Download You are going to toss a rock straight up into the air by placing it on

You are going to toss a rock straight up into the air
by placing it on the palm of your hand (you're not
gripping it), then pushing your hand up very
rapidly. Rank the three times below from the largest
to the smallest number of forces acting on the
stone.
a.  the rock at rest on your palm
b. As your hand is moving up but before the rock
leaves your hand
c. Instantly after the rock leaves your hand.
d. After the rock has reached its highest point and
is now falling straight down.
1.  A=D<C<B
2.  A<D<C<B
3.  A=B<C=D
4.  C=D<A=B
5.  A<B<C<D
Phys 1021 -- 2
1
You are going to toss a rock straight up into the air
by placing it on the palm of your hand (you're not
gripping it), then pushing your hand up very
rapidly. Rank the three times below from the largest
to the smallest number of forces acting on the
stone.
a.  the rock at rest on your palm
b. As your hand is moving up but before the rock
leaves your hand
c. Instantly after the rock leaves your hand.
d. After the rock has reached its highest point and
is now falling straight down.
1.  A=D<C<B
2.  A<D<C<B
3.  A=B<C=D
4.  C=D<A=B
5.  A<B<C<D
For A and B, there are two forces, gravity and the normal force from
the hand. For C and D there is only one for, from gravity.
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New Topic
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5
amoon
g
R
where G = 6.67 x 10 -11 m3 kg-1 s-2
Direction: attractive along the line
between the centers of the two masses
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Which is stronger,
the Earth’s pull on
the Moon, or the
Moon’s pull on the
Earth?
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Which is stronger,
the Earth’s pull on
the Moon, or the
Moon’s pull on the
Earth?
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1
2.  2
Earth
3.  3
4.  4
d 5.  5
2d
1. 
1 2
5
4
2m
3
m
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2d
2m
5
4
d
2
1
3
m
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 Near the earth’s surface...
$
=g
Force of attraction between an object and
an astronomical body (Earth, Moon, etc.)
Weight = Force of gravity:
W = mg
Weight (like any force) is a vector.
scalar
units: newtons (N)
units: kilogram (kg)
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8
Suppose you stand on a spring scale in six identical elevators. Each
elevator moves as shown below. Let the reading of the scale in
elevator n be Sn. Rank in order, from largest to smallest, the six
scale readings S1 to S6. Some may be equal. Give your answer in the
form A>B = C > D.
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New Topic
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9
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Rank order, from largest to smallest,
the size of the friction forces in these
five different situations. The box and
the floor are made of the same
materials in all situations.
A.  fc >
B.  fb >
C.  fb >
D.  fa >
E.  fa =
fd >
fc =
fc >
fc =
fb >
fe >
fd =
fd >
fd =
fc =
fb > fa.
fe > fa .
fe > fa.
fe > fb.
fd = fe.
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Rank order, from largest to smallest,
the size of the friction forces in these
five different situations. The box and
the floor are made of the same
materials in all situations.
A.  fc >
B.  fb >
C.  fb >
D.  fa >
E.  fa =
fd >
fc =
fc >
fc =
fb >
fe >
fd =
fd >
fd =
fc =
fb > fa.
fe > fa.
fe > fa.
fe > fb.
fd = fe.
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1. 
2. 
3. 
4. 
5. 
6. 
1. 
2. 
3. 
4. 
By Group Number:
of a name block on a whiteboard?
of an eraser on a whiteboard?
of an upside-down eraser on a whiteboard?
of a book on a whiteboard?
of a calculator on a whiteboard?
of a pen (not rolling) on a whiteboard?
Steps:
Brainstorm how to do this
Make calculations, estimate the value
Get your materials -- ask me or Deepa for help here
Make measurements
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11
A 2.0 kg wood box slides
down a vertical wood wall
at a constant speed while
you push upward on it at a
45° angle. For wood on
wood, the coefficient of
kinetic friction is µk = 0.20.
What is Fpush?
  What magnitude of force
should you apply to
cause the box to slide
down with a 0.1 g
acceleration?
 
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A 2.0 kg wood box slides
down a vertical wood wall
at a constant speed while
you push upward on it at a
45° angle. For wood on
wood, the coefficient of
kinetic friction is µk = 0.20.
What is Fpush?
  What magnitude of force
should you apply to
cause the box to slide
down with a 0.1 g
acceleration?
 
