# Energy Conservation of a toy gun

1. A toy gun fires a 9.41-g projectile disc by using a compressed sprint (k=1.72 x 10^3 N/m) and a 13.1 cm long barrel. As the disc travvels through the barrel it experiences a constant frictional force of 0.13 N. If the spring is compressed 14 mm what is the speed of the disc as it leaves the gun?

2. Ek = 1/2mv^2
Ee = 1/2kx^2
W = F*d

3. DeltaEe - W = DeltaEk ???

1/2kx^2 - F*d = 1/2mv^2
1/2(1.72 x 10^3)(0.014)^2 - (0.13*0.131) = 1/2(0.00941)(v)^2
v = 5.67 m/s BUT THATS WRONG, I have a physics exam some day this week and i was just reviewin gmy notes and trying old review problems he gave us..and on the review it says the answer to this question is 1.8 m/s...i dont get what im doing wrong :(

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im not whats wrong but after going through it again if i make v = 1.8m/s as it says the answer is then the Ek part equals 0.015 while the left equals 0.15 so maybe imd oing a conversion mistake but i chekced i dont see where :S:S:S

hmm come on guys im sure someones knows what im doing wrong this is grade 12physics :S :S :S

Is it that hard? or no one wants to help lool..

malawi_glenn
Homework Helper
Is it that hard? or no one wants to help lool..
there has been many many problems of this kind posting during these weeks. We who hang around on this forum have studies to do ourselves and can not help everyone all the time.

1. find out how much potential energy there is in the spring when pulled back in the initial position.

2. All this energy is then reformed as kinetic energy + friction work.

3. Find out the work done by the friction, if the force is given and the lenght, what is then the work done by the friction.

4. The energy remained is kinetic energy of bullet.

2. Ek = 1/2mv^2
Ee = 1/2kx^2
W = F*d

3. DeltaEe = DeltaEk + W

1/2kx^2 - F*d = 1/2mv^2

1/2(1.72 x 10^3)(0.014)^2 - (0.13*0.131) = 1/2(0.00941)(v)^2
v = 5.67 m/s

Does that look right to you? cause thats what i did and i dont get 1.8m/s which is supposedly the right answer

cepheid
Staff Emeritus
Gold Member
I checked your arithmetic. I also seem to think that your equation is right. I would take it up with the prof. It's possible his answer to the review problem was wrong.

malawi_glenn