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Happy September
1st!
• 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)
• Velocity at maximum height is 0 m/s
– It has to stop, turn around, then speed back up towards the earth.
𝑎=𝑔=−9.8𝑚 /𝑠2
𝑔≈−10𝑚/𝑠2
Must use g = -9.8 m/s2 on WebAssign (and labs) but you can use g = -10 m/s2 on all quizzes and tests.
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:
• Vertical Equations:
∆ 𝑥=𝑣𝑥𝑡
∆ 𝑦=𝑣 𝑦𝑡+12𝑎𝑡 2
2𝑎∆ 𝑦=𝑣 𝑦𝑓2 −𝑣 𝑦𝑜
2
𝑎=𝑣 𝑦𝑓 −𝑣 𝑦𝑜
𝑡
Direction is important!
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?
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