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Physics; re-up only; not mine
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Lecture 26: Angular velocity and acceleration; Rotation with constant angular acceleration
Lecture Objectives 1. Distinguish rotational and translational quantities. 2. Apply the rotational kinematic relations in rotating objects.
Rigid bodyNeglect deformations
Perfectly definite and unchanging size and shape
Image from http://wiki.blender.org
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Describing rotation
Comparing translational and rotational motions
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Example: Earth undergoes both types of rotational motion.
• It revolves around the sun once every 365 ¼ days. • It rotates around an axis passing through its
geographical poles once every 24 hours.
Rotation and Revolution
An axis is the straight line around which rotation takes place. • When an object turns about an internal axis—that is,
an axis located within the body of the object—the motion is called rotation, or spin.
• When an object turns about an external axis, the motion is called revolution.
Example The turntable rotates around its axis while a ladybug sitting at its edge revolves around the same axis.
Angular measurements
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Angular measurements
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• Kinematics of a rotating body
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Angular velocity (rad/s)•
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Given: t1 = 2.0s t2 = 5.0s
(a) Substitute the values of time t into the given equation:
(b) The flywheel turns through an angular displacement of Δθ = θ2 – θ1 = 250rad – 16rad = 234rad. Since the diameter is 0.36m, r = 0.18m. The distance traveled is therefore:
(c) The angular velocity:
(d) The instantaneous angular velocity at t = 5.00s
Curl (right hand) fingers to direction of rotation, thumb points to direction of angular quantity
Direction of vector quantities
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Angular acceleration (rad/s2)
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Relative directions of angular velocity and acceleration
Same direction, speeding up Different directions, slowing down
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(a) Using the equation for instantaneous angular velocity for time 2.0s and 5.0s:
We will use this to solve for the average angular acceleration:
(b) The instantaneous acceleration at time t = 5.0s is:
Comparing translational and rotational motion
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Constant angular acceleration
Problems to be considered: acceleration = constant
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Sample Problem: Rotation with constant acceleration You have just finished watching a movie on DVD and the disc is slowing to a stop. The angular velocity of the disc at t = 0 is 27.5rad/s and its angular acceleration is constant at -‐10.0rad/s2. A line PQ on the surface of the disc lies along the +x-‐axis at t = 0. (a) What is the disc’s angular velocity at t = 0.300s? (b) What angle does the line PQ make with the x-‐axis at
this time?
Given: ω0 = 27.5rad/s Constant α = -‐10.0rad/s2 (a) ω at t = 0.30s (b) angle at t = 0.30s
(a) Substitute ω0 , α and t to the equation:
Given: ω0 = 27.5rad/s Constant α = -‐10.0rad/s2
(b) To get the angle, first calculate the angular displacement:
Converting into angle:
Seatwork
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• Seatwork 1
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•
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Seatwork 2
Seatwork 3 and 4: The figure shows a graph of ωz and αz versus time for a particular rotating body. SW 3: During which time intervals is the rotating body speeding up? (a) 0 < t < 2s (b) 2s < t < 4s (c) 4s < t < 6s SW4: During which time is the rotation slowing down? (a) 0 < t < 2s (b) 2s < t < 4s (c) 4s < t < 6s
Seatworks 5, 6, 7, 8 and 9
Arc length: s = rθ (with θ in radians) To convert in radians use: πrad = 180o
If angular velocity is constant: θ-‐θo = ωt
Seatwork answers
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• Seatwork 1
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•
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Seatwork 2
Seatwork 3 and 4: The figure shows a graph of ωz and αz versus time for a particular rotating body. SW 3: During which time intervals is the rotating body speeding up? (a) 0 < t < 2s (same sign: both positive) (b) 2s < t < 4s (c) 4s < t < 6s (same sign: both negative) SW4: During which time is the rotation slowing down? (a) 0 < t < 2s (b) 2s < t < 4s (opposite sign) (c) 4s < t < 6s
Seatworks 5, 6, 7, 8 and 9
Arc length: s = rθ (with θ in radians) To convert in radians use: πrad = 180oIf angular velocity is constant: θ-‐θo = ωt
Seatwork 5 to 9: s = rθ (with θ in radians) πrad = 180o
Since angular velocity is constant: θ-‐θo = ωt