Question

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

Answer 1

Do you know how to calculate kinetic energy?

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

so i got
Fg = 64.4
Vf = 56.7

Answer 6

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?

Answer 7

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

Answer 8

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

Answer 9

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.\[F _{g}=mg\]\[m=\frac{ F _{g} }{ g } = \frac{ 2.00 \text{ lb} }{ 32.2 \frac{ \text{ft} }{ \text{s} ^{2}}}\]

Answer 10

Now what do you get for mass?

Answer 11

0.06211 lb

Answer 12

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?

Answer 13

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

Answer 14

Looks good to me. Congrats!

Answer 15

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?

Answer 16

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

Answer 17

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

Answer 18

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

Answer 19

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