Happy
September
st
1 !
• Today:
–Free Fall Motion
• Tomorrow:
–Free Fall Motion – More practice
• Homework: WebAssign, POTW,
Bowling Ball Lab, PHeT Lab
Extension (All due Friday)
• Quiz on Friday
FREEFALL
MOTION
Freefall
Problems
• The kinematic equations still hold.
• Up is positive, down is negative.
• Acceleration is caused by gravity alone
(acceleration due to gravity)
𝑎 = 𝑔 = −9.8 𝑚/𝑠 2
𝑔 ≈ −10 𝑚/𝑠 2
Must use g = -9.8 m/s2 on
WebAssign (and labs) but you
can use g = -10 m/s2 on all
quizzes and tests.
• Velocity at maximum height is 0 m/s
– It has to stop, turn around, then speed back
up towards the earth.
Sample
An object is dropped from a height of
75.0 m above ground level.
(a) Determine the time it takes to reach
the ground.
(b) Determine the final velocity at which
the object hits the ground.
Sample
A rescue helicopter is hovering over a person
whose boat has sunk. One of the rescuers
throws a life preserver straight down to the
victim with an initial velocity of 1.40 m/s and
observes that it takes 1.8 s to reach the water.
How high above the water was the preserver
released?
Note that the downdraft of the helicopter reduces the effects of air
resistance on the falling life preserver, so that an acceleration
equal to that of gravity is reasonable
Sample
A ball is thrown upward at 20.0 m/sec.
a) How high will be the ball when it is
moving at 10.0 m/sec?
b) When will it be there?
Sample
A helicopter is ascending vertically with a
speed of 5.5 m/sec. At a height of 125 m
above the Earth, a package is dropped
from a window.
a) How much time does it take for the package
to reach the ground?
b) How high will it rise before falling?
Thirsty
Thursday…
Make it to
Sonic before
4 PM!
• Today: Horizontal Projectile Motion
• Tomorrow: Quiz
• Next Week: Finish Kinematics and Test
• Homework: EVERYTHING! Get on it!
STEM CLUB meeting in room
117 after school. Be there! We
will be choosing officers.
Warm Up
TWO
DIMENSIONAL
MOTION
Projectile
Motion
• A projectile is an object upon which the
only force acting is gravity
• The horizontal motion is independent
of the vertical motion.
– Horizontally: nothing is acting on the
projectile so it moves at constant velocity in
the x-direction.
– Vertically: gravity (g) acts on the object so it
accelerates in the y-direction.
Equations
of Projectile
Motion
• Since we are moving in the xy-plane we will define the
following:
– Δx is the displacement in the horizontal (x) direction
– Δy is the displacement in the vertical (y) direction
• Horizontal Equation:
∆𝑥 = 𝑣𝑥 𝑡
• Vertical Equations:
Direction is important!
1 2
∆𝑦 = 𝑣𝑦 𝑡 + 𝑎𝑡
2
2
2
2𝑎∆𝑦 = 𝑣𝑦𝑓
− 𝑣𝑦𝑜
𝑣𝑦𝑓 − 𝑣𝑦𝑜
𝑎=
𝑡
Sample
A ball rolls off a table 2.0 m high. If it is
rolling at 4.0 m/sec when it gets to the
end of the table, how far will it travel
horizontally before hitting the ground?
Sample
A cannonball is projected horizontally
from a cliff 40 m high. If the ball lands 65
m from the edge of the cliff, how fast was
the ball moving when it was projected?
Sample
Mr. Mellyn is going to show off his diving
skills from a 10 m high platform. He
dives horizontally with an initial speed of
4.0 m/sec.
a) How far from the edge of the board
does Mr. Mellyn land horizontally?
b) What is Mr. Mellyn’s velocity as he hits
the water?