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12
Myosin motors move along actin fibers in living
things as seen in these videos made by atomic
force microscopy (AFM):
Newton’s Third Law at work to
a) Contract muscles
b) Carry cargo
c) Move cilia
6.8 fps, Scan area, 150 × 75 nm2
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New Topic
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A
B
Is boxing glove A hitting glove B,
or is glove B hitting glove A?
The gloves
hit each other
For every action there is an
equal and opposite reaction.
For the boxing gloves:
Action:
A pushes on B
Reaction: B pushes on A
FA on B = – FB on A
Action and Reaction forces:
• have the same magnitude
• are in opposite directions
• act on different objects
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Action: force of the Earth on the apple (gravity)
Reaction: NOT force of the table on the apple
BUT force of the apple on the Earth
How can a table exert a force on an apple?
No apple, no force
With the apple, the table bends
ever so slightly so there is a force
upward (like a diving board)
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ice
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This is why you need to use
a Free Body Diagram and
consider the net force on
the block… only Fman on block
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Two blocks of masses 2m and m
are in contact on a horizontal
frictionless surface. If a force F
is applied to mass 2m, what is
the force on mass m ?
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Two blocks of masses 2m and m
are in contact on a horizontal
frictionless surface. If a force F
is applied to mass 2m, what is
the force on mass m ?
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17
A block of mass m rests on the
floor of an elevator that is
accelerating upward. What is
the relationship between the
force due to gravity and the
normal force on the block?
a
m
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A block of mass m rests on the
floor of an elevator that is
accelerating upward. What is
the relationship between the
force due to gravity and the
normal force on the block?
N
m
a>0
mg
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T
mg
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 
You tie a rope to a tree and
you pull on the rope with a
1. 
1 of 100 N. What is the
force
2. 
2
tension
in the rope?
3. 
3
4. 
5. 
4
5
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 
You tie a rope to a tree and
you pull on the rope with a
force of 100 N. What is the
tension in the rope?
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20
 
Two tug-of-war opponents each
pull with a force of 100 N on
1.  1
opposite
ends of a rope. What
2.  2
is the tension in the rope?
3. 
4. 
5. 
3
4
5
(1) 0 N
(2) 50 N
(3) 100 N
(4) 150 N
(5) 200 N
Phys 1021 -- 41
 
Two tug-of-war opponents each
pull with a force of 100 N on
opposite ends of a rope. What
is the tension in the rope?
(1) 0 N
(2) 50 N
(3) 100 N
(4) 150 N
(5) 200 N
Phys 1021 -- 42
21
 
 
 
 
Take 5 rubber bands to your table. You will stretch them,
measure the force needed, then combine them end to end
in two ways:
  To make a 5 - band - wide configuration
  To make a 5 - band - long configuration
What is the force needed to stretch a single rubber
band by 1 cm, 2 cm, 4 cm?
What force do you think is needed to stretch the rubber
band by 1 m? This is the spring constant in Hooke’s
Law.
Calculate and then measure:
What is the force needed to stretch each of the
configurations of rubber bands by 1 cm?
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A 200.0-N sign is suspended from a horizontal strut of negligible weight. The
force exerted on the strut by the wall is horizontal. Draw an FBD to show the
forces acting on the strut. Find the tension T in the diagonal cable supporting
the strut.
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 
 
 
In an electricity experiment, an electrically charged plastic
ball (mass = 100 g) is suspended on a 60 cm long string.
When a charged rod is brought near the ball, the rod exerts
a horizontal electrical force Felec on it, causing the ball to
swing out to a 20° angle and remain at rest there.
What is the magnitude of the electric force Felec?
What is the tension in the string?
Phys 1021 -- 45
The terminal speed of a Styrofoam
ball is 15 m/s. Suppose a Styrofoam
ball is shot straight down with an
initial speed of
30 m/s. Which velocity graph is
correct?
Phys 1021 -- 46
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The terminal speed of a Styrofoam
ball is 15 m/s. Suppose a Styrofoam
ball is shot straight down with an
initial speed of
30 m/s. Which velocity graph is
correct?
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  Gravity
  Normal
  Friction
  Tension
  Weight
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