Transcript Measuring Acceleration - St. Louis Area Physics Teachers

Ummm…Isn’t that thing in the bag an accelerometer?
How does that tube thing work?
 It’s called an accelerometer, but it doesn’t really
measure acceleration.
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A clue to what it measures
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 Turn the tube so that it is vertical. Now it is
measuring along the “head-to-toe axis.”
When you stand motionless, the tube looks
like this:
 What does that reading of “1” mean?
 Draw a force diagram for a person standing
motionless. (2)
 How big is the normal force on the person
by the floor compared to the force of gravity
on the person by the earth? (3)
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Are you correct?
FNPS
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FGPE
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A Simple Confirmation of what that
device tells you
 Stand still. Hold the tube so that one end points
directly in front of you. The device is now measuring
along the “front-to-back” axis. The tube looks like
this:
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 On your force diagram for a person standing
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motionless, how big is the force pushing forward or
backward on the person compared to the force of
gravity on the person? (4)
Are you correct?
FNPS
FGPE
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A more correct name than
“Accelerometer”
 The device is actually a “Force Factor Meter.”
 The reading on the device will tell you the size of a
push or pull force on you compared to the size of the
force of gravity on you.
 The key to using it correctly is understanding which
force it is measuring.
 After obtaining the force factor readings, acceleration
can be found by examining a force diagram.
Easy example:
 On Mr. Freeze, a 45 kg rider speeds up while moving
to the right. Predict the direction of movement on
the force factor meter. (5)
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Example (continued)
 The force factor meter looks like this while the rider
accelerates. Draw a force diagram for the rider. (6)
Check your neighbor!
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Are you correct?
FNPS
FPPS
FGPE
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Find the acceleration!
 The rider’s mass is 45 kg. How much does the rider
weigh? (7)
 The force factor meter reads 2.2. How big is the
forward push on the person by the seat? (8)
 What is size of the net force acting on the person?
(9)
 What is the person’s acceleration? (10)
Bonus!
 Hey—the guy sitting next to this rider had a mass of
90 kg. What do you think his force factor meter
read? (11)
Tougher example
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 On the Screamin’ Eagle, a 45 kg rider
goes over the crest of a hill. The FF meter
is held on the head-to-toe axis. Predict the
reading on the meter. (12)
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 Draw a force diagram for the rider. (13)
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Are you correct?
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FGPE
 Can you calculate the net force on the
rider? The acceleration of the rider?
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OMG example
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down at the top of a loop. The FF meter is
held on the head-to-toe axis. Predict the
reading on the meter. (14)
 Draw a force diagram for the rider. (15)
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 On the Ninja, a 45 kg rider goes upside-
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inverted point of view, the FF meter is moving
“down” towards his lap.
 So this FF reading is for a force directed which
way? (16)
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 Before we go on, notice that from the rider’s
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Frame of Reference Check
Are you correct?
FGPE
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FNPS
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 Can you calculate the net force on the
rider? The acceleration of the rider?
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