# Finding recoil speed given force, mass, and velocity

1. Jun 27, 2010

### mandy9008

1. The problem statement, all variables and given/known data
A rifle with a weight of 25 N fires a 5.5 g bullet with a speed of 260 m/s.
(a) Find the recoil speed of the rifle.
(b) If a 725 N man holds the rifle firmly against his shoulder, find the recoil speed of the man and rifle.

2. Relevant equations
p=mv
F=p/t

3. The attempt at a solution
I do not even know where to begin converting the mass to 0.0055 kg

2. Jun 27, 2010

### Staff: Mentor

Hint: What's conserved as the rifle is fired?

3. Jun 27, 2010

### mandy9008

momentum?

4. Jun 27, 2010

### Staff: Mentor

Yep. That's all you need to solve both parts.

5. Jun 27, 2010

### mandy9008

m1v1i + m2v2i = m1v1f + m2v2f

6. Jun 27, 2010

### Staff: Mentor

Sure. What's the initial velocity and momentum before the rifle is fired?

7. Jun 27, 2010

### mandy9008

velocity and momentum is 0

8. Jun 27, 2010

### Staff: Mentor

Right. Keep going. You're given the final speed of the bullet.

9. Jun 27, 2010

### mandy9008

okay, so it will be
mass 1 is 0.0055kg? then what is mass 2?

10. Jun 27, 2010

### Staff: Mentor

Mass 2 is the mass of the rifle (at least in part a). You'll have to figure that out from the given information.

11. Jun 27, 2010

### mandy9008

okay, so
m1v1i + (0.0055 kg) (0 m/s) = m1v1f + (0.0055 kg) (260 m/s)

I am confused now

12. Jun 27, 2010

### inky

You know weight of the rifle. Find mass of the rifle.
Before firing, rifle and bullet haven't moved yet. You can consider their velocities before firing.

13. Jun 28, 2010

### Uku

I suggest you write out all your equations and develop them before putting numbers in. Things are less messy then and thus more elegant.

14. Jun 28, 2010

### Staff: Mentor

You switched them around. You have m2 as the mass of the bullet, which means that m1 is the mass of the rifle (for part a). That's fine.

You are given the weight of the rifle in Newtons, so what's its mass?

Since the initial speed of everything is zero, what does that do to the left hand side of your equation?