# Work-Kinetic Energy Problem

1. Mar 17, 2012

### shaggyace

1. The problem statement, all variables and given/known data

A 0.16 kg pinecone falls 18 m to the ground where it lands with a speed of 11 m/s.

a)How much work was done on the pinecone by air resistance?

b)What was the average for of air resistance exerted on the pine cone?

2. Relevant equations

ƩF=ma

Vf^2=Vi^2 + 2aΔx

W=FΔx=maΔx=1/2(mVf^2)-1/2(mVi^2)

3. The attempt at a solution

I tried using the kinematic equation to find initial velocity while setting the acceleration as gravity. I got 18.78 m/s but that doesn't seem right. I tried using that initial velocity in the work-kinetic energy theorem and it came out to -28.275. However that was the wrong answer. I don't understand where to start with this problem let alone part B. Someone help me please.

2. Mar 17, 2012

### LawrenceC

The 18.78 m/sec represents the speed if there were no wind resistance. Note that it is larger than what was presented in the problem statement.

Write an energy balance such as:

PE(1) + KE(1) = PE(2) + KE(2) + Work

3. Mar 17, 2012

### tal444

The net work done is equivalent to the work by gravity minus the work by friction.

EDIT: My way is essentially a rearrangement of LawrenceC's formula above.