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(a) Find the speed of the asteroid at closest approach, assuming its speed at infinite distance to be zero and considering only its interaction with the Earth.

2.64011 km/s

(b) Observations indicate the asteroid to have a diameter of about 2.0 km. Estimate the kinetic energy of the asteroid at closest approach, assuming it has an average density of 3.33 g/cm3. (For comparison, a 1-megaton nuclear weapon releases about 5.6 1015 J of energy.)

It is part B i am getting stuck with...

I know that density = mv and once i get the mass of the asteriod I can get the KE = .5mv^2...

So D = m/v

3.33 g/cm^3 = x/(4/3)pi r^3

My question is what units do i use here...for instance, should the 3.33 g/cm^3 be converted to meters and the 2.0km converted to meters.....if so, how to i convert cm^3 to m^3....

If this is not correct, could someone please guide me on the correct units to use.

thanks so much