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

1. anonymous

Do you know how to calculate kinetic energy?

2. Summersnow8

@ospreytriple KE = 1/2 m v^2 ?

3. anonymous

That'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

4. anonymous

Then, knowing m and v_f, you'll be able to calculate KE.

5. Summersnow8

so i got Fg = 64.4 Vf = 56.7

6. anonymous

We 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?

7. Summersnow8

the rock is in lbs.... the answer is supposed to be in lb ft

8. Summersnow8

gravity for ft/s ^2 is 32.2 ft/s^2

9. anonymous

OK. 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}}}$

10. anonymous

Now what do you get for mass?

11. Summersnow8

0.06211 lb

12. anonymous

Good. 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?

13. Summersnow8

KE = 1/2 (.06211 lb) (56.7 m/s) = 99.83

14. anonymous

Looks good to me. Congrats!

15. anonymous

One 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?

16. Summersnow8

yes, we need to know how to convert back and forth

17. anonymous

OK. Thanks. And do you call these units of mass SLUGS?

18. Summersnow8

some of the problems I have solved involved slugs, but not all of them

19. anonymous

Well, you've done very well. It's been great working with you.

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