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Lecture 2 Slide 1Physics 218 Fall 2010
Physics 218: Lecture 2
Chapter 2: Motion Forces Newton's laws
Lecture 2 Slide 2Physics 218 Fall 2010
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Lecture 2 Slide 3Physics 218 Fall 2010
http://www.physics.purdue.edu/class/services/HC218220.pdf
Physics 218 Fall 2010 Lecture 2 Slide 4
Position and Displacement
X0 1 2 3 4 5 6
x1 = 1 m
x2 = 5 m
x = x2 – x1 = (5 – 1) m = 4 m
The change in position, x, is called the displacement
x
Physics 218 Fall 2010 Lecture 2 Slide 5
Distance and Displacement
X0 1 2 3 4 5 6
x1 = 1 m
x2 = 3 m
Displacement: x = x2 – x1 = (3 – 1) m = 2 m
Distance: |5-1| m + |3-5| m = 6 m
x
Physics 218 Fall 2010 Lecture 2 Slide 6
Distance and Displacement
Distance (scalar)
Displacement (vector)
Lecture 2 Slide 7Physics 218 Fall 2010
Reading quiz
The reading assignment was Chapters 1 & 2
A. I read the whole assignment for todayB. I read everything except the examplesC. I read some of the assignmentD. I did not read the assignment
Lecture 2 Slide 8Physics 218 Fall 2010
quizYou and your dog go for a walk to a nearby park.
On the way, your dog takes many short side trips to chase squirrels, examine fire hydrants, and so on. When you arrive at the park, do you and your dog have the same displacement? Have you traveled the same distance?
A. Same displacement, same distanceB. Different displacement, same distanceC. Different displacement, different distanceD. Same displacement, different distance
Lecture 2 Slide 9Physics 218 Fall 2010
x po
sitio
n (m
)
Time (sec)
Velocity
Lecture 2 Slide 10Physics 218 Fall 2010
x po
sitio
n (m
)
Time (sec)
t1 t2
x 1x 2
vx = t2 - t1
x2 - x1
“constant velocity”
Velocity
Lecture 2 Slide 11Physics 218 Fall 2010
Bodies at constant velocity do experience forces, however,
the sum of all forces on a body at constant velocity is zero.
Lecture 2 Slide 12Physics 218 Fall 2010
Newton’s First Law
If the total force acting on a body is zero, it will maintain its
velocity* forever.
*The object at rest is just a special case of this with velocity equal to zero.
Lecture 2 Slide 13Physics 218 Fall 2010
Newton’s First Lawin Reverse
If a body is moving at constant velocity*, then the total force
acting on it is zero.
*The object at rest is just a special case of this with velocity equal to zero.
Lecture 2 Slide 14Physics 218 Fall 2010
x po
sitio
n (m
)
Time (sec)
“constant (zero) velocity”
vx = 0
Velocity
“At rest”
Lecture 2 Slide 15Physics 218 Fall 2010
First law
The net force on a moving object suddenly becomes zero. The object then:
A Stops abruptlyB Stops during a short time intervalC Changes directionD Continues at constant velocityE Slows down gradually
Lecture 2 Slide 16Physics 218 Fall 2010
If an object is traveling at a constant velocity, is it necessarily traveling in a straight line?
A. YesB. No
Average velocity 2
Physics 218 Fall 2010 Lecture 2 Slide 17
VelocityWhen an object undergoes a change in position it has a velocity
the magnitude of the velocity is the speed
For motion in a straight line the average velocity is the change in position over a given time interval t
The instantaneous velocity is the limit of the average velocity as the time interval t
0
txv
t
lim0
12
12
ttxx
txvAVG
Physics 218 Fall 2010 Lecture 2 Slide 18
Average Velocity and Average SpeedPosition vs. Time graph
12
12
ttxx
txVAVG
The average velocity is the ratio of the displacement, x, which occurs in time interval t
Lecture 2 Slide 19Physics 218 Fall 2010
If one continues to take average velocitiesover shorter t’s one approaches the limit vinst
Lecture 2 Slide 20Physics 218 Fall 2010
x po
sitio
n (m
)
Time (sec)initially zerovelocity
toward the endfurther along and
moving fast
Acceleration
Physics 218 Fall 2010 Lecture 2 Slide 21
AccelerationWhen an object undergoes a change in velocity it is said to be
acceleratingFor motion in a straight line the average acceleration is the change in
velocity v over a given time interval t
12
12
ttvv
tvaAVG
The instantaneous acceleration is the limit of the average acceleration as the time interval t 0
tva
t
lim0
Lecture 2 Slide 22Physics 218 Fall 2010
Constant accelerationIt is very useful to plot graphs of x, v, a versus t
0 2 4 6 8 100
20
40
60
0 2 4 6 8 100
5
10
0 2 4 6 8 100
1
2
posi
tion
(m)
time (s)
velo
city
(m/s
)
time (s)
acce
lera
tion
(m/s
2 )
time (s)
consttva
txtav
2tatvx
Lecture 2 Slide 23Physics 218 Fall 2010
Acceleration can be both + and -speed can decrease then increasein opposite direction
Lecture 2 Slide 24Physics 218 Fall 2010
Can have v > 0 anda < 0 as in (b).Can have v < 0 andA > 0 as in ( c )
Lecture 2 Slide 25Physics 218 Fall 2010
MassA measure of the inertiaAn intrinsic property of the objectObjects with a lot of inertia (large mass) are
harder to change the state of motion of compared to objects with a small amount of inertia (small mass)The units of mass are grams, kilograms, or
slugs
F
FSmall acceleration
Large acceleration
Lecture 2 Slide 26Physics 218 Fall 2010
WeightWeight is the force of gravity on an object
with massUnits of weight are Newtons or Pounds
Same massDifferent weight!
Lecture 2 Slide 27Physics 218 Fall 2010
Demos: Inertia
Table Cloth JerkEgg drop Inertial Ball
Lecture 2 Slide 28Physics 218 Fall 2010
InertiaYou are a passenger in a car and not
wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation?
A. Before and after the collision, there is a rightward force pushing you into the door.
B. Starting at the time of collision, the door exerts a leftward force on you.
C. Both of the above
