Download Principles of Technology/Physics in Context (PT/PIC)

Physics in Context (PT/PIC)
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Chapter 7
Work and Energy 1
Key Objectives:
Define the term work, and state its SI unit.
Solve problems involving force, displacement,
and work.
Define the term power, and state its SI unit.
Solve problems involving power and work.
7.1 WORK
• Would you pay a
person $5 an hour to
do this?
• The typical answer is
“No!” because the
person isn’t doing any
work.
• What exactly do we
mean by the term
work?
7.1 WORK
Work = Force x Distance
• In physics, work is
defined as the product of
force and displacement,
assuming that both are in
the same direction.
7.1 WORK
• Even though force and displacement are
vector quantities, work is a scalar quantity.
• The unit of work is the newton meter,
which is called a joule (J) in honor of
English scientist James Prescott Joule.
Assessment Question 1
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A.
B.
C.
D.
E.
All of the following statements are TRUE
EXCEPT:
In physics, work is defined as the product of
force and displacement,
Work occurs if force and displacement are in
the same direction.
Force and displacement are scalar quantities.
Work is a scalar quantity.
The unit of work is the newton meter, which is
called a joule (J) in honor of English scientist
James Prescott Joule.
7.1 WORK
• PROBLEM
• How much work is done on an object if a
force of 30 newtons [south] displaces the
object 200 meters [south]
7.1 WORK
• SOLUTION
Assessment Question 2
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A.
B.
C.
D.
E.
How much work (W) is done on an object if a
force of 75 N [south] (F) displaces the object
530 m [south] (d)?
W = F x d = 75 N [S] x 530 m [S] =
0.14 J
7.1 J
460 J
610 J
40,000 J
7.1 WORK
• Now we will suppose that the force and
the displacement are not in the same
direction.
• We define work to be the product of the
component of the force in the direction of
the displacement and the displacement.
7.1 WORK
• The diagram illustrates this relationship.
7.1 WORK
• PROBLEM
• As Alex pulls his red wagon down the
sidewalk, the handle of the wagon makes
an angle of 60° with the pavement.
• If Alex exerts a force of 100 newtons along
the direction of the handle, how much
work is done when the displacement of the
wagon is 20 meters along the ground?
7.1 WORK
• SOLUTION
7.1 WORK
• SOLUTION
Assessment Question 3
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A.
B.
C.
D.
E.
As a Vista Ridgean pulls a red wagon down the
road, the wagon handle makes an angle of
80°(θ) with the ground.
How much work (W) is done if a force of 570 N
(F) displaces the object 95 m (d)?
W = F∙cosθ x d = 570 N∙cos 80° x 95 m =
0.94 J
6.0 J
100 J
9400 J
430,000 J
7.1 WORK
• PROBLEM
• A constant force of 50 newtons is applied
over a distance of 10 meters.
• (a) Prepare a graph of force versus
displacement.
7.1 WORK
• SOLUTION
• (a)
Assessment Question 4
A force of 390 N (F) displaces the object
120 m (d)? Determine the ratio of force
versus displacement.
F/d
A. 3.25 N/m
B. 5.00 N/m
C. 7.5 N/m
D. 10.0 N/m
E. 15.0 N/m
7.1 WORK
• PROBLEM
• A constant force of 50 newtons is applied
over a distance of 10 meters.
• (b) Calculate the area underneath the
curve.
7.1 WORK
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SOLUTION
(b)
Area = base height
Area =(10 m)(50 N)
Area = 500 J
Assessment Question 5
A constant force of 390 N (F) displaces the object
120 m (d)?
Calculate the area underneath the curve in the
graph of force versus displacement?
Area = base x height =(120 m)(390 N) =
A.
B.
C.
D.
E.
3.25 J
510 J
2500 J
47000 J
64000 J
7.1 WORK
• PROBLEM
• A constant force of 50 newtons is applied
over a distance of 10 meters.
• (c) Calculate the work done.
7.1 WORK
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SOLUTION
(c)
W = F∙d
W = (50 N)(10 m) = 500 J
Assessment Question 6
A constant force of 930 N (F) displaces the
object 210 m (d)?
How much work (W) is done?
W = F x d =(930 N)(210 m) =
A.
B.
C.
D.
E.
3500 J
17500 J
200000 J
710000 J
2500000 J
7.1 WORK
• It is evident that the area under the forceversus-displacement curve, shown below,
is equal to the work done on the object.
7.1 WORK
• In general, the area under any force curve
is equal to the work done.
Assessment Question 7
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A.
B.
C.
D.
E.
All of the following statements are TRUE
EXCEPT:
If force and the displacement are not in the
same direction work is the product of the
component of the force in the direction of the
displacement and the displacement.
The area under the force-versus-displacement
curve is equal to the work done on the object.
In general, the area under any force curve is
equal to the work done.
Work may be calculated graphically,
geometrically or algebraically.
Work is a vector quantity.
7.2 POWER
• Power is a term that is frequently misused.
• We define power as the rate at which work
is done:
7.2 POWER
• Power is also a scalar quantity, and its unit
is joules per second (J/s), also known as
the watt (W).
Assessment Question 8
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A.
B.
C.
D.
E.
All of the following statements are TRUE
EXCEPT:
Power is a vector quantity used frequently.
Power is the rate at which work is done.
Power is a scalar quantity,
The unit for power is joules per second (J/s).
The unit for power is the watt (W).
7.2 POWER
• PROBLEM
If 300 joules (W) of work is done on an
object in 1.0 minute (60 seconds) (t).
What is the power (P) expended on the
object?
7.2 POWER
• SOLUTION
Assessment Question 9
If 650 joules (W) of work is done on an object in
(95 seconds) (t).
What is the power (P) expended on the object?
P = W / T = 650 J / 95 s =
A. 0.15 W
B. 6.8 W
C. 555 W
D. 745 W
E. 62000 W
7.2 POWER
• PROBLEM
• A 200-newton force is applied to an object
that moves in the direction of the force.
• If the object travels with a constant velocity
of 10 meters per second, calculate the
power expended on the object.
7.2 POWER
• SOLUTION
Conclusion
• Work is the product of force and the
component of displacement in the
direction of the force; work is a scalar
quantity. Without motion there can be no
work.
• Power is the rate at which work is done,
and it is also a scalar quantity.
Assessment Question 10
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A 880 N force (F) is applied to an object that
moves in the direction of the force.
If the object travels with a constant velocity of
77 m/s (v), calculate the power (P) expended
on the object.
P = W / T = F∙d / T = F∙v =
A.
B.
C.
D.
E.
32000
68000
79000
88000
99000
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