machines change in order to make work easier.
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10. Compare Work is done on a ball when a pitcher throws it. Is the pitcher still doing work on the ball as it
flies through the air? Explain.
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11. Calculate Determine the amount of work done in each situation.
a. A man applies a force of 500 N to push a truck 100 m down the street.
How much work does he do?
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b. When you push a 75-N box 3 m across the floor, how much work has been done?
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c. In which situation do you do more work—when you lift a 75-N bowling ball 2 m off the floor or when
you lift two 40-N bowling balls 1 m off the floor?
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