VCAA 2007 Q9 Motion

[darkmaster25]

In a laboratory class at school, Lee is given a spring with a stiffness of 20 N m–1 and unstretched length of
0.40 m. He hangs it vertically, and attaches a mass to it, so that the new length of the spring is 0.60 m.
Question 9
Assuming the spring has no mass, what was the value of the mass he attached?

Examiners report: When the spring is stretched 0.2 m by hanging a mass on the end of it, the force in the spring, ‘kx’, must equal the gravitational force on the mass ‘mg’. This results in a mass of 0.4 kg.
Many students incorrectly presumed that the change in gravitational potential energy was equal to the energy stored in the spring.

I really don't understand the examiners report to this. Why is k*x=m*g. Please explain. thanks

[Ndon95]

Since the net force (weight force) is downwards which is mg it can be equated to fx which is the force of spring?

[darkmaster25]

Ahhh i see, yes that works out to be 0.4 kg and is correct. But why isn't it corrent when done with the energy formulas. letting 0.5kx^2= change in potential energy, why is this wrong?

Also another question on the same exam paper, Q11 in electronics. Why can't it be A instead of C.
http://www.vcaa.vic.edu.au/vce/studies/physics/pastexams/2007/2007physics1.pdf

[StumbleBum]

Ok the equation they have used is F=kx where F is Force, k is the spring constant and x is the amount the spring has been stretched.
seen as the weight of the mass that is hung is the only thing that will be applying the force then the force is equal to mg or F=mg

so when we put these to formulae's together we get mg=kx and then you can solve from there.

mg=kx
m*10=20*0.2
m*10=4
m=4/10
m=0.4kg

[StumbleBum]

Ahhh i see, yes that works out to be 0.4 kg and is correct. But why isn't it corrent when done with the energy formulas. letting 0.5kx^2= change in potential energy, why is this wrong?

Also another question on the same exam paper, Q11 in electronics. Why can't it be A instead of C.
http://www.vcaa.vic.edu.au/vce/studies/physics/pastexams/2007/2007physics1.pdf

This is because it is not an isolated system and you cannot assume that all the energy has gone from Gravitational potential to kinetic, because there would have most likely been energy lost due to heat and sound in the stretching of the spring.

As for the electronics question this is because there cannot be a negative brightness and when this is demodulated back into an electrical signal it too cannot have a negative as the brightness did not go into the negative.

[darkmaster25]

yeah but isn't the point of modulation to change it back to original signal. I have seen in many books and they show the final voltage output to be the same as the original, in other words it hasn't been translated upwards. It's in A+ Phyiscs Notes.

[StumbleBum]

although i think that is only when modulating say an information signal (audio), but i could be wrong.