Physics 211 Lecture 5, Slide 1

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Physics 211 Lecture 5
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• Today's Concepts
• Free Body Diagrams
• Force due to strings
• Force due to springs
• Force due to gravity

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Stuff you asked about
Nothing was particularly confusing. You guys rock.
The part with the moon and how it proved the
universal law of gravitation.
How to find equations for tension and normal
force when an object is on an angled surface
Calculating the acceleration the moon experienced
from the earth confused me. Did the calculation
mean that the moon experiences the same
acceleration as the apple? How can that be if the
moon is so much farther away from the earth than
an apple on the surface? Can we go over it in
class???
Elevator scenarios
Springs. Also, U of I got DESTROYED by Missouri
despite all of the professor's trash talking.
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Act

• You are traveling on an elevator up the Sears
  tower. As you near the top floor, your weight
  appears to be
• Heavier
• Lighter
• Unchanged

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Preflight
a

• A box of mass m is hung with a string from the
  ceiling of an elevator that is accelerating
  upward. Which of the following best describes the
  tension T in the string
• T mg
• T gt mg
• T lt mg

m
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Preflight
L

• A box of mass m is hung by a spring from the
  ceiling of an elevator. When the elevator is at
  rest the length of the spring is L 1m. If the
  elevator accelerates upward the length of the
  spring will be
• L 1 m
• L lt 1 m
• L gt 1 m

a
m
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Act
This is an explanation from someone who got the
last question right
The fact that the elevator is accelerating
upward will produce more downward force on the
box. This will stretch the spring more than
usual, making the length greater than when the
spring is at rest.
Do you agree A) YES B) NO
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A block sits at rest on a horizontal frictionless
surface. Which of the following sketches most
closely resembles the correct free body diagram
for all forces acting on the block? Each red
arrow represents a force.
Preflight
A B C
D
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A block slides down a frictionless inclined
plane. Which of the following sketches most
closely resembles the correct free body diagram
for all forces acting on the block? Each red
arrow represents a force.
Act
A B C
D
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A cart with mass m2 is connected to a mass m1
using a string that passes over a frictionless
pulley, as shown below. The cart is held
motionless.
Act

• The tension in the string is
• A) m1g
• B) m2g
• C) 0

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A cart with mass m2 is connected to a mass m1
using a string that passes over a frictionless
pulley, as shown below. Initially, the cart is
held motionless, but is then released and starts
to accelerate.
Act
a
m2

• After the cart is released, the tension in the
  string is
• A) m1g
• B) gt m1g
• C) lt m1g

g
m1
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A pair of forces is missing !!
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Act

• An astronaut on Earth kicks a bowling ball
  straight ahead and hurts his foot. A year later,
  the same astronaut kicks a bowling ball on the
  moon in the same manner with the same force.
  
• His foot hurts...

A) more B) less C) the same
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• The masses of both the bowling ball and the
  astronaut remain the same, so his foot will feel
  the same resistance and hurt the same as before.

Ouch.
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Act

• A block weighing 4 lbs is hung from a rope
  attached to a scale. The scale is then attached
  to a wall and reads 4 lbs. What will the scale
  read when it is instead attached to another block
  weighing 4 lbs?

m
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A) 0 lbs. B) 4 lbs. C)
8 lbs.