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St. Mary's Physics Online

 

 

 

b. Objects propelled horizontally also fall vertically at a rate of +9.8 m/s2 (No matter how hard object is thrown)

 

 

Dropped/Projected Horizontally
Rutgers U

 

 

 

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St. Mary's Physics Online

 

 

 

 

 

 

 

 

 

 
A simple projectile has 2 velocities

 

Horizontal and vertical

 

 

 

Force of gravity only
acts vertically

 

 

Vertical Velocity (Vy) - Force of gravity accelerates objects vertically
at a rate of +10 m/s2 (ay)

 

 

 

 

Horizontal velocity?

 

Vx ?

 

 

In absence of air resistance,

Projectiles maintain initial horizontal velocity given. (Inertia)

 

Vx is constant

(not slowed, or sped up)

 

 

 

Viy = ?

(Initial vertical velocity)

 

 

 

 

 

Viy = 0 !!!!!

 

 

object projected horizontally

 

 

 

 

 

Ex1) A rock is thrown horizontally from the top of a cliff @ 150m/s. 

a) How long does it take the rock to fall 45m vertically.

 

 

 

 

Step 1 - Underline

 

Ex1) A rock is thrown horizontally from the top of a cliff @ 150m/s. 

a) How long does it take the rock to fall 45m vertically.

 

 

 

 

 

Step 2 - Turn information into letters

(Don't mix y information with x information)


Horizontal Vertical
Vx = 150 m/s Viy = 0
(thrown horizontally)
t = ? ay=+10 m/s2
  dy = 45 m
  t = ?

 

 

 

 

 

 

Step 3 -  Substitute and Solve

 

 

 

dy = Viyt + 1/2ayt2

 

 

 

45 m = 0 + ½(10 m/s2)t2

 

 

 

45 m = 5t2

 

 

 

 

9 = t2


 

t = 3 seconds

 

 

 

 

 

 

 

b) Find the rock's vertical velocity at 45 meters

 

Horizontal Info Vertical Info
Vx = 150 m/s Viy = 0
(thrown horizontally)
t = ? ay=+10 m/s2
  dy = 45 m
  Vfy = ?

 

 

 

 

 

 

 

Vfy2 = Viy2  + 2ad

 

 

Vfy2 = 02 + 2(+10 m/s2)45m

Vfy2 = 20(45)

 

 

Vfy = 30 m/s

 

 

 

 

 

Only X Equation

** dx = Vxt **

(from Vx = dx/t)

 

dx - Horizontal distance
Vx - horizontal velocity

 

 

 

 

ORIGINAL QUESTION

Ex1) A rock is thrown horizontally from the top of a cliff @ 150m/s.  How long does it take the rock to
fall 45m vertically

 

c) What was the rocks' horizontal displacement after 3 seconds?

 

 

Horizontal Vertical
Vx = 150 m/s Viy= 0
(thrown horizontally)
t = 3 sec ay = 10 m/s2
dx = ? dy = 45 m
  t = 3 sec

 

Given

 

Vx = 150 m/s 

(Object thrown horizontally) 

 

dx = ?

(A.K.A. Range)

 

t = 3 seconds

 

 

 

 

 

 

** dx  = Vxt **

 

 

dx = 150 m/s(3 seconds)

 

 

 

= 450 m

 

 

 

 

 

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