# Changes in Mechanical Energy for Nonconservative Forces: Problem with a toy cannon

A toy cannon uses a spring to project a 5.30 g soft rubber ball. The spring is originally compressed by 5.00 cm and has a force constant of 8.00 N/m. When the cannon is fired, the ball moves 15.0 cm through the horizontal barrel of the cannon and barrel exerts a constant friction force of 0.032 N on the ball. a) With what speed does the projectile leave the barrel of the cannon?

I thought to use the equation 1/2mv + 1/2kx = 1/2mv + 1/2kx
But having the friction with the barrel, I got lost, and have no idea where to go from there. Or actually where to start.

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PhanthomJay
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A toy cannon uses a spring to project a 5.30 g soft rubber ball. The spring is originally compressed by 5.00 cm and has a force constant of 8.00 N/m. When the cannon is fired, the ball moves 15.0 cm through the horizontal barrel of the cannon and barrel exerts a constant friction force of 0.032 N on the ball. a) With what speed does the projectile leave the barrel of the cannon?

I thought to use the equation 1/2mv + 1/2kx = 1/2mv + 1/2kx
But having the friction with the barrel, I got lost, and have no idea where to go from there. Or actually where to start.
Please note that KE is 1/2 mv^2 and PE_spring = 1/2 kx^2. Also you should be familiar with the conservation of total energy equation when non-conservative forces are involved, you know, W_nc = delta KE + delta PE?
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