Section 2: Weight and Drag Force
Newton’s second law can be used to explain
the motion of falling objects.
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4.2 Weight and Drag Force
Essential Questions
• How
are the weight and
the mass of an object
related?
• How
do actual weight
and apparent weight
differ?
• What
effect does air
have on falling objects?
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Weight and Drag Force
4.2 Weight and Drag Force
A. How are the weight and the mass of an object related?
1. Weight is the gravitational force experienced by an
object.
2. Because weight is a force, the proper unit used to
measure weight is the newton.
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4.2 Weight and Drag Force
3. Fg = mg, where m is the
mass of the object and
g is the gravitational
field.
a. Near Earth’s surface, g
is 9.8 N/kg toward
Earth’s center.
b. 9.8 m/s2 = 9.8 N/kg
since
1 N = 1 kg  m/s2
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4.2 Weight and Drag Force
4. The gravitational field is a
vector quantity.
a. Relates the mass of
an object to the
gravitational
force it experiences
at a given location.
b. A field force whose
magnitude is directly
proportional to the
mass of the object
experiencing the
force.
4.2 Weight and Drag Force
Practice Problems
16. You place a watermelon on a spring scale calibrated
to measure in newtons. If the watermelon’s mass is
4.0 kg, what is the scale’s reading?
4.2 Weight and Drag Force
Practice Problems
17. You place a 22.50-kg television on a spring scale. If the
scale reads 235.2 N, what is the gravitational field at
that location?
4.2 Weight and Drag Force
Practice Problems
18. A 0.50-kg guinea pig is lifted up from the ground.
What is the smallest force needed to lift it? Describe
the particular motion resulting from this minimum
force.
4.2 Weight and Drag Force
Practice Problems
19. A grocery sack can withstand a maximum of 230 N
before it rips. Will a bag holding 15 kg of groceries
that is lifted from the checkout counter at an
acceleration of 7.0 m/s2 hold?
4.2 Weight and Drag Force
B. How do actual weight
and apparent
weight differ?
1. Apparent weight
is the support force
exerted on an
object during a
vertical acceleration.
4.2 Weight and Drag Force
2. Depends on how you are accelerating.
a. Not accelerating: apparent weight = actual
weight
b. Accelerating upwards:
apparent weight  actual weight
c. Accelerating downwards:
apparent weight  actual weight
3. Examples
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4.2 Weight and Drag Force
4. Weightlessness
a. the condition
where there are
no contact forces
acting to support
the object
b. the object’s
apparent weight
is zero.
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Weight and Drag Force
4.2 Weight and Drag Force
Practice Problem
You are in an elevator, standing on a bathroom scale.
You notice that the scale reads a weight that is less
than your actual weight. (Assume that the scale is
correctly calibrated.)
a.Is the elevator moving at constant velocity, or is it
accelerating?
b. If it is accelerating, what is the direction of the
acceleration?
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Weight and Drag Force
4.2 Weight and Drag Force
Practice Problems
19. On Earth, a scale shows that you weigh 585 N.
a. What is your mass?
b. What would the scale read on the Moon
(g = 1.60 N/kg)?
4.2 Weight and Drag Force
Practice Problems
20. Use the results from Example Problem 3 to answer
questions about a scale in an elevator on Earth.
What force would be exerted by the scale on a
person in the following situations?
a. The elevator moves upward at constant speed.
4.2 Weight and Drag Force
b. It slows at 2.0 m/s2 while moving downward.
c. It speeds up at 2.0 m/s2 while moving downward.
4.2 Weight and Drag Force
Practice Problem 20
d. It moves downward at constant speed.
e. In what direction is the net force as the elevator slows
to a stop as it is moving down?
4.2 Weight and Drag Force
C. What effect does air have on falling objects?
1. Drag force is the force exerted by a fluid on
an object opposing motion through the
fluid.
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4.2 Weight and Drag Force
2. Drag force is dependent on:
a. the motion of the
object
b. the properties of the
object
c. the properties of the
fluid (viscosity and
temperature) that
the object is moving
through.
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4.2 Weight and Drag Force
3. The constant velocity that is reached
when the drag force equals the
force of gravity is called the terminal
velocity.
a. Drop an object, has a small drag
force.
b. As it falls, drag force increases
as object’s velocity increases.
c. As it continues to fall, drag force
will equal force of gravity.
d. At this point, Fnet = 0, so no
acceleration.
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Weight and Drag Force
4.2 Weight and Drag Force
4.2 Assignment:
p. 105 #22-27
p. 114- 115 #45-54
Due Tuesday, 11/10