Transcript Physics 102 Introduction to Physics

Physics 102-002
Announcements
• WebAssign –
– Chapter 8 and 9 due Wednesday
• Exam #2 corrections due Wednesday
• Revised class schedule posted on website
Picture: Trebuchet at Château des Baux, France
Class Schedule
3/11 –
3/18
Spring Recess
3/19
Chapter 10
Projectile and Satellite Motion (Pg 184-191)
3/21
Chapter 11
Atomic Nature of Matter (Pg 211-223)
3/26
Chapter 22
Electrostatics, Part 1 (Pg 410-419)
3/28
Chapter 22
Electrostatics, Part 2 (Pg 419-430)
4/2
Chapter 23
Electric Current, Part 1 (Pg 436-443)
4/4
Chapter 23
Electric Current, Part 2 (Pg 444-452)
Chapter 10
Projectile and Satellite
Motion
(Not so much)
• Projectile Motion
– Projectiles Launched Horizontally
– Projectiles Launched at an Angle
Projectile Motion
Projectile motion refers to the motion of an object projected into the air at an
angle. A few examples include:
• a soccer ball being kicked
“Height”
• a baseball being thrown
• an athlete long jumping
• fireworks
• water fountains
“Range”
Projectile terminology
Before Galileo, people thought
projectiles followed a straight line until
it lost its “impetus”, then fell to the
Galileo was the first to theorize that a
earth.
projectile was influenced by 2
independent motions, the vertical
motion influenced by the pull of
gravity, and the horizontal motion
which was uniform, consistent with the
law of inertia.
Projectile Motion
Without gravity, you could throw a rock and it would follow a straight line
path at constant speed.
But in the real world, if you throw a rock, it falls to the ground because of
gravity. Since the vertical pull of gravity is the only force acting on the rock
(if we neglect air resistance), this is a sign that the horizontal and vertical
motions are independent.
This figure shows both components. They
change (or stay the same) independently of
each other
Projectile Motion Applet
http://www.walter-fendt.de/ph11e/projectile.htm
Projectiles Launched Horizontally
When throwing a ball horizontally:
The horizontal component of the ball’s velocity doesn’t change as the
falling ball moves forward.
The vertical component changes exactly in the same way as it would if
the ball were dropped vertically.
The curved path is a combination of the uniform horizontal motion and the
vertical acceleration due to gravity
Projectiles Launched Horizontally
Whether an object is
dropped vertically or
launched horizontally, its
rate of fall is the same. It
has the same acceleration
downward due to gravity.
The projectile’s horizontal
speed is constant (unless
there’s air drag).
At a given location on the earth and in the absence of air resistance, all
objects fall with the same uniform acceleration. Thus, two objects of different
sizes and weights, dropped from the same height, will hit the ground at the
same time.
An object is controlled by two independant motions. So an object
projected horizontally will reach the ground in the same time as an
object dropped vertically. No matter how large the horizontal velocity
is, the downward pull of gravity is always the same.
Projectiles Launched Horizontally
Newton’s first law applies: Without gravity, an
object in motion will continue in motion with the
same speed and in the same direction.
With gravity, the object will fall below its “inertial”
path. Gravity will cause a downward acceleration.
No horizontal force is needed to maintain the
cannonball’s horizontal motion … inertia does that.
Physics Place Interactive Figures
http://wps.aw.com/aw_hewitt_cp_10/0,10865,2370903-,00.html
Projectiles Launched at an Angle
A projectile falls from ideal trajectory (the straight-line path that it would follow if
there were no gravity) the same distance over time regardless of the launch
angle.
d = ½ gt2
Where d = the distance fallen
d = ½ (10)t2 = 5 t2
Water Balloons game
http://library.thinkquest.org/2779/Balloon.html
Projectiles Launched at an Angle
Remember: the horizontal component of the
projectile’s velocity doesn’t have any affect on it’s
vertical velocity.
Notice how the vertical velocity changes as the
projectile moves along the trajectory. It’s equal to
zero at the apex. The horizontal velocity vector is
always the same.
Animation: A projectile launched
vertically in a vehicle moving at
uniform speed will come down at the
same spot that it was launched from!
A launch angle of 45 degrees gives the greatest
range (if no air drag). Launching at a higher or
lower angle gives a shorter range. There are
always 2 angles that will give the same range, and
they add to 90 degrees.
Monkey zookeeper animation
http://www.physicsclassroom.com/mmedia/vectors/mzng.html
Projectiles Launched at an Angle
Air Drag has the effect of reducing the
range and making the path of the projectile
altered from the ideal “parabolic” shape.
The projectile has the same deceleration going up
as the acceleration it experiences going down.
The speed it loses going up is the same as the
speed it gains as it comes down.
So, when it reaches it’s starting height, it has the
same speed that it had when it was launched.
Question 1
If you toss a ball upward in a train moving at constant speed, it returns
to its starting place. Will the ball do the same if the train is
accelerating?
A. Yes
B. No
Question 1 Answer
If you toss a ball upward in a train moving at
constant speed, it returns to its starting
place. Will the ball do the same if the train
is accelerating?
A. Yes
 B. No
Question 2
When a rifle is being aimed at a distant
target, the barrel should be lined up so
that it points
A. Slightly higher than the target
B. Exactly at the target
C. Slightly lower than the target
Question 1 Answer
When a rifle is being aimed at a distant
target, the barrel should be lined up so
that it points
 A. Slightly higher than the target
B. Exactly at the target
C. Slightly lower than the target