Transcript PHYS16 – Lecture 4 Looney Tunes Motion Ch. 2 Motion in 1D Before we begin… this class is different • My role vs.

PHYS16 – Lecture 4
Looney Tunes
Motion
Ch. 2 Motion in 1D
Before we begin… this class is different
• My role vs. your role
– My role… Guide, Resource, Demonstrator
– Your role… Thinker, Inquisitor, Observer
• The role of homework
– Physics is about trying to describe our world with
a set of mathematical equations
– Homework is not regurgitating information
• Cracking the problem – which eqs./concepts to use
• Solving the problem – apply the math correctly
Motion
• Ch. 2 Motion in 1D
– Position, Velocity, and Acceleration
– Free Fall
• Ch. 4 Motion in 2D
– Projectile Motion
– Relative Motion
Intro – Motion pre-question
• If a 0.50 kg ball is launched straight up and is given an
initial velocity of 15 m/s, what will be the velocity of
the ball on the way back down at the launch point?
(Note: assume air resistance is negligible and that
g=9.8 m/s2.)
A) It is the same as when it was launched.
B) It is dependent on initial velocity.
C) It is dependent on the acceleration constant.
D) It is dependent on the angle it was shot at.
E) None of the above
Displacement, Velocity, Acceleration
Displacement
• Displacement vs. Distance
– Displacement = change in position
– Distance = total position traversed
x  x f  xi
d
Velocity
• Velocity vs. Speed
– Avg. Velocity –
– Avg. Speed –
– Inst. Velocity –
– Inst. Speed –
x t
d t
dx dt
dx dt
Can the magnitude of average velocity
ever equal the average speed?
Acceleration
• Acceleration
– Avg. Acceleration – v t
– Inst. Acceleration – dv dt
Position graph where
acceleration is not 0
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Displacement, Velocity, Acceleration
dt

dt

X
V
a
d
dt
d
dt
Demo: Remote Control Car
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Questions
• Can displacement be zero and distance be
Yes
positive?
No
• Can distance ever be negative?
No
• Can speed ever be negative?
Yes
• Can acceleration be negative?
• Can an object that is stopped at t=0 have a
Yes
positive acceleration?
• If an object is traveling at a constant velocity can
it have an acceleration?
No
Free Fall
Free Fall
• Object under influence of the Earth’s gravity is
in free fall
• Dropped, thrown up, thrown down ->
acceleration due to gravity is the same
• g=9.80 m/s2, ay=-g
Derive Kinematic Equations
I. v f  vi  a x t
ax 2
II. x f  xi  vxit  t
2
2
2
III. vxf  vxi  2a( x f  xi )
Conclusions
• Displacement and Distance are different!
• Avg. Speed is NOT the magnitude of Avg. Vel.
• Displacement -> Velocity -> Acceleration
(take derivatives)
• Const. acceleration gives the three equations