Is this the Tarzan question?

[HistoRocks]

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This here physics book has so many Tarzan questions, so I'm just curious:

Tarzan, whose mass is 80.0 kg, swings from a 3.00-m vine that is horizontal when he starts. At the bottom of his arc, he picks up 60.0 kg Jane in a perfectly inelastic collision. What is the height of the highest tree limb they can reach on their upward swing?

 

[Freakedout]

that dont sound too bad...just your good ole conservation of energy would do the trick i think...hmmm

 

[MollyMalone]

Nope. Not it.

 

[HistoRocks]

Nope. Not it.

Okay, tks. Just wanted to be sure. I am wondering where the MCAT test writers get their questions from.

 

[OwnageMobile]

Tarzan was an intense complete passage containing most of basic Newtonian physics. We(a previous thread is on this topic) aren't sure what the answer was for one of the questions about momentum and energy conservation, but it was a cool problem.

 

[DrBowtie]

just for S&G's (avoiding studying for my bio lab practical by doing physics on SDN)

Before Jane
Ki+Ui=Kf+Uf
Ui=Kf
80kg(9.8m/s^2)(3m)=.5(80kg)(v^2)
vf=58.8^.5=7.67m/s

Getting Jane
mvi=(mt+mj)vf
80kg(7.67m/s)=(80kg+60kg)vf
vf=4.38

With Jane
Ki+Ui=Kf+Uf
.5(140kg)4.38m/s=140kg(9.8m/s^2)h
h=.22m

Correct me if you see an error

 

[Shrike]

Correct me if you see an error

Perfect, until the last line: you didn't square the velocity in the KE term in the last equation, hence your height is of (low) by a factor of 4.38.

 

[Shrike]

that dont sound too bad...just your good ole conservation of energy would do the trick i think...hmmm

As you see in brettbachelor's writeup, conservation of energy is fine up until the collision, but because it's inelastic, energy is lost -- we use momentum to solve those (as he did). Then, it's conservation of energy again for the trip up.

 

[frankrizzo18]

Shrike - How did you do on your PS section on the real thing? Did you feel like maybe you did not do as well as you may have hoped? Thanks

As you see in brettbachelor's writeup, conservation of energy is fine up until the collision, but because it's inelastic, energy is lost -- we use momentum to solve those (as he did). Then, it's conservation of energy again for the trip up.

 

[DrBowtie]

Well it was at 1:15AM and the answer did seem a little low but thanks for pointing out the correction.

 

[HistoRocks]

Here's the solution (from the instructor's manual):

Use conservation of mechanical energy to find Tarzan's velocity just as he reaches Jane:

v = sq.root (2ghi) = sq.root [(2)(9.80)(3)] = 7.67 m/s

Velocity,V, of Tarzan & Jane just after collision (again using COME):

(M + m)V = Mv + 0, or

V = (M/M+m)v = (80 kg/140 kg)(7.67 m/s) = 4.38 m/s

And finally,

H = [V^2]/2g = [(4.38 m/s)^2]/(2)(9.80) = 0.980 m

 

[Shrike]

Shrike - How did you do on your PS section on the real thing? Did you feel like maybe you did not do as well as you may have hoped? Thanks

I got 14 both times. Durned chemistry -- I can't keep straight what's a cathode and what's an anode, for example. It's just senility setting in.

Yes, I'd have prefered to get a 15, but I pretty much knew I would blow something, particularly as I really don't know chemistry very well (and it's been two decades).