# Work+ Nrg! gr.11 problems

1. Apr 8, 2007

### biochemnerd

1. The problem statement, all variables and given/known data
1. A 95 g arrow is shot by a bow from ground height such that it is moving at 25m/s at a height of 25m. Ignore air resistance.
a) What was the launch speed?
b) How much work did the bow do on the arrow?

m=95g (.095 kg)
V2= 25m/s
d=25m
t=1 s

2. Relevant equations
Ek= ½ mv(squared)

W= deltaE= Fapp(d)

3. The attempt at a solution

a) The launch speed was either 0m/s OR 25 m/s.

b) Ek= ½ mv(squared)
= ½ (0.095)(25)2
=1.1875

W= deltaE= Fapp(d)
= 1.1875(25)
= 29.69 J

* i cant figure out whether the 25m/s is the v2. or is it irrelevant and i should assume v1=0m/s. this is gr. 11 stuff, any suggestions?

2. Apr 8, 2007

### biochemnerd

can i automatically assume that:
deltaE =Ek (d)

and that deltaE=Fapp(d)

or is it not possible for change in energy=Kinetic energy

i'm not sure on the equations. help!

Last edited: Apr 8, 2007
3. Apr 8, 2007

### hage567

I guess you can say the arrow is initially at rest until the bow acts on it, giving the arrow its launch speed. The 25 m/s is important. Think conservation of energy. If the bow has a certain kinetic and potential energy at the 25 m above the ground, what must the initial kinetic energy be for energy to be conserved? (ground level is considered zero potential energy)

Why do you think this? The launch speed can't be zero!

Where did you get this?