What is the average force on Superman's chest when hit by 3g bullets at 500 m/s?

[Shatzkinator]

Homework Statement

Suppose a gangster sprays Superman's chest with 3 g bullets at the rate of 100 bullets/min, and the speed of each bullet is 500 m/s. Suppose too that the bullets rebound straight back with no change in speed. What is the magnitude of the average force on Superman's chest?

Homework Equations

J = FavgT
p = mv
J = pf - pi

The Attempt at a Solution

I don't really understand how to get the delta T. I got J = -3 Ns from equation 3, don't know where to go from there.

 

[LowlyPion]

You have 300 grams of bullets a minute hitting at 5 grams a sec on average.

Taking rebound into account you have a change in velocity of 1000 m/s then don't you have 5k grams/s*m/s? That looks like Newtons to me.

 

[Shatzkinator]

You have 300 grams of bullets a minute hitting at 5 grams a sec on average.

Taking rebound into account you have a change in velocity of 1000 m/s then don't you have 5k grams/s*m/s? That looks like Newtons to me.

hmm interesting. So how would I know to play with the units rather than using typical linear momentum equations?

 

[LowlyPion]

hmm interesting. So how would I know to play with the units rather than using typical linear momentum equations?

Insight. Regardless of the equations you must end with the same units they are asking. Units are often a guide to the answer. But it's not like I'm suggesting you not use the equations, so much as suggesting you understand what the equations are relating to begin with.

F = m*a = m*Δv/Δt = Δmv/Δt = Δp/Δt