Quick question- energy

[joe215]

Homework Statement

If a projectile of mass m leaves the carrell of a cannon with a speed v, at what speed will a projectile of mass 2m leave the barrel.

I think i need to use conservation of energy but I am not sure how!

Thanks

 

[PhanthomJay]

Hint: Since the projectiles are fired over the same length of the carrell with the same force, and since work is force times the distance in the direction of the force, then the work done on the projectiles are the same in each case. Think about the work-energy theorem. Please show your attempt.