Rishi's Physics Thread

[Rod]

Christine is bouncing on her trampoline. At her maximum bounce height the bottom of her feet are 1.5 m above the level of the unstretched trampoline surface. At the lowest point of her bounce the bottom of her feet are 0.4m below the unstretched level of the trampoline surface. Christine weighs a bouncy 48 kg
From this information, what is the best estimate of the spring constant of the trampoline?

Hi Rishi

Egp at max height = 48 x 10 x 1.9 = 912 J

Es = Egp

Es = 1/2kx^2 = 912

Sub 0.4m because this is the amount that has been stretched

0.5xkx0.4^2 = 912

k = 1824/0.16 = 11400 Nm

de

[Rishi97]

Hi Rishi

Egp at max height = 48 x 10 x 1.9 = 912 J

Es = Egp

Es = 1/2kx^2 = 912

Sub 0.4m because this is the amount that has been stretched

0.5xkx0.4^2 = 912

k = 1824/0.16 = 11400 Nm

de

Thanks
Just wondering, wouldn't the max height be 1.5 instead of 1.9?

[Rod]

Thanks
Just wondering, wouldn't the max height be 1.5 instead of 1.9?

Yeah sorry the max height is 1.5. But I reckon you should add 0.4 m because the trampoline is beings stretched by 0.4 m. We can't just leave out the energy from that.

[Rishi97]

Yeah sorry the max height is 1.5. But I reckon you should add 0.4 m because the trampoline is beings stretched by 0.4 m. We can't just leave out the energy from that.

So the stretched length would also be added to the total height? Idk bout this... getting confuzzled now :/

[Rod]

So the stretched length would also be added to the total height? Idk bout this... getting confuzzled now :/

Yeah. Do you have the answer? Is it 11400 Nm or whatever?

[Rishi97]

Yeah. Do you have the answer? Is it 11400 Nm or whatever?

no sorry this was a question from a STAV practise exam 

[Rod]

no sorry this was a question from a STAV practise exam 

I think you do have to add that extra height because it still counts as gravitational potential energy being used up and converted to strain potential energy. I think you'll have to wait for Lxnl and PB to see if I'm correct, if I am then they could explain, I'm running out of ideas.

[Rishi97]

I think you do have to add that extra height because it still counts as gravitational potential energy being used up and converted to strain potential energy. I think you'll have to wait for Lxnl and PB to see if I'm correct, if I am then they could explain, I'm running out of ideas.

yeah true
haha all good Rod. You've helped me heaps. Thanks
Yeah I'll wait for them...

[PB]

Yes, everything you have done is correct Rod, well done! That is a pretty tricky question.  In fact Rishi, this question was similar to that bungee jumping example I gave in my lecture, exactly the same method to solve it.

[Rishi97]

What exactly is the function of a slip ring used in generators?

[lzxnl]

Maintain contact between the rotating wire and the external circuit, maintains current direction so the current is AC

[knightrider]

How would you do this question

I have found out that the balloon takes 10 seconds to reach the ground but i am struggling with the second part of the question

[davomac]

How would you do this question

I have found out that the balloon takes 10 seconds to reach the ground but i am struggling with the second part of the question

using the rule x=ut+.5at^2
80=0+.5x10xt^2
t=4
because the coin takes 4 seconds and the balloon took 10, it landed 6secs after

[Zealous]

How would you do this question

I have found out that the balloon takes 10 seconds to reach the ground but i am struggling with the second part of the question

Find the time it takes for the coin to reach the ground after it is dropped.





Solve for 't' using the quadratic formula:



The coin takes 3.28 seconds to hit the ground. Therefore the balloon lands 10-3.28=6.72 seconds after the coin hits the ground.

[knightrider]

Thanks so much zealous'

 how would you do the following question

I have worked out the initial speed to be 20m/s and the maximum height to be 20m

how would you find the speed of the cork as it returned to its
starting point?