# Physics Plz Help

1. Apr 14, 2008

### LesVampires

Superman leaps in front of Lois Lane to save her from a volley of bullets. In a 1 minute interval, an automatic weapon fires 149 bullets, each of mass 8.0 g, at 430 m/s. The bullets strike his mighty chest, which has an area of 0.74 m2. Find the average force exerted on Superman's chest if the bullets bounce back after an elastic, head-on collision.

i dont c what the area has to do with it i tried multiplyin

number of bullets x mass in kg x velocity / by the time in seconds 60

2. Apr 14, 2008

### D H

Staff Emeritus
Almost! That is the force if the collisions were purely inelastic (i.e., if the bullets stuck to Superman). These are elastic collisions.

3. Apr 14, 2008

### LesVampires

then what would it be? the equation cant figure it out

4. Apr 14, 2008

### D H

Staff Emeritus
What happens in an elastic collision?

5. Apr 14, 2008

### LesVampires

well the v is halved right? so 1/2 mv2

6. Apr 14, 2008

### D H

Staff Emeritus
No. What happens in an elastic collision?

7. Apr 14, 2008

### LesVampires

momentum is conserved??? like KE0=KEf

8. Apr 14, 2008

### D H

Staff Emeritus
You are having conceptual issues, so please take this one step at a time. What happens in an elastic collision?

9. Apr 14, 2008

### LesVampires

im honestly clueless at this point... energy is conserved?

10. Apr 14, 2008

### D H

Staff Emeritus
What happens to the bullets? What is their velocity (NOT SPEED) before and after the collision?

11. Apr 14, 2008

### LesVampires

oh its in the opposite direction

12. Apr 14, 2008

### D H

Staff Emeritus
OK. So how much has the momentum changed? Remember that momentum, like velocity, is a vector quantity.

13. Apr 14, 2008

### LesVampires

twice as much?

14. Apr 14, 2008

### D H

Staff Emeritus
That's right. I don't like the question mark, however. It means you are guessing.

15. Apr 14, 2008

### LesVampires

no im not, coz it acts like distance vs displacement kinda, so i still dont get whats wrong with the equation

16. Apr 14, 2008

### D H

Staff Emeritus
Spell things out then. Draw pictures. Momentum is conserved here, so the momentum changes incurred the bullets also happen to Superman (but in exactly the opposite sense, of course). So what exactly happens to each bullet? Be specific. Don't say "twice as much". (Twice as much as what?)

I have an early meeting tomorrow so I need to go to bed. Good luck!

17. Apr 14, 2008

### LesVampires

never mind i got it!