Acceleration and Velocity: Signs
Today’s Topics
• Sign of acceleration and velocity
• Examples
• Gravity
v v 
s 2  s1   1 2 t
 2 
vavg 
s 2  s1  v1t  12 at 2
Walk the following motion
 Positive velocity, positive acceleration
 Positive velocity, negative acceleration
 Negative velocity, negative acceleration
 Negative velocity, positive acceleration
s2  s1
t
Summary
v v
a  2 1
t
v22  v12
s 2  s1 
2a
From Wikipedia: “In common speech, the term acceleration commonly is
used for an increase in speed (the magnitude of velocity); a decrease in
speed is called deceleration.”
Be careful – in physics, a negative acceleration isn’t always a decrease
in speed!
EF 151 Spring, 2017 Lecture 1-7
Example
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 v  v2 
s2  s1   1
 t
 2 
s2  s1  v 1t  21 at 2
1
s2  s1 
v 22  v 12
2a
EF 151 Spring, 2017 Lecture 1-7
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2
Objects Subject to Gravity Only
A car starts from rest and accelerates at 6 ft/s2 until reaching a
speed of 42 ft/s. 120 ft later the car is traveling at 54 ft/s.
Determine the total time and distance of the trip. Assume constant
acceleration.
 ________ is the only force on object
 Acceleration is constant, and equal to ____, only as
long as:
 changes in ______ are not enough to cause changes in g
 _______________ and other forces are ignored
Which equation will you use for the distance of leg A?
Which equation will you use to solve for the time of leg A?
Which equation will you use to solve for the time of leg B?
EF 151 Spring, 2017 Lecture 1-7
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EF 151 Spring, 2017 Lecture 1-7
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Crushing a Mini Cooper
Crushing a Mini Cooper – Part 2
We want to crush a Mini Cooper. Don’t ask why. A crane lifts a
10 ton weight 50 ft above the Mini Cooper, waits until the driver
looks up, then drops the weight. What is the speed of the weight
when it hits the Mini? How long did the driver have to escape?
To make the effect more dramatic, we mount a cannon 50 ft
above the Cooper and shoot a 50 ton weight straight up at a
speed of 20 mph. What is the speed of the weight when it hits the
Mini Cooper and how long does it take?
What changes if we turn the cannon
and shoot down rather than up?
 v  v2 
s2  s1   1
 t
 2 
EF 151 Spring, 2017 Lecture 1-7
s2  s1  v 1t  21 at 2
s2  s1 
v 22  v 12
2a
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 v  v2 
s2  s1   1
t
 2 
5
Elevator Example
Given: Ball thrown vertically upward from the 40 ft level of an
elevator shaft with an initial velocity of 50 ft/sec. At the same instant,
an open elevator passes the 10 ft level moving upward with a constant
velocity of 5 ft/sec
50 ft/s
Required: When and where the ball hits the elevator
40 ft
What two things are the same when the ball hits the elevator?
Elevator
EF 151 Spring, 2017 Lecture 1-7
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10 ft
5 ft/s
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EF 151 Spring, 2017 Lecture 1-7
How does the time change if
we shoot down instead of up?
s2  s1  v 1t  21 at 2
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s2  s1 
v 22  v 12
2a
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