Kinematics Question

[Biebs]

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A rock is dropped from a 128-ft cliff. How long does it take to reach the ground? (Ignore air resistance and take g = 32 ft/sec2.)

• 2.0 sec
• 2.8 sec
• 4.0 sec
• 5.6 sec

Am I missing something here? If we think about it logically, if you drop a rock and it has an acceleration (of falls down) at a rate of 32 feet per second/second, then in FOUR seconds you have reached 128 feet, i.e falling to the ground.

Using Kinematics equation:
d= ViT = 1/2at^2
Vi= 0

128= 1/2(32) t^2
128= 16 t^2
8= t^2
t= 4 seconds.

However, the answer is 3 seconds. WTF am I doing wrong here?!

Thanks

 

[MedPR]

A rock is dropped from a 128-ft cliff. How long does it take to reach the ground? (Ignore air resistance and take g = 32 ft/sec2.)

• 2.0 sec
• 2.8 sec
• 4.0 sec
• 5.6 sec

Am I missing something here? If we think about it logically, if you drop a rock and it has an acceleration (of falls down) at a rate of 32 feet per second/second, then in FOUR seconds you have reached 128 feet, i.e falling to the ground.

Using Kinematics equation:
d= ViT = 1/2at^2
Vi= 0

128= 1/2(32) t^2
128= 16 t^2
8= t^2
t= 4 seconds.

However, the answer is 3 seconds. WTF am I doing wrong here?!

Thanks

4^2 is 16, not 8. sqrt8 is 2sqrt2 or 2*1.4 or 2.8

 

[Biebs]

Oh damn, you're right. #MATHFAIL.

Haha. Thanks, I appreciate it.

 

[mcloaf]

Yikes, english measurements. No thanks.