Ch. 5 Work and Machines 5 WORK AND MACHINES What Is Work? (pages 156-159) This section explains the scientific meaning of work and describes how to calculate the work done on an object.
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Science Explorer Grade 7 Guided Reading and Study Workbook 53
Name ____________________________________ Date __________ Class ___________________
16. The mechanical advantage that a machine provides in a real situation is
called the mechanical advantage.
17. The mechanical advantage of a machine without friction is called the
machine’s mechanical advantage.ideal
actual
Simple Machines(pages 168-178)
This section describes the six kinds of simple machines. It also explains how tocalculate the ideal mechanical advantage for each simple machine.
Introduction (page 168)
1. What are the six basic kinds of simple machines?
a. b. c.
d. e. f.
Inclined Plane (pages 169–170)
2. What is an inclined plane? An inclined plane is a flat, slanted surface
that allows you to exert your input force over a distance.
3. What formula do you use to determine the ideal mechanical advantageof an inclined plane?
Ideal mechanical advantage = Length of inclineHeight of incline
pulleywheel and axlelever
screwwedgeinclined plane
SECTION
5–3
By looking carefully at photographs and illustrations in textbooks, you can help yourselfunderstand better what you have read. Look carefully at Figure 5 on page 161. What importantidea does this illustration communicate?
Reading Skill Practice
A machine can make a task easier in three ways. It can multiply the input force. It can multiplythe distance over which the force is exerted. Or it can neither multiply force or distance; in thatcase, a machine changes the direction in which you exert your force.
58 Guided Reading and Study Workbook Science Explorer Grade 7
Name ____________________________________ Date __________ Class ___________________
Machines in Human Systems(pages 182-184)
This section describes how the body uses natural levers and wedges.
Body Systems—Living Levers (pages 182–184)
1. What do most of the levers in your body consist of? They consist of
bones and muscles.
2. Your muscles are attached to your bones by tough connective tissue
called .
3. In a living lever in your body, what acts as the lever’s fulcrum?
The joint, near where the tendon is attached to the bone, acts as the fulcrum.
4. On the illustration of a living lever, label each arrow to show where theinput force and the output force are located. Also show where thefulcrum is located.
tendons
SECTION
5–4
Working Wedges (page 184)
5. What simple machines do your incisors resemble?
6. Explain how your front teeth are like an ax. When you bite down on
something, the wedge shape of your teeth produces enough force to break
it in half, just as an ax is used to split a log.
wedges
62 Guided Reading and Study Workbook Science Explorer Grade 7
Name ____________________________________ Date __________ Class ___________________
MathWiseFor the problems below, show your calculations. If you need more space, use another sheet ofpaper. Write the answers for the problems on the lines below.
Calculating Work (pages 158–159)
1. Work = 10 N � 35 m =
2. An elevator lifts a man with a weight of 500 N up three floors, or 30 m.How much work did the elevator do?
Work = 500 N � 30 m = 15,000 J
Answer:
Mechanical Advantage (page 163)
3. Mechanical advantage = =
4. Suppose you exert of force of 2,800 N to lift a desk up onto a porch. Butif you use a ramp, you need to exert a force of only 1,400 N to push it upthe ramp onto the porch. What is the mechanical advantage of the ramp?
Answer:
Calculating Efficiency (pages 164–165)
5. Efficiency = � 100% =
6. You do 4,000 J of work using a sledge hammer. The sledge hammer does3,000 J of work on the spike. What is the efficiency of the sledge hammer?
Answer: Efficiency = 75%
Efficiency = 3,000 J� 100% = 0.75 � 100% = 75%
4,000 J
50%100 J200 J
Mechanical advantage = 2
Mechanical advantage = 2,800 N = 21,400 N
460 N15 N
Work = 15,000 J
350 J
64 Guided Reading and Study Workbook Science Explorer Grade 7
Name ____________________________________ Date __________ Class ___________________
Advantage of an Inclined Plane (page 169)
7. Ideal mechanical advantage = =
8. Suppose you built a ramp to the front door of the post office for peopleusing wheel chairs. The post office door is 3 m above the level of thesidewalk. The ramp you build is 15 m long. What is the idealmechanical advantage of your ramp?
Answer:
Advantage of a Lever (page 172)
9. Ideal mechanical advantage = =
10. Suppose you held the handles of a wheel barrow 2.4 m from where theyare attached to the wheel. The heavy stone in the wheel barrow was 1.2 mfrom the wheel. What is the ideal mechanical advantage of the wheelbarrow?
Answer:
Advantage of a Wheel and Axle (pages 175–176)
11. Ideal mechanical advantage = =
12. Suppose the radius of your bicycle’s wheel is 30 cm. The radius of thebicycle’s axle is just 5 cm. What is the ideal mechanical advantage ofthat wheel and axle?