Welcome Back to PHY211!

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Welcome Back to PHY211!
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Recap from lecture 1

• Motion described by position s(t) (1D)
• Important concept
• displacement DssF-sI
• time interval DttF-tI
• (average) velocity VavDs/Dt
• For v. small Dt defines instantaneous velocity

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Graphing Motion

• Plot displacement vertically, time horizontally
• Discussed how to read motion from curve of s(t)
• Instantaneous velocity is slope of s vs. t curve

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Examples
s
t
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Cart experiment

• Transmitter sends out signal which is reflected
  back by cart - can calculate distance to cart at
  any instant.
• cart is not subject to any forces on track -
  expect constant velocity
• computer shows position vs. time plot for motion
  (and velocity plot)
• 2 different speeds and directions

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Describing Motion

• We have seen that in describing motion its useful
  to introduce the concept of velocity
• v Ds/Dt
• Two velocities encountered
• average velocity vav
• instantaneous velocity vinst
• Motions with constant velocity
• vav vinst

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Acceleration

• Similarly when velocity changes it is useful
  (crucial!) to introduce acceleration a
• a Dv/Dt (vF-vI)/Dt
• Average acceleration - keep time interval Dt
  non-zero
• Instantaneous acceleration
• ainst limDt-gt0 Dv/Dtdv/dt

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Graph interpretation

• Plot now v vs. t
• Slope measures acceleration
• Positive a means v is increasing
• Negative a means v decreasing (deceleration)

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Examples
v
t
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Fan cart experiment

• Attach fan to cart - provides a constant force
  (will see implies constant acceleration)
• Force acts to slow initial motion
• Sketch graphs of position, velocity and
  acceleration for cart that slows down and then
  turns around

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• You are throwing a ball up in the air. At its
• highest point the balls
• Velocity v and acceleration a are zero
• v is non-zero but a is zero
• Acceleration is non-zero but v is zero
• V and a are both non-zero

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Acceleration from s vs t plot ?

• If s vs. t plot is linear zero accel.
• Is s vs. t is curved accel is non-zero
• If slope is going from positive to negative
• a is negative
• If slope is going from negative to positive
• a is positive
• Accel is rate of change of slope!