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## Homework Statement

## Homework Equations

Elastic potential energy = 1/2 * C * u^2

Ek = 1/2 * m * v^2

Efficiency = useful output/total input

## The Attempt at a Solution

**a)**My main problem is to find the length of tension force. From the graph i can see that 12 cm corresponds to 100N, so i just need to find the length of the tension in cm and i can determine it in newton. The solution states the tension forces are approximately 5.1cm long but i really cant figure out why, for me it seems they kinda arbitrarily picked a point on the slingshot but this cant be the reason.

**b)**I got it

**c)**So if i use the formula for kinetic energy i get: Ek = 1/2 * 26*10^-3 kg * (9.3 ms^-1)^2 = 1.12 J.

But i dont know how to calculate the total energy input for the efficiency in this question. The solution says its 7.2J but i thought it should be the 6.0J from b)

I hope someone can explain it to me, thanks for every answer.