+0  
 
0
56
3
avatar

A Stone is thrown downward from a height of 900 feet with a velocity of 30 ft/sec. Find its distance above the ground after t seconds. What is its velocity after 5 seconds? When will it strike the ground? What it's velocity be when it hits the ground?

Guest Nov 15, 2018
 #1
avatar+92785 
+1

We have this

 

H   =   -16t^2  - 30t + 900

Where H is the height after t seconds

 

The derivative of this is the velocity

H'  =  -32t - 30

 

After 5 seconds, the velocity is

 

-32(5) - 30   =

 

-190 ft/ s

 

To find the time to hit the ground....set H = 0

 

-16t^2 - 30t + 900   = 0

 

-8t^2  - 15t  + 450  =  0

 

8t^2 + 15t - 450  = 0

 

The positive solution to this is ≈ 6.62 sec

 

The velocity at this time is    -32(6.62) - 30 ≈   -241.8  ft/sec

 

 

cool cool cool

CPhill  Nov 15, 2018
 #2
avatar
0

Thanks

Guest Nov 16, 2018
 #3
avatar+14579 
0

Approx   330 fps is ARROW velocity from a compound bow.....that rock is moving!   In reality, air friction will keep it from coming down that fast!   

  Terminal velocity for a body thrown out of an airplane (think skydiver) is ~ 120 mph , which is

120 m/hr x 5280 ft/m x 1hr/3600s = 120*5280/3600 = 176 ft/sec   (been there, done that)    just an FYI.....    ~ EP

ElectricPavlov  Nov 16, 2018

4 Online Users

New Privacy Policy

We use cookies to personalise content and advertisements and to analyse access to our website. Furthermore, our partners for online advertising receive information about your use of our website.
For more information: our cookie policy and privacy policy.