SOVLED(adsbygoogle = window.adsbygoogle || []).push({});

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

Bob, who has a mass of 75 kg, can throw a 600 g rock with a speed of 35 m/s. The distance through which his hand moves as he accelerates the rock forward from rest until he releases it is 1.6 m.

(a) What constant force must Bob exert on the rock to throw it with this speed?

229.7 N

(b) If Bob is standing on frictionless ice, what is his recoil speed after releasing the rock?

___ m/s

2. Relevant equations

1. F=ma

2. Vf=Vo+at

3. X=Vot+.5a(t^2)

4. Vf^2=(Vo)^2+2ax

3. The attempt at a solution

Well, I got the first part of the problem correct by using the fourth equation and solving for acceleration which equals 382.8. I then applied that to the first equation, because the mass (.6) was given, to find the force which equals 229.7. But, I can't seem to figure out how to start the second equation. I thought that the force would be the same for Bob because of Newton's third law and so I found the acceleration using equation 1, and then applied that to the fourth equation again to find the final velocity. That number kept coming out around 3.13 but that answer isn't correct. Any idea what I'm doing incorrectly and what I should be doing? Thank you in advance!

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Bob throwing a rock

**Physics Forums | Science Articles, Homework Help, Discussion**