When bullets are shot on Superman, they bounce off of his chest. 100 Bullets will be shot on his chest per minute, each with a mass of 9g and v= 1000m/s. Assume that the bullets will bounce off elastically back in x-direction.
a) What is the average power of all of the bullets?
b) What work should superman perform
i) when he stands still?
ii) when he moves towards the shooter with a speed of 20km/h?
The Attempt at a Solution
a) P= [ (1/2mv^2)*100 ] / 60 = 7500watt
For a) I found the total kinetic energy of all the bullets and divided by the total time, 60s, to find the average power.
I found b) confusing
so in i) since he stands without moving ( unaffected by the bullets) I thought the work he is doing must be equal with the work all the bullets are doing and in opposite direction. So that he opposes all that work by the bullets. However as I realized, an object usually does move to do some work. so can he still be doing work standing still there? ( like if a ball hits a wall, I guess the ball is doing work but the wall is not? )
my suspected answer would be W= [ (1/2mv^2)*100 ] = 450,000watt
for ii) then for ii I would assume the work is the sum of work done in i) plus the work done for just moving which is m*v ( v is known but mass of the guy isn't). That would be my idea but seemingly it wouldn't work since I don't even know the guy's mass.
Can you please help me about this question?