What is the launch speed and work done by the bow?

[biochemnerd]

Homework Statement

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

Homework Equations

Ek= ½ mv(squared)

W= deltaE= Fapp(d)

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 can't 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?

 

[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!

 

[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)

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

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

t=1 s

Where did you get this?