# Kinetic Energy

## Homework Statement

A 3 m long canon fires a 14.0 kg projectile with a velocity of 630 m/s [F]. Calculate.
A) The kinetic energy of the projectile as it leaves the canon barrel.
B) The average force acting on the projectile in the barrel.

## The Attempt at a Solution

A) Ek=1/2*14kg*630m/s
Ek=2.8*10^6j

B)2.8*10^6j=Fnet*3m
Fnet=2.8*10^6N/3m
Fnet=9.3*10^5N
Is that Right?

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## Homework Statement

A 3 m long canon fires a 14.0 kg projectile with a velocity of 630 m/s [F]. Calculate.
A) The kinetic energy of the projectile as it leaves the canon barrel.
B) The average force acting on the projectile in the barrel.

## The Attempt at a Solution

A) Ek=1/2*14kg*630m/s
Ek=2.8*10^6N

B)2.8*10^6N=Fnet*3m
Fnet=2.8*10^6N/3m
Fnet=9.3*10^5N
Is that Right?
Kinetic energy is not $\frac{mv}{2}$ (though your final number is correct), and its unit is the Joule, not the Newton. Other than that, your approach seems correct.

arildno
Homework Helper
Gold Member
Dearly Missed
I can't possibly see the point in exercise B.
Sure, you can average whatever numbers you want, but that doesn't mean anything if the deviations from that average is humungous, as in this case.

The rate of momentum transfer in the initial phase (from the explosion of gun powder) is orders of magnitude higher than the forces acting from the barrel upon the ball during its motion through it.
In addition, those negligible barrel forces are resistive, rather than contributive.

I can't possibly see the point in exercise B.
Sure, you can average whatever numbers you want, but that doesn't mean anything if the deviations from that average is humungous, as in this case.

The rate of momentum transfer in the initial phase (from the explosion of gun powder) is orders of magnitude higher than the forces acting from the barrel upon the ball during its motion through it.
In addition, those negligible barrel forces are resistive, rather than contributive.

arildno
Homework Helper
Gold Member
Dearly Missed
No, I mean that the exercise is dumb, not that you have made a mistake.