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Summersnow8
 one year ago
A 2.00 lb rock is dropped from a bridge to the water 50.0 ft below. What is its kinetic energy (in ft lb) as it hits the water.
book says 99.8 lb ft
@radar
Summersnow8
 one year ago
A 2.00 lb rock is dropped from a bridge to the water 50.0 ft below. What is its kinetic energy (in ft lb) as it hits the water. book says 99.8 lb ft @radar

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anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Do you know how to calculate kinetic energy?

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0@ospreytriple KE = 1/2 m v^2 ?

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0That's right. The mass can be determined from the given weight of 2 lb using\[F _{g} = mg\]I assume you're OK with doing that. What's left, then is to determine the velocity of the rock when it hits the water. Knowing that acceleration is 32.2 ft/s^2 we could use\[v _{f}^{2} = v _{i}^{2} + 2a \Delta d\]where v_i = 0, a = 32.2, and d = 50

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Then, knowing m and v_f, you'll be able to calculate KE.

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0so i got Fg = 64.4 Vf = 56.7

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0We agree on the Vf. But I don't use the Imperial system of units often. Does the rock have a weight of 2 lbf or a mass of 2 lbm?

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0the rock is in lbs.... the answer is supposed to be in lb ft

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0gravity for ft/s ^2 is 32.2 ft/s^2

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0OK. So if the weight (Fg) of the rock is 2.00 lb, we need to rearrange the weight formula to solve for mass, i.e.\[F _{g}=mg\]\[m=\frac{ F _{g} }{ g } = \frac{ 2.00 \text{ lb} }{ 32.2 \frac{ \text{ft} }{ \text{s} ^{2}}}\]

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Now what do you get for mass?

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Good. If my memory is correct, the units of mass in this system are slugs. Anyway, doesn;t matter. Use this value of mass and the value of Vf you calculated previously and use the KE equation. What do you get?

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0KE = 1/2 (.06211 lb) (56.7 m/s) = 99.83

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Looks good to me. Congrats!

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0One quick question before you go. I assume you're in the U.S. I'm in Canada. Do you ever work in the metric system in your physics class?

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0yes, we need to know how to convert back and forth

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0OK. Thanks. And do you call these units of mass SLUGS?

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0some of the problems I have solved involved slugs, but not all of them

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Well, you've done very well. It's been great working with you.
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