A limb with mass 14 kg falls out of a tree from a height of 3.5 m. How much mechanical energy does the limb have just before it hits the ground? (Assume there is no air resistance.)

A. 480.2 J

B. 355.3 J

C. 0 J

D. 49 J

Question

A limb with mass 14 kg falls out of a tree from a height of 3.5 m. How much mechanical energy does the limb have just before it hits the ground? (Assume there is no air resistance.)

A. 480.2 J

B. 355.3 J

C. 0 J

D. 49 J

anonymous · Answer

There are two ways you could go about this: Calculating the kinetic energy as a function of the limb's end-velocity, or calculating the limb's potential energy from its point of origin somewhere above the ground. The latter is the easiest. $$U=F*d$$ U is the potential energy of an object with force F at a distance d above the ground. When the limb falls toward the ground, the potential energy is very rapidly converted to mechanical energy - they are two sides of the same coin, which is why we can find the size of one by calculating the other.

anonymous · Answer

$$U=K$$ when the limb is infinitesimally close to the ground.

anonymous · Answer

I don't understand anything you said. I'm sorry, I really just can't understand physics I guess.

anonymous · Answer

limb possess potential energy,
P.E.=mgh
     $$=14\times9.8\times3.5$$
=480.2 J
Hence the answer is A
I guess so, but please confirm.

anonymous · Answer

That's exactly it!